[PATCH v3 00/14] perf/hw_breakpoint: Optimize for thousands of tasks

[PATCH v3 00/14] perf/hw_breakpoint: Optimize for thousands of tasks

[Marco Elver]

The hw_breakpoint subsystem's code has seen little change in over 10
years. In that time, systems with >100s of CPUs have become common,
along with improvements to the perf subsystem: using breakpoints on
thousands of concurrent tasks should be a supported usecase.

The breakpoint constraints accounting algorithm is the major bottleneck
in doing so:

  1. toggle_bp_slot() and fetch_bp_busy_slots() are O(#cpus * #tasks):
     Both iterate through all CPUs and call task_bp_pinned(), which is
     O(#tasks).

  2. Everything is serialized on a global mutex, 'nr_bp_mutex'.

The series progresses with the simpler optimizations and finishes with
the more complex optimizations:

 1. We first optimize task_bp_pinned() to only take O(1) on average.

 2. Rework synchronization to allow concurrency when checking and
    updating breakpoint constraints for tasks.

 3. Eliminate the O(#cpus) loops in the CPU-independent case.

Along the way, smaller micro-optimizations and cleanups are done as they
seemed obvious when staring at the code (but likely insignificant).

The result is (on a system with 256 CPUs) that we go from:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
	 	[ ^ more aggressive benchmark parameters took too long ]
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 236.418 [sec]
 |
 |   123134.794271 usecs/op
 |  7880626.833333 usecs/op/cpu

... to the following with all optimizations:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 0.067 [sec]
 |
 |       35.292187 usecs/op
 |     2258.700000 usecs/op/cpu

On the used test system, that's an effective speedup of ~3490x per op.

Which is on par with the theoretical ideal performance through
optimizations in hw_breakpoint.c (constraints accounting disabled), and
only 12% slower than no breakpoints at all.

Changelog
---------

v3:
* Fix typos.
* Introduce hw_breakpoint_is_used() for the test.
* Add WARN_ON in bp_blots_histogram_add().
* Don't use raw_smp_processor_id() in test.
* Apply Acked-by/Reviewed-by given in v2 for mostly unchanged patches.

v2: https://lkml.kernel.org/r/20220628095833.2579903-1-elver@google.com
 * Add KUnit test suite.
 * Remove struct bp_busy_slots and simplify functions.
 * Add "powerpc/hw_breakpoint: Avoid relying on caller synchronization".
 * Add "locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked()".
 * Use percpu-rwsem instead of rwlock.
 * Use task_struct::perf_event_mutex instead of sharded mutex.
 * Drop v1 "perf/hw_breakpoint: Optimize task_bp_pinned() if CPU-independent".
 * Add "perf/hw_breakpoint: Introduce bp_slots_histogram".
 * Add "perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets".
 * Add "perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets".
 * Apply Acked-by/Reviewed-by given in v1 for unchanged patches.
==> Speedup of ~3490x (vs. ~3315x in v1).

v1: https://lore.kernel.org/all/20220609113046.780504-1-elver@google.com/

Marco Elver (14):
  perf/hw_breakpoint: Add KUnit test for constraints accounting
  perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test
  perf/hw_breakpoint: Clean up headers
  perf/hw_breakpoint: Optimize list of per-task breakpoints
  perf/hw_breakpoint: Mark data __ro_after_init
  perf/hw_breakpoint: Optimize constant number of breakpoint slots
  perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable
  perf/hw_breakpoint: Remove useless code related to flexible
    breakpoints
  powerpc/hw_breakpoint: Avoid relying on caller synchronization
  locking/percpu-rwsem: Add percpu_is_write_locked() and
    percpu_is_read_locked()
  perf/hw_breakpoint: Reduce contention with large number of tasks
  perf/hw_breakpoint: Introduce bp_slots_histogram
  perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent
    task targets
  perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task
    targets

 arch/powerpc/kernel/hw_breakpoint.c  |  53 ++-
 arch/sh/include/asm/hw_breakpoint.h  |   5 +-
 arch/x86/include/asm/hw_breakpoint.h |   5 +-
 include/linux/hw_breakpoint.h        |   4 +-
 include/linux/percpu-rwsem.h         |   6 +
 include/linux/perf_event.h           |   3 +-
 kernel/events/Makefile               |   1 +
 kernel/events/hw_breakpoint.c        | 638 ++++++++++++++++++++-------
 kernel/events/hw_breakpoint_test.c   | 333 ++++++++++++++
 kernel/locking/percpu-rwsem.c        |   6 +
 lib/Kconfig.debug                    |  10 +
 11 files changed, 885 insertions(+), 179 deletions(-)
 create mode 100644 kernel/events/hw_breakpoint_test.c

-- 
2.37.0.rc0.161.g10f37bed90-goog

[PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Marco Elver]

Add KUnit test for hw_breakpoint constraints accounting, with various
interesting mixes of breakpoint targets (some care was taken to catch
interesting corner cases via bug-injection).

The test cannot be built as a module because it requires access to
hw_breakpoint_slots(), which is not inlinable or exported on all
architectures.

Signed-off-by: Marco Elver <elver@google.com>
---
v3:
* Don't use raw_smp_processor_id().

v2:
* New patch.
---
 kernel/events/Makefile             |   1 +
 kernel/events/hw_breakpoint_test.c | 323 +++++++++++++++++++++++++++++
 lib/Kconfig.debug                  |  10 +
 3 files changed, 334 insertions(+)
 create mode 100644 kernel/events/hw_breakpoint_test.c

diff --git a/kernel/events/Makefile b/kernel/events/Makefile
index 8591c180b52b..91a62f566743 100644
--- a/kernel/events/Makefile
+++ b/kernel/events/Makefile
@@ -2,4 +2,5 @@
 obj-y := core.o ring_buffer.o callchain.o
 
 obj-$(CONFIG_HAVE_HW_BREAKPOINT) += hw_breakpoint.o
+obj-$(CONFIG_HW_BREAKPOINT_KUNIT_TEST) += hw_breakpoint_test.o
 obj-$(CONFIG_UPROBES) += uprobes.o
diff --git a/kernel/events/hw_breakpoint_test.c b/kernel/events/hw_breakpoint_test.c
new file mode 100644
index 000000000000..433c5c45e2a5
--- /dev/null
+++ b/kernel/events/hw_breakpoint_test.c
@@ -0,0 +1,323 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit test for hw_breakpoint constraints accounting logic.
+ *
+ * Copyright (C) 2022, Google LLC.
+ */
+
+#include <kunit/test.h>
+#include <linux/cpumask.h>
+#include <linux/hw_breakpoint.h>
+#include <linux/kthread.h>
+#include <linux/perf_event.h>
+#include <asm/hw_breakpoint.h>
+
+#define TEST_REQUIRES_BP_SLOTS(test, slots)						\
+	do {										\
+		if ((slots) > get_test_bp_slots()) {					\
+			kunit_skip((test), "Requires breakpoint slots: %d > %d", slots,	\
+				   get_test_bp_slots());				\
+		}									\
+	} while (0)
+
+#define TEST_EXPECT_NOSPC(expr) KUNIT_EXPECT_EQ(test, -ENOSPC, PTR_ERR(expr))
+
+#define MAX_TEST_BREAKPOINTS 512
+
+static char break_vars[MAX_TEST_BREAKPOINTS];
+static struct perf_event *test_bps[MAX_TEST_BREAKPOINTS];
+static struct task_struct *__other_task;
+
+static struct perf_event *register_test_bp(int cpu, struct task_struct *tsk, int idx)
+{
+	struct perf_event_attr attr = {};
+
+	if (WARN_ON(idx < 0 || idx >= MAX_TEST_BREAKPOINTS))
+		return NULL;
+
+	hw_breakpoint_init(&attr);
+	attr.bp_addr = (unsigned long)&break_vars[idx];
+	attr.bp_len = HW_BREAKPOINT_LEN_1;
+	attr.bp_type = HW_BREAKPOINT_RW;
+	return perf_event_create_kernel_counter(&attr, cpu, tsk, NULL, NULL);
+}
+
+static void unregister_test_bp(struct perf_event **bp)
+{
+	if (WARN_ON(IS_ERR(*bp)))
+		return;
+	if (WARN_ON(!*bp))
+		return;
+	unregister_hw_breakpoint(*bp);
+	*bp = NULL;
+}
+
+static int get_test_bp_slots(void)
+{
+	static int slots;
+
+	if (!slots)
+		slots = hw_breakpoint_slots(TYPE_DATA);
+
+	return slots;
+}
+
+static void fill_one_bp_slot(struct kunit *test, int *id, int cpu, struct task_struct *tsk)
+{
+	struct perf_event *bp = register_test_bp(cpu, tsk, *id);
+
+	KUNIT_ASSERT_NOT_NULL(test, bp);
+	KUNIT_ASSERT_FALSE(test, IS_ERR(bp));
+	KUNIT_ASSERT_NULL(test, test_bps[*id]);
+	test_bps[(*id)++] = bp;
+}
+
+/*
+ * Fills up the given @cpu/@tsk with breakpoints, only leaving @skip slots free.
+ *
+ * Returns true if this can be called again, continuing at @id.
+ */
+static bool fill_bp_slots(struct kunit *test, int *id, int cpu, struct task_struct *tsk, int skip)
+{
+	for (int i = 0; i < get_test_bp_slots() - skip; ++i)
+		fill_one_bp_slot(test, id, cpu, tsk);
+
+	return *id + get_test_bp_slots() <= MAX_TEST_BREAKPOINTS;
+}
+
+static int dummy_kthread(void *arg)
+{
+	return 0;
+}
+
+static struct task_struct *get_other_task(struct kunit *test)
+{
+	struct task_struct *tsk;
+
+	if (__other_task)
+		return __other_task;
+
+	tsk = kthread_create(dummy_kthread, NULL, "hw_breakpoint_dummy_task");
+	KUNIT_ASSERT_FALSE(test, IS_ERR(tsk));
+	__other_task = tsk;
+	return __other_task;
+}
+
+static int get_test_cpu(int num)
+{
+	int cpu;
+
+	WARN_ON(num < 0);
+
+	for_each_online_cpu(cpu) {
+		if (num-- <= 0)
+			break;
+	}
+
+	return cpu;
+}
+
+/* ===== Test cases ===== */
+
+static void test_one_cpu(struct kunit *test)
+{
+	int idx = 0;
+
+	fill_bp_slots(test, &idx, get_test_cpu(0), NULL, 0);
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+}
+
+static void test_many_cpus(struct kunit *test)
+{
+	int idx = 0;
+	int cpu;
+
+	/* Test that CPUs are independent. */
+	for_each_online_cpu(cpu) {
+		bool do_continue = fill_bp_slots(test, &idx, cpu, NULL, 0);
+
+		TEST_EXPECT_NOSPC(register_test_bp(cpu, NULL, idx));
+		if (!do_continue)
+			break;
+	}
+}
+
+static void test_one_task_on_all_cpus(struct kunit *test)
+{
+	int idx = 0;
+
+	fill_bp_slots(test, &idx, -1, current, 0);
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+	/* Remove one and adding back CPU-target should work. */
+	unregister_test_bp(&test_bps[0]);
+	fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
+}
+
+static void test_two_tasks_on_all_cpus(struct kunit *test)
+{
+	int idx = 0;
+
+	/* Test that tasks are independent. */
+	fill_bp_slots(test, &idx, -1, current, 0);
+	fill_bp_slots(test, &idx, -1, get_other_task(test), 0);
+
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+	/* Remove one from first task and adding back CPU-target should not work. */
+	unregister_test_bp(&test_bps[0]);
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+}
+
+static void test_one_task_on_one_cpu(struct kunit *test)
+{
+	int idx = 0;
+
+	fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+	/*
+	 * Remove one and adding back CPU-target should work; this case is
+	 * special vs. above because the task's constraints are CPU-dependent.
+	 */
+	unregister_test_bp(&test_bps[0]);
+	fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
+}
+
+static void test_one_task_mixed(struct kunit *test)
+{
+	int idx = 0;
+
+	TEST_REQUIRES_BP_SLOTS(test, 3);
+
+	fill_one_bp_slot(test, &idx, get_test_cpu(0), current);
+	fill_bp_slots(test, &idx, -1, current, 1);
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+
+	/* Transition from CPU-dependent pinned count to CPU-independent. */
+	unregister_test_bp(&test_bps[0]);
+	unregister_test_bp(&test_bps[1]);
+	fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
+	fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+}
+
+static void test_two_tasks_on_one_cpu(struct kunit *test)
+{
+	int idx = 0;
+
+	fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
+	fill_bp_slots(test, &idx, get_test_cpu(0), get_other_task(test), 0);
+
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+	/* Can still create breakpoints on some other CPU. */
+	fill_bp_slots(test, &idx, get_test_cpu(1), NULL, 0);
+}
+
+static void test_two_tasks_on_one_all_cpus(struct kunit *test)
+{
+	int idx = 0;
+
+	fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
+	fill_bp_slots(test, &idx, -1, get_other_task(test), 0);
+
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+	/* Cannot create breakpoints on some other CPU either. */
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
+}
+
+static void test_task_on_all_and_one_cpu(struct kunit *test)
+{
+	int tsk_on_cpu_idx, cpu_idx;
+	int idx = 0;
+
+	TEST_REQUIRES_BP_SLOTS(test, 3);
+
+	fill_bp_slots(test, &idx, -1, current, 2);
+	/* Transitioning from only all CPU breakpoints to mixed. */
+	tsk_on_cpu_idx = idx;
+	fill_one_bp_slot(test, &idx, get_test_cpu(0), current);
+	fill_one_bp_slot(test, &idx, -1, current);
+
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+
+	/* We should still be able to use up another CPU's slots. */
+	cpu_idx = idx;
+	fill_one_bp_slot(test, &idx, get_test_cpu(1), NULL);
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
+
+	/* Transitioning back to task target on all CPUs. */
+	unregister_test_bp(&test_bps[tsk_on_cpu_idx]);
+	/* Still have a CPU target breakpoint in get_test_cpu(1). */
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	/* Remove it and try again. */
+	unregister_test_bp(&test_bps[cpu_idx]);
+	fill_one_bp_slot(test, &idx, -1, current);
+
+	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
+	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
+}
+
+static struct kunit_case hw_breakpoint_test_cases[] = {
+	KUNIT_CASE(test_one_cpu),
+	KUNIT_CASE(test_many_cpus),
+	KUNIT_CASE(test_one_task_on_all_cpus),
+	KUNIT_CASE(test_two_tasks_on_all_cpus),
+	KUNIT_CASE(test_one_task_on_one_cpu),
+	KUNIT_CASE(test_one_task_mixed),
+	KUNIT_CASE(test_two_tasks_on_one_cpu),
+	KUNIT_CASE(test_two_tasks_on_one_all_cpus),
+	KUNIT_CASE(test_task_on_all_and_one_cpu),
+	{},
+};
+
+static int test_init(struct kunit *test)
+{
+	/* Most test cases want 2 distinct CPUs. */
+	return num_online_cpus() < 2 ? -EINVAL : 0;
+}
+
+static void test_exit(struct kunit *test)
+{
+	for (int i = 0; i < MAX_TEST_BREAKPOINTS; ++i) {
+		if (test_bps[i])
+			unregister_test_bp(&test_bps[i]);
+	}
+
+	if (__other_task) {
+		kthread_stop(__other_task);
+		__other_task = NULL;
+	}
+}
+
+static struct kunit_suite hw_breakpoint_test_suite = {
+	.name = "hw_breakpoint",
+	.test_cases = hw_breakpoint_test_cases,
+	.init = test_init,
+	.exit = test_exit,
+};
+
+kunit_test_suites(&hw_breakpoint_test_suite);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Marco Elver <elver@google.com>");
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 2e24db4bff19..4c87a6edf046 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2513,6 +2513,16 @@ config STACKINIT_KUNIT_TEST
 	  CONFIG_GCC_PLUGIN_STRUCTLEAK, CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF,
 	  or CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL.
 
+config HW_BREAKPOINT_KUNIT_TEST
+	bool "Test hw_breakpoint constraints accounting" if !KUNIT_ALL_TESTS
+	depends on HAVE_HW_BREAKPOINT
+	depends on KUNIT=y
+	default KUNIT_ALL_TESTS
+	help
+	  Tests for hw_breakpoint constraints accounting.
+
+	  If unsure, say N.
+
 config TEST_UDELAY
 	tristate "udelay test driver"
 	help
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Dmitry Vyukov]

On Mon, 4 Jul 2022 at 17:06, Marco Elver <elver@google.com> wrote:
>
> Add KUnit test for hw_breakpoint constraints accounting, with various
> interesting mixes of breakpoint targets (some care was taken to catch
> interesting corner cases via bug-injection).
>
> The test cannot be built as a module because it requires access to
> hw_breakpoint_slots(), which is not inlinable or exported on all
> architectures.
>
> Signed-off-by: Marco Elver <elver@google.com>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
> v3:
> * Don't use raw_smp_processor_id().
>
> v2:
> * New patch.
> ---
>  kernel/events/Makefile             |   1 +
>  kernel/events/hw_breakpoint_test.c | 323 +++++++++++++++++++++++++++++
>  lib/Kconfig.debug                  |  10 +
>  3 files changed, 334 insertions(+)
>  create mode 100644 kernel/events/hw_breakpoint_test.c
>
> diff --git a/kernel/events/Makefile b/kernel/events/Makefile
> index 8591c180b52b..91a62f566743 100644
> --- a/kernel/events/Makefile
> +++ b/kernel/events/Makefile
> @@ -2,4 +2,5 @@
>  obj-y := core.o ring_buffer.o callchain.o
>
>  obj-$(CONFIG_HAVE_HW_BREAKPOINT) += hw_breakpoint.o
> +obj-$(CONFIG_HW_BREAKPOINT_KUNIT_TEST) += hw_breakpoint_test.o
>  obj-$(CONFIG_UPROBES) += uprobes.o
> diff --git a/kernel/events/hw_breakpoint_test.c b/kernel/events/hw_breakpoint_test.c
> new file mode 100644
> index 000000000000..433c5c45e2a5
> --- /dev/null
> +++ b/kernel/events/hw_breakpoint_test.c
> @@ -0,0 +1,323 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * KUnit test for hw_breakpoint constraints accounting logic.
> + *
> + * Copyright (C) 2022, Google LLC.
> + */
> +
> +#include <kunit/test.h>
> +#include <linux/cpumask.h>
> +#include <linux/hw_breakpoint.h>
> +#include <linux/kthread.h>
> +#include <linux/perf_event.h>
> +#include <asm/hw_breakpoint.h>
> +
> +#define TEST_REQUIRES_BP_SLOTS(test, slots)                                            \
> +       do {                                                                            \
> +               if ((slots) > get_test_bp_slots()) {                                    \
> +                       kunit_skip((test), "Requires breakpoint slots: %d > %d", slots, \
> +                                  get_test_bp_slots());                                \
> +               }                                                                       \
> +       } while (0)
> +
> +#define TEST_EXPECT_NOSPC(expr) KUNIT_EXPECT_EQ(test, -ENOSPC, PTR_ERR(expr))
> +
> +#define MAX_TEST_BREAKPOINTS 512
> +
> +static char break_vars[MAX_TEST_BREAKPOINTS];
> +static struct perf_event *test_bps[MAX_TEST_BREAKPOINTS];
> +static struct task_struct *__other_task;
> +
> +static struct perf_event *register_test_bp(int cpu, struct task_struct *tsk, int idx)
> +{
> +       struct perf_event_attr attr = {};
> +
> +       if (WARN_ON(idx < 0 || idx >= MAX_TEST_BREAKPOINTS))
> +               return NULL;
> +
> +       hw_breakpoint_init(&attr);
> +       attr.bp_addr = (unsigned long)&break_vars[idx];
> +       attr.bp_len = HW_BREAKPOINT_LEN_1;
> +       attr.bp_type = HW_BREAKPOINT_RW;
> +       return perf_event_create_kernel_counter(&attr, cpu, tsk, NULL, NULL);
> +}
> +
> +static void unregister_test_bp(struct perf_event **bp)
> +{
> +       if (WARN_ON(IS_ERR(*bp)))
> +               return;
> +       if (WARN_ON(!*bp))
> +               return;
> +       unregister_hw_breakpoint(*bp);
> +       *bp = NULL;
> +}
> +
> +static int get_test_bp_slots(void)
> +{
> +       static int slots;
> +
> +       if (!slots)
> +               slots = hw_breakpoint_slots(TYPE_DATA);
> +
> +       return slots;
> +}
> +
> +static void fill_one_bp_slot(struct kunit *test, int *id, int cpu, struct task_struct *tsk)
> +{
> +       struct perf_event *bp = register_test_bp(cpu, tsk, *id);
> +
> +       KUNIT_ASSERT_NOT_NULL(test, bp);
> +       KUNIT_ASSERT_FALSE(test, IS_ERR(bp));
> +       KUNIT_ASSERT_NULL(test, test_bps[*id]);
> +       test_bps[(*id)++] = bp;
> +}
> +
> +/*
> + * Fills up the given @cpu/@tsk with breakpoints, only leaving @skip slots free.
> + *
> + * Returns true if this can be called again, continuing at @id.
> + */
> +static bool fill_bp_slots(struct kunit *test, int *id, int cpu, struct task_struct *tsk, int skip)
> +{
> +       for (int i = 0; i < get_test_bp_slots() - skip; ++i)
> +               fill_one_bp_slot(test, id, cpu, tsk);
> +
> +       return *id + get_test_bp_slots() <= MAX_TEST_BREAKPOINTS;
> +}
> +
> +static int dummy_kthread(void *arg)
> +{
> +       return 0;
> +}
> +
> +static struct task_struct *get_other_task(struct kunit *test)
> +{
> +       struct task_struct *tsk;
> +
> +       if (__other_task)
> +               return __other_task;
> +
> +       tsk = kthread_create(dummy_kthread, NULL, "hw_breakpoint_dummy_task");
> +       KUNIT_ASSERT_FALSE(test, IS_ERR(tsk));
> +       __other_task = tsk;
> +       return __other_task;
> +}
> +
> +static int get_test_cpu(int num)
> +{
> +       int cpu;
> +
> +       WARN_ON(num < 0);
> +
> +       for_each_online_cpu(cpu) {
> +               if (num-- <= 0)
> +                       break;
> +       }
> +
> +       return cpu;
> +}
> +
> +/* ===== Test cases ===== */
> +
> +static void test_one_cpu(struct kunit *test)
> +{
> +       int idx = 0;
> +
> +       fill_bp_slots(test, &idx, get_test_cpu(0), NULL, 0);
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +}
> +
> +static void test_many_cpus(struct kunit *test)
> +{
> +       int idx = 0;
> +       int cpu;
> +
> +       /* Test that CPUs are independent. */
> +       for_each_online_cpu(cpu) {
> +               bool do_continue = fill_bp_slots(test, &idx, cpu, NULL, 0);
> +
> +               TEST_EXPECT_NOSPC(register_test_bp(cpu, NULL, idx));
> +               if (!do_continue)
> +                       break;
> +       }
> +}
> +
> +static void test_one_task_on_all_cpus(struct kunit *test)
> +{
> +       int idx = 0;
> +
> +       fill_bp_slots(test, &idx, -1, current, 0);
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +       /* Remove one and adding back CPU-target should work. */
> +       unregister_test_bp(&test_bps[0]);
> +       fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> +}
> +
> +static void test_two_tasks_on_all_cpus(struct kunit *test)
> +{
> +       int idx = 0;
> +
> +       /* Test that tasks are independent. */
> +       fill_bp_slots(test, &idx, -1, current, 0);
> +       fill_bp_slots(test, &idx, -1, get_other_task(test), 0);
> +
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +       /* Remove one from first task and adding back CPU-target should not work. */
> +       unregister_test_bp(&test_bps[0]);
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +}
> +
> +static void test_one_task_on_one_cpu(struct kunit *test)
> +{
> +       int idx = 0;
> +
> +       fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +       /*
> +        * Remove one and adding back CPU-target should work; this case is
> +        * special vs. above because the task's constraints are CPU-dependent.
> +        */
> +       unregister_test_bp(&test_bps[0]);
> +       fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> +}
> +
> +static void test_one_task_mixed(struct kunit *test)
> +{
> +       int idx = 0;
> +
> +       TEST_REQUIRES_BP_SLOTS(test, 3);
> +
> +       fill_one_bp_slot(test, &idx, get_test_cpu(0), current);
> +       fill_bp_slots(test, &idx, -1, current, 1);
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +
> +       /* Transition from CPU-dependent pinned count to CPU-independent. */
> +       unregister_test_bp(&test_bps[0]);
> +       unregister_test_bp(&test_bps[1]);
> +       fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> +       fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +}
> +
> +static void test_two_tasks_on_one_cpu(struct kunit *test)
> +{
> +       int idx = 0;
> +
> +       fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
> +       fill_bp_slots(test, &idx, get_test_cpu(0), get_other_task(test), 0);
> +
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +       /* Can still create breakpoints on some other CPU. */
> +       fill_bp_slots(test, &idx, get_test_cpu(1), NULL, 0);
> +}
> +
> +static void test_two_tasks_on_one_all_cpus(struct kunit *test)
> +{
> +       int idx = 0;
> +
> +       fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
> +       fill_bp_slots(test, &idx, -1, get_other_task(test), 0);
> +
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +       /* Cannot create breakpoints on some other CPU either. */
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
> +}
> +
> +static void test_task_on_all_and_one_cpu(struct kunit *test)
> +{
> +       int tsk_on_cpu_idx, cpu_idx;
> +       int idx = 0;
> +
> +       TEST_REQUIRES_BP_SLOTS(test, 3);
> +
> +       fill_bp_slots(test, &idx, -1, current, 2);
> +       /* Transitioning from only all CPU breakpoints to mixed. */
> +       tsk_on_cpu_idx = idx;
> +       fill_one_bp_slot(test, &idx, get_test_cpu(0), current);
> +       fill_one_bp_slot(test, &idx, -1, current);
> +
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +
> +       /* We should still be able to use up another CPU's slots. */
> +       cpu_idx = idx;
> +       fill_one_bp_slot(test, &idx, get_test_cpu(1), NULL);
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
> +
> +       /* Transitioning back to task target on all CPUs. */
> +       unregister_test_bp(&test_bps[tsk_on_cpu_idx]);
> +       /* Still have a CPU target breakpoint in get_test_cpu(1). */
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       /* Remove it and try again. */
> +       unregister_test_bp(&test_bps[cpu_idx]);
> +       fill_one_bp_slot(test, &idx, -1, current);
> +
> +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
> +}
> +
> +static struct kunit_case hw_breakpoint_test_cases[] = {
> +       KUNIT_CASE(test_one_cpu),
> +       KUNIT_CASE(test_many_cpus),
> +       KUNIT_CASE(test_one_task_on_all_cpus),
> +       KUNIT_CASE(test_two_tasks_on_all_cpus),
> +       KUNIT_CASE(test_one_task_on_one_cpu),
> +       KUNIT_CASE(test_one_task_mixed),
> +       KUNIT_CASE(test_two_tasks_on_one_cpu),
> +       KUNIT_CASE(test_two_tasks_on_one_all_cpus),
> +       KUNIT_CASE(test_task_on_all_and_one_cpu),
> +       {},
> +};
> +
> +static int test_init(struct kunit *test)
> +{
> +       /* Most test cases want 2 distinct CPUs. */
> +       return num_online_cpus() < 2 ? -EINVAL : 0;
> +}
> +
> +static void test_exit(struct kunit *test)
> +{
> +       for (int i = 0; i < MAX_TEST_BREAKPOINTS; ++i) {
> +               if (test_bps[i])
> +                       unregister_test_bp(&test_bps[i]);
> +       }
> +
> +       if (__other_task) {
> +               kthread_stop(__other_task);
> +               __other_task = NULL;
> +       }
> +}
> +
> +static struct kunit_suite hw_breakpoint_test_suite = {
> +       .name = "hw_breakpoint",
> +       .test_cases = hw_breakpoint_test_cases,
> +       .init = test_init,
> +       .exit = test_exit,
> +};
> +
> +kunit_test_suites(&hw_breakpoint_test_suite);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Marco Elver <elver@google.com>");
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 2e24db4bff19..4c87a6edf046 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2513,6 +2513,16 @@ config STACKINIT_KUNIT_TEST
>           CONFIG_GCC_PLUGIN_STRUCTLEAK, CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF,
>           or CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL.
>
> +config HW_BREAKPOINT_KUNIT_TEST
> +       bool "Test hw_breakpoint constraints accounting" if !KUNIT_ALL_TESTS
> +       depends on HAVE_HW_BREAKPOINT
> +       depends on KUNIT=y
> +       default KUNIT_ALL_TESTS
> +       help
> +         Tests for hw_breakpoint constraints accounting.
> +
> +         If unsure, say N.
> +
>  config TEST_UDELAY
>         tristate "udelay test driver"
>         help
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:11 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Mon, 4 Jul 2022 at 17:06, Marco Elver <elver@google.com> wrote:
> >
> > Add KUnit test for hw_breakpoint constraints accounting, with various
> > interesting mixes of breakpoint targets (some care was taken to catch
> > interesting corner cases via bug-injection).
> >
> > The test cannot be built as a module because it requires access to
> > hw_breakpoint_slots(), which is not inlinable or exported on all
> > architectures.
> >
> > Signed-off-by: Marco Elver <elver@google.com>
>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> > ---
> > v3:
> > * Don't use raw_smp_processor_id().
> >
> > v2:
> > * New patch.
> > ---
> >  kernel/events/Makefile             |   1 +
> >  kernel/events/hw_breakpoint_test.c | 323 +++++++++++++++++++++++++++++
> >  lib/Kconfig.debug                  |  10 +
> >  3 files changed, 334 insertions(+)
> >  create mode 100644 kernel/events/hw_breakpoint_test.c
> >
> > diff --git a/kernel/events/Makefile b/kernel/events/Makefile
> > index 8591c180b52b..91a62f566743 100644
> > --- a/kernel/events/Makefile
> > +++ b/kernel/events/Makefile
> > @@ -2,4 +2,5 @@
> >  obj-y := core.o ring_buffer.o callchain.o
> >
> >  obj-$(CONFIG_HAVE_HW_BREAKPOINT) += hw_breakpoint.o
> > +obj-$(CONFIG_HW_BREAKPOINT_KUNIT_TEST) += hw_breakpoint_test.o
> >  obj-$(CONFIG_UPROBES) += uprobes.o
> > diff --git a/kernel/events/hw_breakpoint_test.c b/kernel/events/hw_breakpoint_test.c
> > new file mode 100644
> > index 000000000000..433c5c45e2a5
> > --- /dev/null
> > +++ b/kernel/events/hw_breakpoint_test.c
> > @@ -0,0 +1,323 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * KUnit test for hw_breakpoint constraints accounting logic.
> > + *
> > + * Copyright (C) 2022, Google LLC.
> > + */
> > +
> > +#include <kunit/test.h>
> > +#include <linux/cpumask.h>
> > +#include <linux/hw_breakpoint.h>
> > +#include <linux/kthread.h>
> > +#include <linux/perf_event.h>
> > +#include <asm/hw_breakpoint.h>
> > +
> > +#define TEST_REQUIRES_BP_SLOTS(test, slots)                                            \
> > +       do {                                                                            \
> > +               if ((slots) > get_test_bp_slots()) {                                    \
> > +                       kunit_skip((test), "Requires breakpoint slots: %d > %d", slots, \
> > +                                  get_test_bp_slots());                                \
> > +               }                                                                       \
> > +       } while (0)
> > +
> > +#define TEST_EXPECT_NOSPC(expr) KUNIT_EXPECT_EQ(test, -ENOSPC, PTR_ERR(expr))
> > +
> > +#define MAX_TEST_BREAKPOINTS 512
> > +
> > +static char break_vars[MAX_TEST_BREAKPOINTS];
> > +static struct perf_event *test_bps[MAX_TEST_BREAKPOINTS];
> > +static struct task_struct *__other_task;
> > +
> > +static struct perf_event *register_test_bp(int cpu, struct task_struct *tsk, int idx)
> > +{
> > +       struct perf_event_attr attr = {};
> > +
> > +       if (WARN_ON(idx < 0 || idx >= MAX_TEST_BREAKPOINTS))
> > +               return NULL;
> > +
> > +       hw_breakpoint_init(&attr);
> > +       attr.bp_addr = (unsigned long)&break_vars[idx];
> > +       attr.bp_len = HW_BREAKPOINT_LEN_1;
> > +       attr.bp_type = HW_BREAKPOINT_RW;
> > +       return perf_event_create_kernel_counter(&attr, cpu, tsk, NULL, NULL);
> > +}
> > +
> > +static void unregister_test_bp(struct perf_event **bp)
> > +{
> > +       if (WARN_ON(IS_ERR(*bp)))
> > +               return;
> > +       if (WARN_ON(!*bp))
> > +               return;
> > +       unregister_hw_breakpoint(*bp);
> > +       *bp = NULL;
> > +}
> > +
> > +static int get_test_bp_slots(void)
> > +{
> > +       static int slots;
> > +
> > +       if (!slots)
> > +               slots = hw_breakpoint_slots(TYPE_DATA);
> > +
> > +       return slots;
> > +}
> > +
> > +static void fill_one_bp_slot(struct kunit *test, int *id, int cpu, struct task_struct *tsk)
> > +{
> > +       struct perf_event *bp = register_test_bp(cpu, tsk, *id);
> > +
> > +       KUNIT_ASSERT_NOT_NULL(test, bp);
> > +       KUNIT_ASSERT_FALSE(test, IS_ERR(bp));
> > +       KUNIT_ASSERT_NULL(test, test_bps[*id]);
> > +       test_bps[(*id)++] = bp;
> > +}
> > +
> > +/*
> > + * Fills up the given @cpu/@tsk with breakpoints, only leaving @skip slots free.
> > + *
> > + * Returns true if this can be called again, continuing at @id.
> > + */
> > +static bool fill_bp_slots(struct kunit *test, int *id, int cpu, struct task_struct *tsk, int skip)
> > +{
> > +       for (int i = 0; i < get_test_bp_slots() - skip; ++i)
> > +               fill_one_bp_slot(test, id, cpu, tsk);
> > +
> > +       return *id + get_test_bp_slots() <= MAX_TEST_BREAKPOINTS;
> > +}
> > +
> > +static int dummy_kthread(void *arg)
> > +{
> > +       return 0;
> > +}
> > +
> > +static struct task_struct *get_other_task(struct kunit *test)
> > +{
> > +       struct task_struct *tsk;
> > +
> > +       if (__other_task)
> > +               return __other_task;
> > +
> > +       tsk = kthread_create(dummy_kthread, NULL, "hw_breakpoint_dummy_task");
> > +       KUNIT_ASSERT_FALSE(test, IS_ERR(tsk));
> > +       __other_task = tsk;
> > +       return __other_task;
> > +}
> > +
> > +static int get_test_cpu(int num)
> > +{
> > +       int cpu;
> > +
> > +       WARN_ON(num < 0);
> > +
> > +       for_each_online_cpu(cpu) {
> > +               if (num-- <= 0)
> > +                       break;
> > +       }
> > +
> > +       return cpu;
> > +}
> > +
> > +/* ===== Test cases ===== */
> > +
> > +static void test_one_cpu(struct kunit *test)
> > +{
> > +       int idx = 0;
> > +
> > +       fill_bp_slots(test, &idx, get_test_cpu(0), NULL, 0);
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +}
> > +
> > +static void test_many_cpus(struct kunit *test)
> > +{
> > +       int idx = 0;
> > +       int cpu;
> > +
> > +       /* Test that CPUs are independent. */
> > +       for_each_online_cpu(cpu) {
> > +               bool do_continue = fill_bp_slots(test, &idx, cpu, NULL, 0);
> > +
> > +               TEST_EXPECT_NOSPC(register_test_bp(cpu, NULL, idx));
> > +               if (!do_continue)
> > +                       break;
> > +       }
> > +}
> > +
> > +static void test_one_task_on_all_cpus(struct kunit *test)
> > +{
> > +       int idx = 0;
> > +
> > +       fill_bp_slots(test, &idx, -1, current, 0);
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +       /* Remove one and adding back CPU-target should work. */
> > +       unregister_test_bp(&test_bps[0]);
> > +       fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> > +}
> > +
> > +static void test_two_tasks_on_all_cpus(struct kunit *test)
> > +{
> > +       int idx = 0;
> > +
> > +       /* Test that tasks are independent. */
> > +       fill_bp_slots(test, &idx, -1, current, 0);
> > +       fill_bp_slots(test, &idx, -1, get_other_task(test), 0);
> > +
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +       /* Remove one from first task and adding back CPU-target should not work. */
> > +       unregister_test_bp(&test_bps[0]);
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +}
> > +
> > +static void test_one_task_on_one_cpu(struct kunit *test)
> > +{
> > +       int idx = 0;
> > +
> > +       fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +       /*
> > +        * Remove one and adding back CPU-target should work; this case is
> > +        * special vs. above because the task's constraints are CPU-dependent.
> > +        */
> > +       unregister_test_bp(&test_bps[0]);
> > +       fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> > +}
> > +
> > +static void test_one_task_mixed(struct kunit *test)
> > +{
> > +       int idx = 0;
> > +
> > +       TEST_REQUIRES_BP_SLOTS(test, 3);
> > +
> > +       fill_one_bp_slot(test, &idx, get_test_cpu(0), current);
> > +       fill_bp_slots(test, &idx, -1, current, 1);
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +
> > +       /* Transition from CPU-dependent pinned count to CPU-independent. */
> > +       unregister_test_bp(&test_bps[0]);
> > +       unregister_test_bp(&test_bps[1]);
> > +       fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> > +       fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +}
> > +
> > +static void test_two_tasks_on_one_cpu(struct kunit *test)
> > +{
> > +       int idx = 0;
> > +
> > +       fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
> > +       fill_bp_slots(test, &idx, get_test_cpu(0), get_other_task(test), 0);
> > +
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +       /* Can still create breakpoints on some other CPU. */
> > +       fill_bp_slots(test, &idx, get_test_cpu(1), NULL, 0);
> > +}
> > +
> > +static void test_two_tasks_on_one_all_cpus(struct kunit *test)
> > +{
> > +       int idx = 0;
> > +
> > +       fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
> > +       fill_bp_slots(test, &idx, -1, get_other_task(test), 0);
> > +
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +       /* Cannot create breakpoints on some other CPU either. */
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
> > +}
> > +
> > +static void test_task_on_all_and_one_cpu(struct kunit *test)
> > +{
> > +       int tsk_on_cpu_idx, cpu_idx;
> > +       int idx = 0;
> > +
> > +       TEST_REQUIRES_BP_SLOTS(test, 3);
> > +
> > +       fill_bp_slots(test, &idx, -1, current, 2);
> > +       /* Transitioning from only all CPU breakpoints to mixed. */
> > +       tsk_on_cpu_idx = idx;
> > +       fill_one_bp_slot(test, &idx, get_test_cpu(0), current);
> > +       fill_one_bp_slot(test, &idx, -1, current);
> > +
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +
> > +       /* We should still be able to use up another CPU's slots. */
> > +       cpu_idx = idx;
> > +       fill_one_bp_slot(test, &idx, get_test_cpu(1), NULL);
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
> > +
> > +       /* Transitioning back to task target on all CPUs. */
> > +       unregister_test_bp(&test_bps[tsk_on_cpu_idx]);
> > +       /* Still have a CPU target breakpoint in get_test_cpu(1). */
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       /* Remove it and try again. */
> > +       unregister_test_bp(&test_bps[cpu_idx]);
> > +       fill_one_bp_slot(test, &idx, -1, current);
> > +
> > +       TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> > +       TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
> > +}
> > +
> > +static struct kunit_case hw_breakpoint_test_cases[] = {
> > +       KUNIT_CASE(test_one_cpu),
> > +       KUNIT_CASE(test_many_cpus),
> > +       KUNIT_CASE(test_one_task_on_all_cpus),
> > +       KUNIT_CASE(test_two_tasks_on_all_cpus),
> > +       KUNIT_CASE(test_one_task_on_one_cpu),
> > +       KUNIT_CASE(test_one_task_mixed),
> > +       KUNIT_CASE(test_two_tasks_on_one_cpu),
> > +       KUNIT_CASE(test_two_tasks_on_one_all_cpus),
> > +       KUNIT_CASE(test_task_on_all_and_one_cpu),
> > +       {},
> > +};
> > +
> > +static int test_init(struct kunit *test)
> > +{
> > +       /* Most test cases want 2 distinct CPUs. */
> > +       return num_online_cpus() < 2 ? -EINVAL : 0;
> > +}
> > +
> > +static void test_exit(struct kunit *test)
> > +{
> > +       for (int i = 0; i < MAX_TEST_BREAKPOINTS; ++i) {
> > +               if (test_bps[i])
> > +                       unregister_test_bp(&test_bps[i]);
> > +       }
> > +
> > +       if (__other_task) {
> > +               kthread_stop(__other_task);
> > +               __other_task = NULL;
> > +       }
> > +}
> > +
> > +static struct kunit_suite hw_breakpoint_test_suite = {
> > +       .name = "hw_breakpoint",
> > +       .test_cases = hw_breakpoint_test_cases,
> > +       .init = test_init,
> > +       .exit = test_exit,
> > +};
> > +
> > +kunit_test_suites(&hw_breakpoint_test_suite);
> > +
> > +MODULE_LICENSE("GPL");
> > +MODULE_AUTHOR("Marco Elver <elver@google.com>");
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 2e24db4bff19..4c87a6edf046 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -2513,6 +2513,16 @@ config STACKINIT_KUNIT_TEST
> >           CONFIG_GCC_PLUGIN_STRUCTLEAK, CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF,
> >           or CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL.
> >
> > +config HW_BREAKPOINT_KUNIT_TEST
> > +       bool "Test hw_breakpoint constraints accounting" if !KUNIT_ALL_TESTS
> > +       depends on HAVE_HW_BREAKPOINT
> > +       depends on KUNIT=y
> > +       default KUNIT_ALL_TESTS
> > +       help
> > +         Tests for hw_breakpoint constraints accounting.
> > +
> > +         If unsure, say N.
> > +
> >  config TEST_UDELAY
> >         tristate "udelay test driver"
> >         help
> > --
> > 2.37.0.rc0.161.g10f37bed90-goog
> >

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Mark Rutland]

Hi Marco,

[adding Will]

On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> Add KUnit test for hw_breakpoint constraints accounting, with various
> interesting mixes of breakpoint targets (some care was taken to catch
> interesting corner cases via bug-injection).
> 
> The test cannot be built as a module because it requires access to
> hw_breakpoint_slots(), which is not inlinable or exported on all
> architectures.
> 
> Signed-off-by: Marco Elver <elver@google.com>

As mentioned on IRC, I'm seeing these tests fail on arm64 when applied atop
v5.19-rc7:

| TAP version 14
| 1..1
|     # Subtest: hw_breakpoint
|     1..9
|     ok 1 - test_one_cpu
|     ok 2 - test_many_cpus
|     # test_one_task_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 3 - test_one_task_on_all_cpus
|     # test_two_tasks_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 4 - test_two_tasks_on_all_cpus
|     # test_one_task_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 5 - test_one_task_on_one_cpu
|     # test_one_task_mixed: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 6 - test_one_task_mixed
|     # test_two_tasks_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 7 - test_two_tasks_on_one_cpu
|     # test_two_tasks_on_one_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 8 - test_two_tasks_on_one_all_cpus
|     # test_task_on_all_and_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 9 - test_task_on_all_and_one_cpu
| # hw_breakpoint: pass:2 fail:7 skip:0 total:9
| # Totals: pass:2 fail:7 skip:0 total:9

... which seems to be becasue arm64 currently forbids per-task
breakpoints/watchpoints in hw_breakpoint_arch_parse(), where we have:

        /*
         * Disallow per-task kernel breakpoints since these would
         * complicate the stepping code.
         */
        if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
                return -EINVAL;

... which has been the case since day one in commit:

  478fcb2cdb2351dc ("arm64: Debugging support")

I'm not immediately sure what would be necessary to support per-task kernel
breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
invasive.

Mark.

> ---
> v3:
> * Don't use raw_smp_processor_id().
> 
> v2:
> * New patch.
> ---
>  kernel/events/Makefile             |   1 +
>  kernel/events/hw_breakpoint_test.c | 323 +++++++++++++++++++++++++++++
>  lib/Kconfig.debug                  |  10 +
>  3 files changed, 334 insertions(+)
>  create mode 100644 kernel/events/hw_breakpoint_test.c
> 
> diff --git a/kernel/events/Makefile b/kernel/events/Makefile
> index 8591c180b52b..91a62f566743 100644
> --- a/kernel/events/Makefile
> +++ b/kernel/events/Makefile
> @@ -2,4 +2,5 @@
>  obj-y := core.o ring_buffer.o callchain.o
>  
>  obj-$(CONFIG_HAVE_HW_BREAKPOINT) += hw_breakpoint.o
> +obj-$(CONFIG_HW_BREAKPOINT_KUNIT_TEST) += hw_breakpoint_test.o
>  obj-$(CONFIG_UPROBES) += uprobes.o
> diff --git a/kernel/events/hw_breakpoint_test.c b/kernel/events/hw_breakpoint_test.c
> new file mode 100644
> index 000000000000..433c5c45e2a5
> --- /dev/null
> +++ b/kernel/events/hw_breakpoint_test.c
> @@ -0,0 +1,323 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * KUnit test for hw_breakpoint constraints accounting logic.
> + *
> + * Copyright (C) 2022, Google LLC.
> + */
> +
> +#include <kunit/test.h>
> +#include <linux/cpumask.h>
> +#include <linux/hw_breakpoint.h>
> +#include <linux/kthread.h>
> +#include <linux/perf_event.h>
> +#include <asm/hw_breakpoint.h>
> +
> +#define TEST_REQUIRES_BP_SLOTS(test, slots)						\
> +	do {										\
> +		if ((slots) > get_test_bp_slots()) {					\
> +			kunit_skip((test), "Requires breakpoint slots: %d > %d", slots,	\
> +				   get_test_bp_slots());				\
> +		}									\
> +	} while (0)
> +
> +#define TEST_EXPECT_NOSPC(expr) KUNIT_EXPECT_EQ(test, -ENOSPC, PTR_ERR(expr))
> +
> +#define MAX_TEST_BREAKPOINTS 512
> +
> +static char break_vars[MAX_TEST_BREAKPOINTS];
> +static struct perf_event *test_bps[MAX_TEST_BREAKPOINTS];
> +static struct task_struct *__other_task;
> +
> +static struct perf_event *register_test_bp(int cpu, struct task_struct *tsk, int idx)
> +{
> +	struct perf_event_attr attr = {};
> +
> +	if (WARN_ON(idx < 0 || idx >= MAX_TEST_BREAKPOINTS))
> +		return NULL;
> +
> +	hw_breakpoint_init(&attr);
> +	attr.bp_addr = (unsigned long)&break_vars[idx];
> +	attr.bp_len = HW_BREAKPOINT_LEN_1;
> +	attr.bp_type = HW_BREAKPOINT_RW;
> +	return perf_event_create_kernel_counter(&attr, cpu, tsk, NULL, NULL);
> +}
> +
> +static void unregister_test_bp(struct perf_event **bp)
> +{
> +	if (WARN_ON(IS_ERR(*bp)))
> +		return;
> +	if (WARN_ON(!*bp))
> +		return;
> +	unregister_hw_breakpoint(*bp);
> +	*bp = NULL;
> +}
> +
> +static int get_test_bp_slots(void)
> +{
> +	static int slots;
> +
> +	if (!slots)
> +		slots = hw_breakpoint_slots(TYPE_DATA);
> +
> +	return slots;
> +}
> +
> +static void fill_one_bp_slot(struct kunit *test, int *id, int cpu, struct task_struct *tsk)
> +{
> +	struct perf_event *bp = register_test_bp(cpu, tsk, *id);
> +
> +	KUNIT_ASSERT_NOT_NULL(test, bp);
> +	KUNIT_ASSERT_FALSE(test, IS_ERR(bp));
> +	KUNIT_ASSERT_NULL(test, test_bps[*id]);
> +	test_bps[(*id)++] = bp;
> +}
> +
> +/*
> + * Fills up the given @cpu/@tsk with breakpoints, only leaving @skip slots free.
> + *
> + * Returns true if this can be called again, continuing at @id.
> + */
> +static bool fill_bp_slots(struct kunit *test, int *id, int cpu, struct task_struct *tsk, int skip)
> +{
> +	for (int i = 0; i < get_test_bp_slots() - skip; ++i)
> +		fill_one_bp_slot(test, id, cpu, tsk);
> +
> +	return *id + get_test_bp_slots() <= MAX_TEST_BREAKPOINTS;
> +}
> +
> +static int dummy_kthread(void *arg)
> +{
> +	return 0;
> +}
> +
> +static struct task_struct *get_other_task(struct kunit *test)
> +{
> +	struct task_struct *tsk;
> +
> +	if (__other_task)
> +		return __other_task;
> +
> +	tsk = kthread_create(dummy_kthread, NULL, "hw_breakpoint_dummy_task");
> +	KUNIT_ASSERT_FALSE(test, IS_ERR(tsk));
> +	__other_task = tsk;
> +	return __other_task;
> +}
> +
> +static int get_test_cpu(int num)
> +{
> +	int cpu;
> +
> +	WARN_ON(num < 0);
> +
> +	for_each_online_cpu(cpu) {
> +		if (num-- <= 0)
> +			break;
> +	}
> +
> +	return cpu;
> +}
> +
> +/* ===== Test cases ===== */
> +
> +static void test_one_cpu(struct kunit *test)
> +{
> +	int idx = 0;
> +
> +	fill_bp_slots(test, &idx, get_test_cpu(0), NULL, 0);
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +}
> +
> +static void test_many_cpus(struct kunit *test)
> +{
> +	int idx = 0;
> +	int cpu;
> +
> +	/* Test that CPUs are independent. */
> +	for_each_online_cpu(cpu) {
> +		bool do_continue = fill_bp_slots(test, &idx, cpu, NULL, 0);
> +
> +		TEST_EXPECT_NOSPC(register_test_bp(cpu, NULL, idx));
> +		if (!do_continue)
> +			break;
> +	}
> +}
> +
> +static void test_one_task_on_all_cpus(struct kunit *test)
> +{
> +	int idx = 0;
> +
> +	fill_bp_slots(test, &idx, -1, current, 0);
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +	/* Remove one and adding back CPU-target should work. */
> +	unregister_test_bp(&test_bps[0]);
> +	fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> +}
> +
> +static void test_two_tasks_on_all_cpus(struct kunit *test)
> +{
> +	int idx = 0;
> +
> +	/* Test that tasks are independent. */
> +	fill_bp_slots(test, &idx, -1, current, 0);
> +	fill_bp_slots(test, &idx, -1, get_other_task(test), 0);
> +
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +	/* Remove one from first task and adding back CPU-target should not work. */
> +	unregister_test_bp(&test_bps[0]);
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +}
> +
> +static void test_one_task_on_one_cpu(struct kunit *test)
> +{
> +	int idx = 0;
> +
> +	fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +	/*
> +	 * Remove one and adding back CPU-target should work; this case is
> +	 * special vs. above because the task's constraints are CPU-dependent.
> +	 */
> +	unregister_test_bp(&test_bps[0]);
> +	fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> +}
> +
> +static void test_one_task_mixed(struct kunit *test)
> +{
> +	int idx = 0;
> +
> +	TEST_REQUIRES_BP_SLOTS(test, 3);
> +
> +	fill_one_bp_slot(test, &idx, get_test_cpu(0), current);
> +	fill_bp_slots(test, &idx, -1, current, 1);
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +
> +	/* Transition from CPU-dependent pinned count to CPU-independent. */
> +	unregister_test_bp(&test_bps[0]);
> +	unregister_test_bp(&test_bps[1]);
> +	fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> +	fill_one_bp_slot(test, &idx, get_test_cpu(0), NULL);
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +}
> +
> +static void test_two_tasks_on_one_cpu(struct kunit *test)
> +{
> +	int idx = 0;
> +
> +	fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
> +	fill_bp_slots(test, &idx, get_test_cpu(0), get_other_task(test), 0);
> +
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +	/* Can still create breakpoints on some other CPU. */
> +	fill_bp_slots(test, &idx, get_test_cpu(1), NULL, 0);
> +}
> +
> +static void test_two_tasks_on_one_all_cpus(struct kunit *test)
> +{
> +	int idx = 0;
> +
> +	fill_bp_slots(test, &idx, get_test_cpu(0), current, 0);
> +	fill_bp_slots(test, &idx, -1, get_other_task(test), 0);
> +
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, get_other_task(test), idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), get_other_task(test), idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +	/* Cannot create breakpoints on some other CPU either. */
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
> +}
> +
> +static void test_task_on_all_and_one_cpu(struct kunit *test)
> +{
> +	int tsk_on_cpu_idx, cpu_idx;
> +	int idx = 0;
> +
> +	TEST_REQUIRES_BP_SLOTS(test, 3);
> +
> +	fill_bp_slots(test, &idx, -1, current, 2);
> +	/* Transitioning from only all CPU breakpoints to mixed. */
> +	tsk_on_cpu_idx = idx;
> +	fill_one_bp_slot(test, &idx, get_test_cpu(0), current);
> +	fill_one_bp_slot(test, &idx, -1, current);
> +
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +
> +	/* We should still be able to use up another CPU's slots. */
> +	cpu_idx = idx;
> +	fill_one_bp_slot(test, &idx, get_test_cpu(1), NULL);
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
> +
> +	/* Transitioning back to task target on all CPUs. */
> +	unregister_test_bp(&test_bps[tsk_on_cpu_idx]);
> +	/* Still have a CPU target breakpoint in get_test_cpu(1). */
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	/* Remove it and try again. */
> +	unregister_test_bp(&test_bps[cpu_idx]);
> +	fill_one_bp_slot(test, &idx, -1, current);
> +
> +	TEST_EXPECT_NOSPC(register_test_bp(-1, current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), current, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(0), NULL, idx));
> +	TEST_EXPECT_NOSPC(register_test_bp(get_test_cpu(1), NULL, idx));
> +}
> +
> +static struct kunit_case hw_breakpoint_test_cases[] = {
> +	KUNIT_CASE(test_one_cpu),
> +	KUNIT_CASE(test_many_cpus),
> +	KUNIT_CASE(test_one_task_on_all_cpus),
> +	KUNIT_CASE(test_two_tasks_on_all_cpus),
> +	KUNIT_CASE(test_one_task_on_one_cpu),
> +	KUNIT_CASE(test_one_task_mixed),
> +	KUNIT_CASE(test_two_tasks_on_one_cpu),
> +	KUNIT_CASE(test_two_tasks_on_one_all_cpus),
> +	KUNIT_CASE(test_task_on_all_and_one_cpu),
> +	{},
> +};
> +
> +static int test_init(struct kunit *test)
> +{
> +	/* Most test cases want 2 distinct CPUs. */
> +	return num_online_cpus() < 2 ? -EINVAL : 0;
> +}
> +
> +static void test_exit(struct kunit *test)
> +{
> +	for (int i = 0; i < MAX_TEST_BREAKPOINTS; ++i) {
> +		if (test_bps[i])
> +			unregister_test_bp(&test_bps[i]);
> +	}
> +
> +	if (__other_task) {
> +		kthread_stop(__other_task);
> +		__other_task = NULL;
> +	}
> +}
> +
> +static struct kunit_suite hw_breakpoint_test_suite = {
> +	.name = "hw_breakpoint",
> +	.test_cases = hw_breakpoint_test_cases,
> +	.init = test_init,
> +	.exit = test_exit,
> +};
> +
> +kunit_test_suites(&hw_breakpoint_test_suite);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Marco Elver <elver@google.com>");
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 2e24db4bff19..4c87a6edf046 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2513,6 +2513,16 @@ config STACKINIT_KUNIT_TEST
>  	  CONFIG_GCC_PLUGIN_STRUCTLEAK, CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF,
>  	  or CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL.
>  
> +config HW_BREAKPOINT_KUNIT_TEST
> +	bool "Test hw_breakpoint constraints accounting" if !KUNIT_ALL_TESTS
> +	depends on HAVE_HW_BREAKPOINT
> +	depends on KUNIT=y
> +	default KUNIT_ALL_TESTS
> +	help
> +	  Tests for hw_breakpoint constraints accounting.
> +
> +	  If unsure, say N.
> +
>  config TEST_UDELAY
>  	tristate "udelay test driver"
>  	help
> -- 
> 2.37.0.rc0.161.g10f37bed90-goog
> 

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Will Deacon]

On Thu, Jul 21, 2022 at 05:22:07PM +0100, Mark Rutland wrote:
> Hi Marco,
> 
> [adding Will]
> 
> On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > Add KUnit test for hw_breakpoint constraints accounting, with various
> > interesting mixes of breakpoint targets (some care was taken to catch
> > interesting corner cases via bug-injection).
> > 
> > The test cannot be built as a module because it requires access to
> > hw_breakpoint_slots(), which is not inlinable or exported on all
> > architectures.
> > 
> > Signed-off-by: Marco Elver <elver@google.com>
> 
> As mentioned on IRC, I'm seeing these tests fail on arm64 when applied atop
> v5.19-rc7:
> 
> | TAP version 14
> | 1..1
> |     # Subtest: hw_breakpoint
> |     1..9
> |     ok 1 - test_one_cpu
> |     ok 2 - test_many_cpus
> |     # test_one_task_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 3 - test_one_task_on_all_cpus
> |     # test_two_tasks_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 4 - test_two_tasks_on_all_cpus
> |     # test_one_task_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 5 - test_one_task_on_one_cpu
> |     # test_one_task_mixed: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 6 - test_one_task_mixed
> |     # test_two_tasks_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 7 - test_two_tasks_on_one_cpu
> |     # test_two_tasks_on_one_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 8 - test_two_tasks_on_one_all_cpus
> |     # test_task_on_all_and_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 9 - test_task_on_all_and_one_cpu
> | # hw_breakpoint: pass:2 fail:7 skip:0 total:9
> | # Totals: pass:2 fail:7 skip:0 total:9
> 
> ... which seems to be becasue arm64 currently forbids per-task
> breakpoints/watchpoints in hw_breakpoint_arch_parse(), where we have:
> 
>         /*
>          * Disallow per-task kernel breakpoints since these would
>          * complicate the stepping code.
>          */
>         if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
>                 return -EINVAL;
> 
> ... which has been the case since day one in commit:
> 
>   478fcb2cdb2351dc ("arm64: Debugging support")
> 
> I'm not immediately sure what would be necessary to support per-task kernel
> breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
> invasive.

I would actually like to remove HW_BREAKPOINT completely for arm64 as it
doesn't really work and causes problems for other interfaces such as ptrace
and kgdb.

Will

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Dmitry Vyukov]

On Fri, 22 Jul 2022 at 11:10, Will Deacon <will@kernel.org> wrote:
> > [adding Will]
> >
> > On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > > Add KUnit test for hw_breakpoint constraints accounting, with various
> > > interesting mixes of breakpoint targets (some care was taken to catch
> > > interesting corner cases via bug-injection).
> > >
> > > The test cannot be built as a module because it requires access to
> > > hw_breakpoint_slots(), which is not inlinable or exported on all
> > > architectures.
> > >
> > > Signed-off-by: Marco Elver <elver@google.com>
> >
> > As mentioned on IRC, I'm seeing these tests fail on arm64 when applied atop
> > v5.19-rc7:
> >
> > | TAP version 14
> > | 1..1
> > |     # Subtest: hw_breakpoint
> > |     1..9
> > |     ok 1 - test_one_cpu
> > |     ok 2 - test_many_cpus
> > |     # test_one_task_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > |     Expected IS_ERR(bp) to be false, but is true
> > |     not ok 3 - test_one_task_on_all_cpus
> > |     # test_two_tasks_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > |     Expected IS_ERR(bp) to be false, but is true
> > |     not ok 4 - test_two_tasks_on_all_cpus
> > |     # test_one_task_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > |     Expected IS_ERR(bp) to be false, but is true
> > |     not ok 5 - test_one_task_on_one_cpu
> > |     # test_one_task_mixed: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > |     Expected IS_ERR(bp) to be false, but is true
> > |     not ok 6 - test_one_task_mixed
> > |     # test_two_tasks_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > |     Expected IS_ERR(bp) to be false, but is true
> > |     not ok 7 - test_two_tasks_on_one_cpu
> > |     # test_two_tasks_on_one_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > |     Expected IS_ERR(bp) to be false, but is true
> > |     not ok 8 - test_two_tasks_on_one_all_cpus
> > |     # test_task_on_all_and_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > |     Expected IS_ERR(bp) to be false, but is true
> > |     not ok 9 - test_task_on_all_and_one_cpu
> > | # hw_breakpoint: pass:2 fail:7 skip:0 total:9
> > | # Totals: pass:2 fail:7 skip:0 total:9
> >
> > ... which seems to be becasue arm64 currently forbids per-task
> > breakpoints/watchpoints in hw_breakpoint_arch_parse(), where we have:
> >
> >         /*
> >          * Disallow per-task kernel breakpoints since these would
> >          * complicate the stepping code.
> >          */
> >         if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
> >                 return -EINVAL;
> >
> > ... which has been the case since day one in commit:
> >
> >   478fcb2cdb2351dc ("arm64: Debugging support")
> >
> > I'm not immediately sure what would be necessary to support per-task kernel
> > breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
> > invasive.
>
> I would actually like to remove HW_BREAKPOINT completely for arm64 as it
> doesn't really work and causes problems for other interfaces such as ptrace
> and kgdb.

Will it be a localized removal of code that will be easy to revert in
future? Or will it touch lots of code here and there?
Let's say we come up with a very important use case for HW_BREAKPOINT
and will need to make it work on arm64 as well in future.

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Will Deacon]

On Fri, Jul 22, 2022 at 11:20:25AM +0200, Dmitry Vyukov wrote:
> On Fri, 22 Jul 2022 at 11:10, Will Deacon <will@kernel.org> wrote:
> > > [adding Will]
> > >
> > > On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > > > Add KUnit test for hw_breakpoint constraints accounting, with various
> > > > interesting mixes of breakpoint targets (some care was taken to catch
> > > > interesting corner cases via bug-injection).
> > > >
> > > > The test cannot be built as a module because it requires access to
> > > > hw_breakpoint_slots(), which is not inlinable or exported on all
> > > > architectures.
> > > >
> > > > Signed-off-by: Marco Elver <elver@google.com>
> > >
> > > As mentioned on IRC, I'm seeing these tests fail on arm64 when applied atop
> > > v5.19-rc7:
> > >
> > > | TAP version 14
> > > | 1..1
> > > |     # Subtest: hw_breakpoint
> > > |     1..9
> > > |     ok 1 - test_one_cpu
> > > |     ok 2 - test_many_cpus
> > > |     # test_one_task_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > |     Expected IS_ERR(bp) to be false, but is true
> > > |     not ok 3 - test_one_task_on_all_cpus
> > > |     # test_two_tasks_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > |     Expected IS_ERR(bp) to be false, but is true
> > > |     not ok 4 - test_two_tasks_on_all_cpus
> > > |     # test_one_task_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > |     Expected IS_ERR(bp) to be false, but is true
> > > |     not ok 5 - test_one_task_on_one_cpu
> > > |     # test_one_task_mixed: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > |     Expected IS_ERR(bp) to be false, but is true
> > > |     not ok 6 - test_one_task_mixed
> > > |     # test_two_tasks_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > |     Expected IS_ERR(bp) to be false, but is true
> > > |     not ok 7 - test_two_tasks_on_one_cpu
> > > |     # test_two_tasks_on_one_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > |     Expected IS_ERR(bp) to be false, but is true
> > > |     not ok 8 - test_two_tasks_on_one_all_cpus
> > > |     # test_task_on_all_and_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > |     Expected IS_ERR(bp) to be false, but is true
> > > |     not ok 9 - test_task_on_all_and_one_cpu
> > > | # hw_breakpoint: pass:2 fail:7 skip:0 total:9
> > > | # Totals: pass:2 fail:7 skip:0 total:9
> > >
> > > ... which seems to be becasue arm64 currently forbids per-task
> > > breakpoints/watchpoints in hw_breakpoint_arch_parse(), where we have:
> > >
> > >         /*
> > >          * Disallow per-task kernel breakpoints since these would
> > >          * complicate the stepping code.
> > >          */
> > >         if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
> > >                 return -EINVAL;
> > >
> > > ... which has been the case since day one in commit:
> > >
> > >   478fcb2cdb2351dc ("arm64: Debugging support")
> > >
> > > I'm not immediately sure what would be necessary to support per-task kernel
> > > breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
> > > invasive.
> >
> > I would actually like to remove HW_BREAKPOINT completely for arm64 as it
> > doesn't really work and causes problems for other interfaces such as ptrace
> > and kgdb.
> 
> Will it be a localized removal of code that will be easy to revert in
> future? Or will it touch lots of code here and there?
> Let's say we come up with a very important use case for HW_BREAKPOINT
> and will need to make it work on arm64 as well in future.

My (rough) plan is to implement a lower-level abstraction for handling the
underlying hardware resources, so we can layer consumers on top of that
instead of funneling through hw_breakpoint. So if we figure out how to make
bits of hw_breakpoint work on arm64, then it should just go on top.

The main pain point for hw_breakpoint is kernel-side {break,watch}points
and I think there are open design questions about how they should work
on arm64, particularly when considering the interaction with user
watchpoints triggering on uaccess routines and the possibility of hitting
a kernel watchpoint in irq context.

Will

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Dmitry Vyukov]

On Fri, 22 Jul 2022 at 12:11, Will Deacon <will@kernel.org> wrote:
> > > > [adding Will]
> > > >
> > > > On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > > > > Add KUnit test for hw_breakpoint constraints accounting, with various
> > > > > interesting mixes of breakpoint targets (some care was taken to catch
> > > > > interesting corner cases via bug-injection).
> > > > >
> > > > > The test cannot be built as a module because it requires access to
> > > > > hw_breakpoint_slots(), which is not inlinable or exported on all
> > > > > architectures.
> > > > >
> > > > > Signed-off-by: Marco Elver <elver@google.com>
> > > >
> > > > As mentioned on IRC, I'm seeing these tests fail on arm64 when applied atop
> > > > v5.19-rc7:
> > > >
> > > > | TAP version 14
> > > > | 1..1
> > > > |     # Subtest: hw_breakpoint
> > > > |     1..9
> > > > |     ok 1 - test_one_cpu
> > > > |     ok 2 - test_many_cpus
> > > > |     # test_one_task_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > > |     Expected IS_ERR(bp) to be false, but is true
> > > > |     not ok 3 - test_one_task_on_all_cpus
> > > > |     # test_two_tasks_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > > |     Expected IS_ERR(bp) to be false, but is true
> > > > |     not ok 4 - test_two_tasks_on_all_cpus
> > > > |     # test_one_task_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > > |     Expected IS_ERR(bp) to be false, but is true
> > > > |     not ok 5 - test_one_task_on_one_cpu
> > > > |     # test_one_task_mixed: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > > |     Expected IS_ERR(bp) to be false, but is true
> > > > |     not ok 6 - test_one_task_mixed
> > > > |     # test_two_tasks_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > > |     Expected IS_ERR(bp) to be false, but is true
> > > > |     not ok 7 - test_two_tasks_on_one_cpu
> > > > |     # test_two_tasks_on_one_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > > |     Expected IS_ERR(bp) to be false, but is true
> > > > |     not ok 8 - test_two_tasks_on_one_all_cpus
> > > > |     # test_task_on_all_and_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> > > > |     Expected IS_ERR(bp) to be false, but is true
> > > > |     not ok 9 - test_task_on_all_and_one_cpu
> > > > | # hw_breakpoint: pass:2 fail:7 skip:0 total:9
> > > > | # Totals: pass:2 fail:7 skip:0 total:9
> > > >
> > > > ... which seems to be becasue arm64 currently forbids per-task
> > > > breakpoints/watchpoints in hw_breakpoint_arch_parse(), where we have:
> > > >
> > > >         /*
> > > >          * Disallow per-task kernel breakpoints since these would
> > > >          * complicate the stepping code.
> > > >          */
> > > >         if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
> > > >                 return -EINVAL;
> > > >
> > > > ... which has been the case since day one in commit:
> > > >
> > > >   478fcb2cdb2351dc ("arm64: Debugging support")
> > > >
> > > > I'm not immediately sure what would be necessary to support per-task kernel
> > > > breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
> > > > invasive.
> > >
> > > I would actually like to remove HW_BREAKPOINT completely for arm64 as it
> > > doesn't really work and causes problems for other interfaces such as ptrace
> > > and kgdb.
> >
> > Will it be a localized removal of code that will be easy to revert in
> > future? Or will it touch lots of code here and there?
> > Let's say we come up with a very important use case for HW_BREAKPOINT
> > and will need to make it work on arm64 as well in future.
>
> My (rough) plan is to implement a lower-level abstraction for handling the
> underlying hardware resources, so we can layer consumers on top of that
> instead of funneling through hw_breakpoint. So if we figure out how to make
> bits of hw_breakpoint work on arm64, then it should just go on top.
>
> The main pain point for hw_breakpoint is kernel-side {break,watch}points
> and I think there are open design questions about how they should work
> on arm64, particularly when considering the interaction with user
> watchpoints triggering on uaccess routines and the possibility of hitting
> a kernel watchpoint in irq context.

I see. Our main interest would be break/watchpoints on user addresses
firing from both user-space and kernel (uaccess), so at least on irqs.

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Will Deacon]

On Fri, Jul 22, 2022 at 12:31:45PM +0200, Dmitry Vyukov wrote:
> On Fri, 22 Jul 2022 at 12:11, Will Deacon <will@kernel.org> wrote:
> > > > > On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > > > > I'm not immediately sure what would be necessary to support per-task kernel
> > > > > breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
> > > > > invasive.
> > > >
> > > > I would actually like to remove HW_BREAKPOINT completely for arm64 as it
> > > > doesn't really work and causes problems for other interfaces such as ptrace
> > > > and kgdb.
> > >
> > > Will it be a localized removal of code that will be easy to revert in
> > > future? Or will it touch lots of code here and there?
> > > Let's say we come up with a very important use case for HW_BREAKPOINT
> > > and will need to make it work on arm64 as well in future.
> >
> > My (rough) plan is to implement a lower-level abstraction for handling the
> > underlying hardware resources, so we can layer consumers on top of that
> > instead of funneling through hw_breakpoint. So if we figure out how to make
> > bits of hw_breakpoint work on arm64, then it should just go on top.
> >
> > The main pain point for hw_breakpoint is kernel-side {break,watch}points
> > and I think there are open design questions about how they should work
> > on arm64, particularly when considering the interaction with user
> > watchpoints triggering on uaccess routines and the possibility of hitting
> > a kernel watchpoint in irq context.
> 
> I see. Our main interest would be break/watchpoints on user addresses
> firing from both user-space and kernel (uaccess), so at least on irqs.

Interesting. Do other architectures report watchpoint hits on user
addresses from kernel uaccess? It feels like this might be surprising to
some users, and it opens up questions about accesses using different virtual
aliases (e.g. via GUP) or from other entities as well (e.g. firmware,
KVM guests, DMA).

Will

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Dmitry Vyukov]

On Fri, 22 Jul 2022 at 13:03, Will Deacon <will@kernel.org> wrote:
> > > > > > On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > > > > > I'm not immediately sure what would be necessary to support per-task kernel
> > > > > > breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
> > > > > > invasive.
> > > > >
> > > > > I would actually like to remove HW_BREAKPOINT completely for arm64 as it
> > > > > doesn't really work and causes problems for other interfaces such as ptrace
> > > > > and kgdb.
> > > >
> > > > Will it be a localized removal of code that will be easy to revert in
> > > > future? Or will it touch lots of code here and there?
> > > > Let's say we come up with a very important use case for HW_BREAKPOINT
> > > > and will need to make it work on arm64 as well in future.
> > >
> > > My (rough) plan is to implement a lower-level abstraction for handling the
> > > underlying hardware resources, so we can layer consumers on top of that
> > > instead of funneling through hw_breakpoint. So if we figure out how to make
> > > bits of hw_breakpoint work on arm64, then it should just go on top.
> > >
> > > The main pain point for hw_breakpoint is kernel-side {break,watch}points
> > > and I think there are open design questions about how they should work
> > > on arm64, particularly when considering the interaction with user
> > > watchpoints triggering on uaccess routines and the possibility of hitting
> > > a kernel watchpoint in irq context.
> >
> > I see. Our main interest would be break/watchpoints on user addresses
> > firing from both user-space and kernel (uaccess), so at least on irqs.
>
> Interesting. Do other architectures report watchpoint hits on user
> addresses from kernel uaccess? It feels like this might be surprising to
> some users, and it opens up questions about accesses using different virtual
> aliases (e.g. via GUP) or from other entities as well (e.g. firmware,
> KVM guests, DMA).

x86 supports this.
There is that attr.exclude_kernel flag that requires special permissions:
https://elixir.bootlin.com/linux/v5.19-rc7/source/kernel/events/core.c#L12061
https://elixir.bootlin.com/linux/v5.19-rc7/source/kernel/events/core.c#L9323
But if I understand correctly, it only filters out delivery, the HW
breakpoint fires even if attr.exclude_kernel is set.

We also wanted to relax this permission check somewhat:
https://lore.kernel.org/all/20220601093502.364142-1-elver@google.com/

Yes, if the kernel maps the page at a different virtual address, then
the breakpoint won't fire I think.
Don't know what are the issues with firmware/KVM.

Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Marco Elver]

On Thu, 21 Jul 2022 at 18:22, Mark Rutland <mark.rutland@arm.com> wrote:
>
> Hi Marco,
>
> [adding Will]
>
> On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > Add KUnit test for hw_breakpoint constraints accounting, with various
> > interesting mixes of breakpoint targets (some care was taken to catch
> > interesting corner cases via bug-injection).
> >
> > The test cannot be built as a module because it requires access to
> > hw_breakpoint_slots(), which is not inlinable or exported on all
> > architectures.
> >
> > Signed-off-by: Marco Elver <elver@google.com>
>
> As mentioned on IRC, I'm seeing these tests fail on arm64 when applied atop
> v5.19-rc7:
>
> | TAP version 14
> | 1..1
> |     # Subtest: hw_breakpoint
> |     1..9
> |     ok 1 - test_one_cpu
> |     ok 2 - test_many_cpus
> |     # test_one_task_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 3 - test_one_task_on_all_cpus
> |     # test_two_tasks_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 4 - test_two_tasks_on_all_cpus
> |     # test_one_task_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 5 - test_one_task_on_one_cpu
> |     # test_one_task_mixed: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 6 - test_one_task_mixed
> |     # test_two_tasks_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 7 - test_two_tasks_on_one_cpu
> |     # test_two_tasks_on_one_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 8 - test_two_tasks_on_one_all_cpus
> |     # test_task_on_all_and_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 9 - test_task_on_all_and_one_cpu
> | # hw_breakpoint: pass:2 fail:7 skip:0 total:9
> | # Totals: pass:2 fail:7 skip:0 total:9
>
> ... which seems to be becasue arm64 currently forbids per-task
> breakpoints/watchpoints in hw_breakpoint_arch_parse(), where we have:
>
>         /*
>          * Disallow per-task kernel breakpoints since these would
>          * complicate the stepping code.
>          */
>         if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
>                 return -EINVAL;
>
> ... which has been the case since day one in commit:
>
>   478fcb2cdb2351dc ("arm64: Debugging support")
>
> I'm not immediately sure what would be necessary to support per-task kernel
> breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
> invasive.

Thanks for investigating - so the test is working as intended. ;-)

However it's a shame that arm64's support is limited. And what Will
said about possible removal/rework of arm64 hw_breakpoint support
doesn't sound too reassuring.

We will definitely want to revisit arm64's hw_breakpoint support in future.

Thanks,
-- Marco

[PATCH v3 02/14] perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test

[Marco Elver]

Provide hw_breakpoint_is_used() to check if breakpoints are in use on
the system.

Use it in the KUnit test to verify the global state before and after a
test case.

Signed-off-by: Marco Elver <elver@google.com>
---
v3:
* New patch.
---
 include/linux/hw_breakpoint.h      |  3 +++
 kernel/events/hw_breakpoint.c      | 29 +++++++++++++++++++++++++++++
 kernel/events/hw_breakpoint_test.c | 12 +++++++++++-
 3 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
index 78dd7035d1e5..a3fb846705eb 100644
--- a/include/linux/hw_breakpoint.h
+++ b/include/linux/hw_breakpoint.h
@@ -74,6 +74,7 @@ register_wide_hw_breakpoint(struct perf_event_attr *attr,
 extern int register_perf_hw_breakpoint(struct perf_event *bp);
 extern void unregister_hw_breakpoint(struct perf_event *bp);
 extern void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events);
+extern bool hw_breakpoint_is_used(void);
 
 extern int dbg_reserve_bp_slot(struct perf_event *bp);
 extern int dbg_release_bp_slot(struct perf_event *bp);
@@ -121,6 +122,8 @@ register_perf_hw_breakpoint(struct perf_event *bp)	{ return -ENOSYS; }
 static inline void unregister_hw_breakpoint(struct perf_event *bp)	{ }
 static inline void
 unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events)	{ }
+static inline bool hw_breakpoint_is_used(void)		{ return false; }
+
 static inline int
 reserve_bp_slot(struct perf_event *bp)			{return -ENOSYS; }
 static inline void release_bp_slot(struct perf_event *bp) 		{ }
diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index f32320ac02fd..fd5cd1f9e7fc 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -604,6 +604,35 @@ void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events)
 }
 EXPORT_SYMBOL_GPL(unregister_wide_hw_breakpoint);
 
+/**
+ * hw_breakpoint_is_used - check if breakpoints are currently used
+ *
+ * Returns: true if breakpoints are used, false otherwise.
+ */
+bool hw_breakpoint_is_used(void)
+{
+	int cpu;
+
+	if (!constraints_initialized)
+		return false;
+
+	for_each_possible_cpu(cpu) {
+		for (int type = 0; type < TYPE_MAX; ++type) {
+			struct bp_cpuinfo *info = get_bp_info(cpu, type);
+
+			if (info->cpu_pinned)
+				return true;
+
+			for (int slot = 0; slot < nr_slots[type]; ++slot) {
+				if (info->tsk_pinned[slot])
+					return true;
+			}
+		}
+	}
+
+	return false;
+}
+
 static struct notifier_block hw_breakpoint_exceptions_nb = {
 	.notifier_call = hw_breakpoint_exceptions_notify,
 	/* we need to be notified first */
diff --git a/kernel/events/hw_breakpoint_test.c b/kernel/events/hw_breakpoint_test.c
index 433c5c45e2a5..5ced822df788 100644
--- a/kernel/events/hw_breakpoint_test.c
+++ b/kernel/events/hw_breakpoint_test.c
@@ -294,7 +294,14 @@ static struct kunit_case hw_breakpoint_test_cases[] = {
 static int test_init(struct kunit *test)
 {
 	/* Most test cases want 2 distinct CPUs. */
-	return num_online_cpus() < 2 ? -EINVAL : 0;
+	if (num_online_cpus() < 2)
+		return -EINVAL;
+
+	/* Want the system to not use breakpoints elsewhere. */
+	if (hw_breakpoint_is_used())
+		return -EBUSY;
+
+	return 0;
 }
 
 static void test_exit(struct kunit *test)
@@ -308,6 +315,9 @@ static void test_exit(struct kunit *test)
 		kthread_stop(__other_task);
 		__other_task = NULL;
 	}
+
+	/* Verify that internal state agrees that no breakpoints are in use. */
+	KUNIT_EXPECT_FALSE(test, hw_breakpoint_is_used());
 }
 
 static struct kunit_suite hw_breakpoint_test_suite = {
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 02/14] perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test

[Dmitry Vyukov]

On Mon, 4 Jul 2022 at 17:06, Marco Elver <elver@google.com> wrote:
>
> Provide hw_breakpoint_is_used() to check if breakpoints are in use on
> the system.
>
> Use it in the KUnit test to verify the global state before and after a
> test case.
>
> Signed-off-by: Marco Elver <elver@google.com>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
> v3:
> * New patch.
> ---
>  include/linux/hw_breakpoint.h      |  3 +++
>  kernel/events/hw_breakpoint.c      | 29 +++++++++++++++++++++++++++++
>  kernel/events/hw_breakpoint_test.c | 12 +++++++++++-
>  3 files changed, 43 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
> index 78dd7035d1e5..a3fb846705eb 100644
> --- a/include/linux/hw_breakpoint.h
> +++ b/include/linux/hw_breakpoint.h
> @@ -74,6 +74,7 @@ register_wide_hw_breakpoint(struct perf_event_attr *attr,
>  extern int register_perf_hw_breakpoint(struct perf_event *bp);
>  extern void unregister_hw_breakpoint(struct perf_event *bp);
>  extern void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events);
> +extern bool hw_breakpoint_is_used(void);
>
>  extern int dbg_reserve_bp_slot(struct perf_event *bp);
>  extern int dbg_release_bp_slot(struct perf_event *bp);
> @@ -121,6 +122,8 @@ register_perf_hw_breakpoint(struct perf_event *bp)  { return -ENOSYS; }
>  static inline void unregister_hw_breakpoint(struct perf_event *bp)     { }
>  static inline void
>  unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events)        { }
> +static inline bool hw_breakpoint_is_used(void)         { return false; }
> +
>  static inline int
>  reserve_bp_slot(struct perf_event *bp)                 {return -ENOSYS; }
>  static inline void release_bp_slot(struct perf_event *bp)              { }
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index f32320ac02fd..fd5cd1f9e7fc 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -604,6 +604,35 @@ void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events)
>  }
>  EXPORT_SYMBOL_GPL(unregister_wide_hw_breakpoint);
>
> +/**
> + * hw_breakpoint_is_used - check if breakpoints are currently used
> + *
> + * Returns: true if breakpoints are used, false otherwise.
> + */
> +bool hw_breakpoint_is_used(void)
> +{
> +       int cpu;
> +
> +       if (!constraints_initialized)
> +               return false;
> +
> +       for_each_possible_cpu(cpu) {
> +               for (int type = 0; type < TYPE_MAX; ++type) {
> +                       struct bp_cpuinfo *info = get_bp_info(cpu, type);
> +
> +                       if (info->cpu_pinned)
> +                               return true;
> +
> +                       for (int slot = 0; slot < nr_slots[type]; ++slot) {
> +                               if (info->tsk_pinned[slot])
> +                                       return true;
> +                       }
> +               }
> +       }
> +
> +       return false;
> +}
> +
>  static struct notifier_block hw_breakpoint_exceptions_nb = {
>         .notifier_call = hw_breakpoint_exceptions_notify,
>         /* we need to be notified first */
> diff --git a/kernel/events/hw_breakpoint_test.c b/kernel/events/hw_breakpoint_test.c
> index 433c5c45e2a5..5ced822df788 100644
> --- a/kernel/events/hw_breakpoint_test.c
> +++ b/kernel/events/hw_breakpoint_test.c
> @@ -294,7 +294,14 @@ static struct kunit_case hw_breakpoint_test_cases[] = {
>  static int test_init(struct kunit *test)
>  {
>         /* Most test cases want 2 distinct CPUs. */
> -       return num_online_cpus() < 2 ? -EINVAL : 0;
> +       if (num_online_cpus() < 2)
> +               return -EINVAL;
> +
> +       /* Want the system to not use breakpoints elsewhere. */
> +       if (hw_breakpoint_is_used())
> +               return -EBUSY;
> +
> +       return 0;
>  }
>
>  static void test_exit(struct kunit *test)
> @@ -308,6 +315,9 @@ static void test_exit(struct kunit *test)
>                 kthread_stop(__other_task);
>                 __other_task = NULL;
>         }
> +
> +       /* Verify that internal state agrees that no breakpoints are in use. */
> +       KUNIT_EXPECT_FALSE(test, hw_breakpoint_is_used());
>  }
>
>  static struct kunit_suite hw_breakpoint_test_suite = {
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

Re: [PATCH v3 02/14] perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:10 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Mon, 4 Jul 2022 at 17:06, Marco Elver <elver@google.com> wrote:
> >
> > Provide hw_breakpoint_is_used() to check if breakpoints are in use on
> > the system.
> >
> > Use it in the KUnit test to verify the global state before and after a
> > test case.
> >
> > Signed-off-by: Marco Elver <elver@google.com>
>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> > ---
> > v3:
> > * New patch.
> > ---
> >  include/linux/hw_breakpoint.h      |  3 +++
> >  kernel/events/hw_breakpoint.c      | 29 +++++++++++++++++++++++++++++
> >  kernel/events/hw_breakpoint_test.c | 12 +++++++++++-
> >  3 files changed, 43 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
> > index 78dd7035d1e5..a3fb846705eb 100644
> > --- a/include/linux/hw_breakpoint.h
> > +++ b/include/linux/hw_breakpoint.h
> > @@ -74,6 +74,7 @@ register_wide_hw_breakpoint(struct perf_event_attr *attr,
> >  extern int register_perf_hw_breakpoint(struct perf_event *bp);
> >  extern void unregister_hw_breakpoint(struct perf_event *bp);
> >  extern void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events);
> > +extern bool hw_breakpoint_is_used(void);
> >
> >  extern int dbg_reserve_bp_slot(struct perf_event *bp);
> >  extern int dbg_release_bp_slot(struct perf_event *bp);
> > @@ -121,6 +122,8 @@ register_perf_hw_breakpoint(struct perf_event *bp)  { return -ENOSYS; }
> >  static inline void unregister_hw_breakpoint(struct perf_event *bp)     { }
> >  static inline void
> >  unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events)        { }
> > +static inline bool hw_breakpoint_is_used(void)         { return false; }
> > +
> >  static inline int
> >  reserve_bp_slot(struct perf_event *bp)                 {return -ENOSYS; }
> >  static inline void release_bp_slot(struct perf_event *bp)              { }
> > diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> > index f32320ac02fd..fd5cd1f9e7fc 100644
> > --- a/kernel/events/hw_breakpoint.c
> > +++ b/kernel/events/hw_breakpoint.c
> > @@ -604,6 +604,35 @@ void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events)
> >  }
> >  EXPORT_SYMBOL_GPL(unregister_wide_hw_breakpoint);
> >
> > +/**
> > + * hw_breakpoint_is_used - check if breakpoints are currently used
> > + *
> > + * Returns: true if breakpoints are used, false otherwise.
> > + */
> > +bool hw_breakpoint_is_used(void)
> > +{
> > +       int cpu;
> > +
> > +       if (!constraints_initialized)
> > +               return false;
> > +
> > +       for_each_possible_cpu(cpu) {
> > +               for (int type = 0; type < TYPE_MAX; ++type) {
> > +                       struct bp_cpuinfo *info = get_bp_info(cpu, type);
> > +
> > +                       if (info->cpu_pinned)
> > +                               return true;
> > +
> > +                       for (int slot = 0; slot < nr_slots[type]; ++slot) {
> > +                               if (info->tsk_pinned[slot])
> > +                                       return true;
> > +                       }
> > +               }
> > +       }
> > +
> > +       return false;
> > +}
> > +
> >  static struct notifier_block hw_breakpoint_exceptions_nb = {
> >         .notifier_call = hw_breakpoint_exceptions_notify,
> >         /* we need to be notified first */
> > diff --git a/kernel/events/hw_breakpoint_test.c b/kernel/events/hw_breakpoint_test.c
> > index 433c5c45e2a5..5ced822df788 100644
> > --- a/kernel/events/hw_breakpoint_test.c
> > +++ b/kernel/events/hw_breakpoint_test.c
> > @@ -294,7 +294,14 @@ static struct kunit_case hw_breakpoint_test_cases[] = {
> >  static int test_init(struct kunit *test)
> >  {
> >         /* Most test cases want 2 distinct CPUs. */
> > -       return num_online_cpus() < 2 ? -EINVAL : 0;
> > +       if (num_online_cpus() < 2)
> > +               return -EINVAL;
> > +
> > +       /* Want the system to not use breakpoints elsewhere. */
> > +       if (hw_breakpoint_is_used())
> > +               return -EBUSY;
> > +
> > +       return 0;
> >  }
> >
> >  static void test_exit(struct kunit *test)
> > @@ -308,6 +315,9 @@ static void test_exit(struct kunit *test)
> >                 kthread_stop(__other_task);
> >                 __other_task = NULL;
> >         }
> > +
> > +       /* Verify that internal state agrees that no breakpoints are in use. */
> > +       KUNIT_EXPECT_FALSE(test, hw_breakpoint_is_used());
> >  }
> >
> >  static struct kunit_suite hw_breakpoint_test_suite = {
> > --
> > 2.37.0.rc0.161.g10f37bed90-goog
> >

[PATCH v3 03/14] perf/hw_breakpoint: Clean up headers

[Marco Elver]

Clean up headers:

 - Remove unused <linux/kallsyms.h>

 - Remove unused <linux/kprobes.h>

 - Remove unused <linux/module.h>

 - Remove unused <linux/smp.h>

 - Add <linux/export.h> for EXPORT_SYMBOL_GPL().

 - Add <linux/mutex.h> for mutex.

 - Sort alphabetically.

 - Move <linux/hw_breakpoint.h> to top to test it compiles on its own.

Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
---
v2:
* Move to start of series.
---
 kernel/events/hw_breakpoint.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index fd5cd1f9e7fc..6076c6346291 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -17,23 +17,22 @@
  * This file contains the arch-independent routines.
  */
 
+#include <linux/hw_breakpoint.h>
+
+#include <linux/bug.h>
+#include <linux/cpu.h>
+#include <linux/export.h>
+#include <linux/init.h>
 #include <linux/irqflags.h>
-#include <linux/kallsyms.h>
-#include <linux/notifier.h>
-#include <linux/kprobes.h>
 #include <linux/kdebug.h>
 #include <linux/kernel.h>
-#include <linux/module.h>
+#include <linux/list.h>
+#include <linux/mutex.h>
+#include <linux/notifier.h>
 #include <linux/percpu.h>
 #include <linux/sched.h>
-#include <linux/init.h>
 #include <linux/slab.h>
-#include <linux/list.h>
-#include <linux/cpu.h>
-#include <linux/smp.h>
-#include <linux/bug.h>
 
-#include <linux/hw_breakpoint.h>
 /*
  * Constraints data
  */
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 03/14] perf/hw_breakpoint: Clean up headers

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:06 AM Marco Elver <elver@google.com> wrote:
>
> Clean up headers:
>
>  - Remove unused <linux/kallsyms.h>
>
>  - Remove unused <linux/kprobes.h>
>
>  - Remove unused <linux/module.h>
>
>  - Remove unused <linux/smp.h>
>
>  - Add <linux/export.h> for EXPORT_SYMBOL_GPL().
>
>  - Add <linux/mutex.h> for mutex.
>
>  - Sort alphabetically.
>
>  - Move <linux/hw_breakpoint.h> to top to test it compiles on its own.
>
> Signed-off-by: Marco Elver <elver@google.com>
> Acked-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
> v2:
> * Move to start of series.
> ---
>  kernel/events/hw_breakpoint.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index fd5cd1f9e7fc..6076c6346291 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -17,23 +17,22 @@
>   * This file contains the arch-independent routines.
>   */
>
> +#include <linux/hw_breakpoint.h>
> +
> +#include <linux/bug.h>
> +#include <linux/cpu.h>
> +#include <linux/export.h>
> +#include <linux/init.h>
>  #include <linux/irqflags.h>
> -#include <linux/kallsyms.h>
> -#include <linux/notifier.h>
> -#include <linux/kprobes.h>
>  #include <linux/kdebug.h>
>  #include <linux/kernel.h>
> -#include <linux/module.h>
> +#include <linux/list.h>
> +#include <linux/mutex.h>
> +#include <linux/notifier.h>
>  #include <linux/percpu.h>
>  #include <linux/sched.h>
> -#include <linux/init.h>
>  #include <linux/slab.h>
> -#include <linux/list.h>
> -#include <linux/cpu.h>
> -#include <linux/smp.h>
> -#include <linux/bug.h>
>
> -#include <linux/hw_breakpoint.h>
>  /*
>   * Constraints data
>   */
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

[PATCH v3 04/14] perf/hw_breakpoint: Optimize list of per-task breakpoints

[Marco Elver]

On a machine with 256 CPUs, running the recently added perf breakpoint
benchmark results in:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 236.418 [sec]
 |
 |   123134.794271 usecs/op
 |  7880626.833333 usecs/op/cpu

The benchmark tests inherited breakpoint perf events across many
threads.

Looking at a perf profile, we can see that the majority of the time is
spent in various hw_breakpoint.c functions, which execute within the
'nr_bp_mutex' critical sections which then results in contention on that
mutex as well:

    37.27%  [kernel]       [k] osq_lock
    34.92%  [kernel]       [k] mutex_spin_on_owner
    12.15%  [kernel]       [k] toggle_bp_slot
    11.90%  [kernel]       [k] __reserve_bp_slot

The culprit here is task_bp_pinned(), which has a runtime complexity of
O(#tasks) due to storing all task breakpoints in the same list and
iterating through that list looking for a matching task. Clearly, this
does not scale to thousands of tasks.

Instead, make use of the "rhashtable" variant "rhltable" which stores
multiple items with the same key in a list. This results in average
runtime complexity of O(1) for task_bp_pinned().

With the optimization, the benchmark shows:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 0.208 [sec]
 |
 |      108.422396 usecs/op
 |     6939.033333 usecs/op/cpu

On this particular setup that's a speedup of ~1135x.

While one option would be to make task_struct a breakpoint list node,
this would only further bloat task_struct for infrequently used data.
Furthermore, after all optimizations in this series, there's no evidence
it would result in better performance: later optimizations make the time
spent looking up entries in the hash table negligible (we'll reach the
theoretical ideal performance i.e. no constraints).

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
v2:
* Commit message tweaks.
---
 include/linux/perf_event.h    |  3 +-
 kernel/events/hw_breakpoint.c | 56 ++++++++++++++++++++++-------------
 2 files changed, 37 insertions(+), 22 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 01231f1d976c..e27360436dc6 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -36,6 +36,7 @@ struct perf_guest_info_callbacks {
 };
 
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
+#include <linux/rhashtable-types.h>
 #include <asm/hw_breakpoint.h>
 #endif
 
@@ -178,7 +179,7 @@ struct hw_perf_event {
 			 * creation and event initalization.
 			 */
 			struct arch_hw_breakpoint	info;
-			struct list_head		bp_list;
+			struct rhlist_head		bp_list;
 		};
 #endif
 		struct { /* amd_iommu */
diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 6076c6346291..6d09edc80d19 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -26,10 +26,10 @@
 #include <linux/irqflags.h>
 #include <linux/kdebug.h>
 #include <linux/kernel.h>
-#include <linux/list.h>
 #include <linux/mutex.h>
 #include <linux/notifier.h>
 #include <linux/percpu.h>
+#include <linux/rhashtable.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
 
@@ -54,7 +54,13 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
 }
 
 /* Keep track of the breakpoints attached to tasks */
-static LIST_HEAD(bp_task_head);
+static struct rhltable task_bps_ht;
+static const struct rhashtable_params task_bps_ht_params = {
+	.head_offset = offsetof(struct hw_perf_event, bp_list),
+	.key_offset = offsetof(struct hw_perf_event, target),
+	.key_len = sizeof_field(struct hw_perf_event, target),
+	.automatic_shrinking = true,
+};
 
 static int constraints_initialized;
 
@@ -103,17 +109,23 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
  */
 static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
 {
-	struct task_struct *tsk = bp->hw.target;
+	struct rhlist_head *head, *pos;
 	struct perf_event *iter;
 	int count = 0;
 
-	list_for_each_entry(iter, &bp_task_head, hw.bp_list) {
-		if (iter->hw.target == tsk &&
-		    find_slot_idx(iter->attr.bp_type) == type &&
+	rcu_read_lock();
+	head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params);
+	if (!head)
+		goto out;
+
+	rhl_for_each_entry_rcu(iter, pos, head, hw.bp_list) {
+		if (find_slot_idx(iter->attr.bp_type) == type &&
 		    (iter->cpu < 0 || cpu == iter->cpu))
 			count += hw_breakpoint_weight(iter);
 	}
 
+out:
+	rcu_read_unlock();
 	return count;
 }
 
@@ -186,7 +198,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
 /*
  * Add/remove the given breakpoint in our constraint table
  */
-static void
+static int
 toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
 	       int weight)
 {
@@ -199,7 +211,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
 	/* Pinned counter cpu profiling */
 	if (!bp->hw.target) {
 		get_bp_info(bp->cpu, type)->cpu_pinned += weight;
-		return;
+		return 0;
 	}
 
 	/* Pinned counter task profiling */
@@ -207,9 +219,9 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
 		toggle_bp_task_slot(bp, cpu, type, weight);
 
 	if (enable)
-		list_add_tail(&bp->hw.bp_list, &bp_task_head);
+		return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
 	else
-		list_del(&bp->hw.bp_list);
+		return rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
 }
 
 __weak int arch_reserve_bp_slot(struct perf_event *bp)
@@ -307,9 +319,7 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
 	if (ret)
 		return ret;
 
-	toggle_bp_slot(bp, true, type, weight);
-
-	return 0;
+	return toggle_bp_slot(bp, true, type, weight);
 }
 
 int reserve_bp_slot(struct perf_event *bp)
@@ -334,7 +344,7 @@ static void __release_bp_slot(struct perf_event *bp, u64 bp_type)
 
 	type = find_slot_idx(bp_type);
 	weight = hw_breakpoint_weight(bp);
-	toggle_bp_slot(bp, false, type, weight);
+	WARN_ON(toggle_bp_slot(bp, false, type, weight));
 }
 
 void release_bp_slot(struct perf_event *bp)
@@ -707,7 +717,7 @@ static struct pmu perf_breakpoint = {
 int __init init_hw_breakpoint(void)
 {
 	int cpu, err_cpu;
-	int i;
+	int i, ret;
 
 	for (i = 0; i < TYPE_MAX; i++)
 		nr_slots[i] = hw_breakpoint_slots(i);
@@ -718,18 +728,24 @@ int __init init_hw_breakpoint(void)
 
 			info->tsk_pinned = kcalloc(nr_slots[i], sizeof(int),
 							GFP_KERNEL);
-			if (!info->tsk_pinned)
-				goto err_alloc;
+			if (!info->tsk_pinned) {
+				ret = -ENOMEM;
+				goto err;
+			}
 		}
 	}
 
+	ret = rhltable_init(&task_bps_ht, &task_bps_ht_params);
+	if (ret)
+		goto err;
+
 	constraints_initialized = 1;
 
 	perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT);
 
 	return register_die_notifier(&hw_breakpoint_exceptions_nb);
 
- err_alloc:
+err:
 	for_each_possible_cpu(err_cpu) {
 		for (i = 0; i < TYPE_MAX; i++)
 			kfree(get_bp_info(err_cpu, i)->tsk_pinned);
@@ -737,7 +753,5 @@ int __init init_hw_breakpoint(void)
 			break;
 	}
 
-	return -ENOMEM;
+	return ret;
 }
-
-
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 04/14] perf/hw_breakpoint: Optimize list of per-task breakpoints

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:06 AM Marco Elver <elver@google.com> wrote:
>
> On a machine with 256 CPUs, running the recently added perf breakpoint
> benchmark results in:
>
>  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
>  |      Total time: 236.418 [sec]
>  |
>  |   123134.794271 usecs/op
>  |  7880626.833333 usecs/op/cpu
>
> The benchmark tests inherited breakpoint perf events across many
> threads.
>
> Looking at a perf profile, we can see that the majority of the time is
> spent in various hw_breakpoint.c functions, which execute within the
> 'nr_bp_mutex' critical sections which then results in contention on that
> mutex as well:
>
>     37.27%  [kernel]       [k] osq_lock
>     34.92%  [kernel]       [k] mutex_spin_on_owner
>     12.15%  [kernel]       [k] toggle_bp_slot
>     11.90%  [kernel]       [k] __reserve_bp_slot
>
> The culprit here is task_bp_pinned(), which has a runtime complexity of
> O(#tasks) due to storing all task breakpoints in the same list and
> iterating through that list looking for a matching task. Clearly, this
> does not scale to thousands of tasks.
>
> Instead, make use of the "rhashtable" variant "rhltable" which stores
> multiple items with the same key in a list. This results in average
> runtime complexity of O(1) for task_bp_pinned().
>
> With the optimization, the benchmark shows:
>
>  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
>  |      Total time: 0.208 [sec]
>  |
>  |      108.422396 usecs/op
>  |     6939.033333 usecs/op/cpu
>
> On this particular setup that's a speedup of ~1135x.
>
> While one option would be to make task_struct a breakpoint list node,
> this would only further bloat task_struct for infrequently used data.
> Furthermore, after all optimizations in this series, there's no evidence
> it would result in better performance: later optimizations make the time
> spent looking up entries in the hash table negligible (we'll reach the
> theoretical ideal performance i.e. no constraints).
>
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> ---
> v2:
> * Commit message tweaks.
> ---
>  include/linux/perf_event.h    |  3 +-
>  kernel/events/hw_breakpoint.c | 56 ++++++++++++++++++++++-------------
>  2 files changed, 37 insertions(+), 22 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 01231f1d976c..e27360436dc6 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -36,6 +36,7 @@ struct perf_guest_info_callbacks {
>  };
>
>  #ifdef CONFIG_HAVE_HW_BREAKPOINT
> +#include <linux/rhashtable-types.h>
>  #include <asm/hw_breakpoint.h>
>  #endif
>
> @@ -178,7 +179,7 @@ struct hw_perf_event {
>                          * creation and event initalization.
>                          */
>                         struct arch_hw_breakpoint       info;
> -                       struct list_head                bp_list;
> +                       struct rhlist_head              bp_list;

nit: perhaps it would be more intention revealing here to rename this
to bp_hashtable?

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

>                 };
>  #endif
>                 struct { /* amd_iommu */
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 6076c6346291..6d09edc80d19 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -26,10 +26,10 @@
>  #include <linux/irqflags.h>
>  #include <linux/kdebug.h>
>  #include <linux/kernel.h>
> -#include <linux/list.h>
>  #include <linux/mutex.h>
>  #include <linux/notifier.h>
>  #include <linux/percpu.h>
> +#include <linux/rhashtable.h>
>  #include <linux/sched.h>
>  #include <linux/slab.h>
>
> @@ -54,7 +54,13 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
>  }
>
>  /* Keep track of the breakpoints attached to tasks */
> -static LIST_HEAD(bp_task_head);
> +static struct rhltable task_bps_ht;
> +static const struct rhashtable_params task_bps_ht_params = {
> +       .head_offset = offsetof(struct hw_perf_event, bp_list),
> +       .key_offset = offsetof(struct hw_perf_event, target),
> +       .key_len = sizeof_field(struct hw_perf_event, target),
> +       .automatic_shrinking = true,
> +};
>
>  static int constraints_initialized;
>
> @@ -103,17 +109,23 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>   */
>  static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
>  {
> -       struct task_struct *tsk = bp->hw.target;
> +       struct rhlist_head *head, *pos;
>         struct perf_event *iter;
>         int count = 0;
>
> -       list_for_each_entry(iter, &bp_task_head, hw.bp_list) {
> -               if (iter->hw.target == tsk &&
> -                   find_slot_idx(iter->attr.bp_type) == type &&
> +       rcu_read_lock();
> +       head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params);
> +       if (!head)
> +               goto out;
> +
> +       rhl_for_each_entry_rcu(iter, pos, head, hw.bp_list) {
> +               if (find_slot_idx(iter->attr.bp_type) == type &&
>                     (iter->cpu < 0 || cpu == iter->cpu))
>                         count += hw_breakpoint_weight(iter);
>         }
>
> +out:
> +       rcu_read_unlock();
>         return count;
>  }
>
> @@ -186,7 +198,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
>  /*
>   * Add/remove the given breakpoint in our constraint table
>   */
> -static void
> +static int
>  toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>                int weight)
>  {
> @@ -199,7 +211,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>         /* Pinned counter cpu profiling */
>         if (!bp->hw.target) {
>                 get_bp_info(bp->cpu, type)->cpu_pinned += weight;
> -               return;
> +               return 0;
>         }
>
>         /* Pinned counter task profiling */
> @@ -207,9 +219,9 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>                 toggle_bp_task_slot(bp, cpu, type, weight);
>
>         if (enable)
> -               list_add_tail(&bp->hw.bp_list, &bp_task_head);
> +               return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
>         else
> -               list_del(&bp->hw.bp_list);
> +               return rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
>  }
>
>  __weak int arch_reserve_bp_slot(struct perf_event *bp)
> @@ -307,9 +319,7 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
>         if (ret)
>                 return ret;
>
> -       toggle_bp_slot(bp, true, type, weight);
> -
> -       return 0;
> +       return toggle_bp_slot(bp, true, type, weight);
>  }
>
>  int reserve_bp_slot(struct perf_event *bp)
> @@ -334,7 +344,7 @@ static void __release_bp_slot(struct perf_event *bp, u64 bp_type)
>
>         type = find_slot_idx(bp_type);
>         weight = hw_breakpoint_weight(bp);
> -       toggle_bp_slot(bp, false, type, weight);
> +       WARN_ON(toggle_bp_slot(bp, false, type, weight));
>  }
>
>  void release_bp_slot(struct perf_event *bp)
> @@ -707,7 +717,7 @@ static struct pmu perf_breakpoint = {
>  int __init init_hw_breakpoint(void)
>  {
>         int cpu, err_cpu;
> -       int i;
> +       int i, ret;
>
>         for (i = 0; i < TYPE_MAX; i++)
>                 nr_slots[i] = hw_breakpoint_slots(i);
> @@ -718,18 +728,24 @@ int __init init_hw_breakpoint(void)
>
>                         info->tsk_pinned = kcalloc(nr_slots[i], sizeof(int),
>                                                         GFP_KERNEL);
> -                       if (!info->tsk_pinned)
> -                               goto err_alloc;
> +                       if (!info->tsk_pinned) {
> +                               ret = -ENOMEM;
> +                               goto err;
> +                       }
>                 }
>         }
>
> +       ret = rhltable_init(&task_bps_ht, &task_bps_ht_params);
> +       if (ret)
> +               goto err;
> +
>         constraints_initialized = 1;
>
>         perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT);
>
>         return register_die_notifier(&hw_breakpoint_exceptions_nb);
>
> - err_alloc:
> +err:
>         for_each_possible_cpu(err_cpu) {
>                 for (i = 0; i < TYPE_MAX; i++)
>                         kfree(get_bp_info(err_cpu, i)->tsk_pinned);
> @@ -737,7 +753,5 @@ int __init init_hw_breakpoint(void)
>                         break;
>         }
>
> -       return -ENOMEM;
> +       return ret;
>  }
> -
> -
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

Re: [PATCH v3 04/14] perf/hw_breakpoint: Optimize list of per-task breakpoints

[Marco Elver]

On Wed, 20 Jul 2022 at 17:29, Ian Rogers <irogers@google.com> wrote:
>
> On Mon, Jul 4, 2022 at 8:06 AM Marco Elver <elver@google.com> wrote:
> >
> > On a machine with 256 CPUs, running the recently added perf breakpoint
> > benchmark results in:
> >
> >  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
> >  | # Running 'breakpoint/thread' benchmark:
> >  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
> >  |      Total time: 236.418 [sec]
> >  |
> >  |   123134.794271 usecs/op
> >  |  7880626.833333 usecs/op/cpu
> >
> > The benchmark tests inherited breakpoint perf events across many
> > threads.
> >
> > Looking at a perf profile, we can see that the majority of the time is
> > spent in various hw_breakpoint.c functions, which execute within the
> > 'nr_bp_mutex' critical sections which then results in contention on that
> > mutex as well:
> >
> >     37.27%  [kernel]       [k] osq_lock
> >     34.92%  [kernel]       [k] mutex_spin_on_owner
> >     12.15%  [kernel]       [k] toggle_bp_slot
> >     11.90%  [kernel]       [k] __reserve_bp_slot
> >
> > The culprit here is task_bp_pinned(), which has a runtime complexity of
> > O(#tasks) due to storing all task breakpoints in the same list and
> > iterating through that list looking for a matching task. Clearly, this
> > does not scale to thousands of tasks.
> >
> > Instead, make use of the "rhashtable" variant "rhltable" which stores
> > multiple items with the same key in a list. This results in average
> > runtime complexity of O(1) for task_bp_pinned().
> >
> > With the optimization, the benchmark shows:
> >
> >  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
> >  | # Running 'breakpoint/thread' benchmark:
> >  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
> >  |      Total time: 0.208 [sec]
> >  |
> >  |      108.422396 usecs/op
> >  |     6939.033333 usecs/op/cpu
> >
> > On this particular setup that's a speedup of ~1135x.
> >
> > While one option would be to make task_struct a breakpoint list node,
> > this would only further bloat task_struct for infrequently used data.
> > Furthermore, after all optimizations in this series, there's no evidence
> > it would result in better performance: later optimizations make the time
> > spent looking up entries in the hash table negligible (we'll reach the
> > theoretical ideal performance i.e. no constraints).
> >
> > Signed-off-by: Marco Elver <elver@google.com>
> > Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> > ---
> > v2:
> > * Commit message tweaks.
> > ---
> >  include/linux/perf_event.h    |  3 +-
> >  kernel/events/hw_breakpoint.c | 56 ++++++++++++++++++++++-------------
> >  2 files changed, 37 insertions(+), 22 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 01231f1d976c..e27360436dc6 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -36,6 +36,7 @@ struct perf_guest_info_callbacks {
> >  };
> >
> >  #ifdef CONFIG_HAVE_HW_BREAKPOINT
> > +#include <linux/rhashtable-types.h>
> >  #include <asm/hw_breakpoint.h>
> >  #endif
> >
> > @@ -178,7 +179,7 @@ struct hw_perf_event {
> >                          * creation and event initalization.
> >                          */
> >                         struct arch_hw_breakpoint       info;
> > -                       struct list_head                bp_list;
> > +                       struct rhlist_head              bp_list;
>
> nit: perhaps it would be more intention revealing here to rename this
> to bp_hashtable?

The naming convention for uses of rhlist_head appears to be either
'list' or 'node' (also inside lib/rhashtable.c). I think this makes
sense because internally this struct is used to just append to the
bucket's list.

> Acked-by: Ian Rogers <irogers@google.com>

Thanks!
-- Marco

[PATCH v3 05/14] perf/hw_breakpoint: Mark data __ro_after_init

[Marco Elver]

Mark read-only data after initialization as __ro_after_init.

While we are here, turn 'constraints_initialized' into a bool.

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
 kernel/events/hw_breakpoint.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 6d09edc80d19..7df46b276452 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -46,7 +46,7 @@ struct bp_cpuinfo {
 };
 
 static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
-static int nr_slots[TYPE_MAX];
+static int nr_slots[TYPE_MAX] __ro_after_init;
 
 static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
 {
@@ -62,7 +62,7 @@ static const struct rhashtable_params task_bps_ht_params = {
 	.automatic_shrinking = true,
 };
 
-static int constraints_initialized;
+static bool constraints_initialized __ro_after_init;
 
 /* Gather the number of total pinned and un-pinned bp in a cpuset */
 struct bp_busy_slots {
@@ -739,7 +739,7 @@ int __init init_hw_breakpoint(void)
 	if (ret)
 		goto err;
 
-	constraints_initialized = 1;
+	constraints_initialized = true;
 
 	perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT);
 
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 05/14] perf/hw_breakpoint: Mark data __ro_after_init

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:06 AM Marco Elver <elver@google.com> wrote:
>
> Mark read-only data after initialization as __ro_after_init.
>
> While we are here, turn 'constraints_initialized' into a bool.
>
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
>  kernel/events/hw_breakpoint.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 6d09edc80d19..7df46b276452 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -46,7 +46,7 @@ struct bp_cpuinfo {
>  };
>
>  static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
> -static int nr_slots[TYPE_MAX];
> +static int nr_slots[TYPE_MAX] __ro_after_init;
>
>  static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
>  {
> @@ -62,7 +62,7 @@ static const struct rhashtable_params task_bps_ht_params = {
>         .automatic_shrinking = true,
>  };
>
> -static int constraints_initialized;
> +static bool constraints_initialized __ro_after_init;
>
>  /* Gather the number of total pinned and un-pinned bp in a cpuset */
>  struct bp_busy_slots {
> @@ -739,7 +739,7 @@ int __init init_hw_breakpoint(void)
>         if (ret)
>                 goto err;
>
> -       constraints_initialized = 1;
> +       constraints_initialized = true;
>
>         perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT);
>
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

[PATCH v3 06/14] perf/hw_breakpoint: Optimize constant number of breakpoint slots

[Marco Elver]

Optimize internal hw_breakpoint state if the architecture's number of
breakpoint slots is constant. This avoids several kmalloc() calls and
potentially unnecessary failures if the allocations fail, as well as
subtly improves code generation and cache locality.

The protocol is that if an architecture defines hw_breakpoint_slots via
the preprocessor, it must be constant and the same for all types.

Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
---
 arch/sh/include/asm/hw_breakpoint.h  |  5 +-
 arch/x86/include/asm/hw_breakpoint.h |  5 +-
 kernel/events/hw_breakpoint.c        | 94 ++++++++++++++++++----------
 3 files changed, 63 insertions(+), 41 deletions(-)

diff --git a/arch/sh/include/asm/hw_breakpoint.h b/arch/sh/include/asm/hw_breakpoint.h
index 199d17b765f2..361a0f57bdeb 100644
--- a/arch/sh/include/asm/hw_breakpoint.h
+++ b/arch/sh/include/asm/hw_breakpoint.h
@@ -48,10 +48,7 @@ struct pmu;
 /* Maximum number of UBC channels */
 #define HBP_NUM		2
 
-static inline int hw_breakpoint_slots(int type)
-{
-	return HBP_NUM;
-}
+#define hw_breakpoint_slots(type) (HBP_NUM)
 
 /* arch/sh/kernel/hw_breakpoint.c */
 extern int arch_check_bp_in_kernelspace(struct arch_hw_breakpoint *hw);
diff --git a/arch/x86/include/asm/hw_breakpoint.h b/arch/x86/include/asm/hw_breakpoint.h
index a1f0e90d0818..0bc931cd0698 100644
--- a/arch/x86/include/asm/hw_breakpoint.h
+++ b/arch/x86/include/asm/hw_breakpoint.h
@@ -44,10 +44,7 @@ struct arch_hw_breakpoint {
 /* Total number of available HW breakpoint registers */
 #define HBP_NUM 4
 
-static inline int hw_breakpoint_slots(int type)
-{
-	return HBP_NUM;
-}
+#define hw_breakpoint_slots(type) (HBP_NUM)
 
 struct perf_event_attr;
 struct perf_event;
diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 7df46b276452..9fb66d358d81 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -40,13 +40,16 @@ struct bp_cpuinfo {
 	/* Number of pinned cpu breakpoints in a cpu */
 	unsigned int	cpu_pinned;
 	/* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
+#ifdef hw_breakpoint_slots
+	unsigned int	tsk_pinned[hw_breakpoint_slots(0)];
+#else
 	unsigned int	*tsk_pinned;
+#endif
 	/* Number of non-pinned cpu/task breakpoints in a cpu */
 	unsigned int	flexible; /* XXX: placeholder, see fetch_this_slot() */
 };
 
 static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
-static int nr_slots[TYPE_MAX] __ro_after_init;
 
 static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
 {
@@ -73,6 +76,54 @@ struct bp_busy_slots {
 /* Serialize accesses to the above constraints */
 static DEFINE_MUTEX(nr_bp_mutex);
 
+#ifdef hw_breakpoint_slots
+/*
+ * Number of breakpoint slots is constant, and the same for all types.
+ */
+static_assert(hw_breakpoint_slots(TYPE_INST) == hw_breakpoint_slots(TYPE_DATA));
+static inline int hw_breakpoint_slots_cached(int type)	{ return hw_breakpoint_slots(type); }
+static inline int init_breakpoint_slots(void)		{ return 0; }
+#else
+/*
+ * Dynamic number of breakpoint slots.
+ */
+static int __nr_bp_slots[TYPE_MAX] __ro_after_init;
+
+static inline int hw_breakpoint_slots_cached(int type)
+{
+	return __nr_bp_slots[type];
+}
+
+static __init int init_breakpoint_slots(void)
+{
+	int i, cpu, err_cpu;
+
+	for (i = 0; i < TYPE_MAX; i++)
+		__nr_bp_slots[i] = hw_breakpoint_slots(i);
+
+	for_each_possible_cpu(cpu) {
+		for (i = 0; i < TYPE_MAX; i++) {
+			struct bp_cpuinfo *info = get_bp_info(cpu, i);
+
+			info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(int), GFP_KERNEL);
+			if (!info->tsk_pinned)
+				goto err;
+		}
+	}
+
+	return 0;
+err:
+	for_each_possible_cpu(err_cpu) {
+		for (i = 0; i < TYPE_MAX; i++)
+			kfree(get_bp_info(err_cpu, i)->tsk_pinned);
+		if (err_cpu == cpu)
+			break;
+	}
+
+	return -ENOMEM;
+}
+#endif
+
 __weak int hw_breakpoint_weight(struct perf_event *bp)
 {
 	return 1;
@@ -95,7 +146,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
 	unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
 	int i;
 
-	for (i = nr_slots[type] - 1; i >= 0; i--) {
+	for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
 		if (tsk_pinned[i] > 0)
 			return i + 1;
 	}
@@ -312,7 +363,7 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
 	fetch_this_slot(&slots, weight);
 
 	/* Flexible counters need to keep at least one slot */
-	if (slots.pinned + (!!slots.flexible) > nr_slots[type])
+	if (slots.pinned + (!!slots.flexible) > hw_breakpoint_slots_cached(type))
 		return -ENOSPC;
 
 	ret = arch_reserve_bp_slot(bp);
@@ -632,7 +683,7 @@ bool hw_breakpoint_is_used(void)
 			if (info->cpu_pinned)
 				return true;
 
-			for (int slot = 0; slot < nr_slots[type]; ++slot) {
+			for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) {
 				if (info->tsk_pinned[slot])
 					return true;
 			}
@@ -716,42 +767,19 @@ static struct pmu perf_breakpoint = {
 
 int __init init_hw_breakpoint(void)
 {
-	int cpu, err_cpu;
-	int i, ret;
-
-	for (i = 0; i < TYPE_MAX; i++)
-		nr_slots[i] = hw_breakpoint_slots(i);
-
-	for_each_possible_cpu(cpu) {
-		for (i = 0; i < TYPE_MAX; i++) {
-			struct bp_cpuinfo *info = get_bp_info(cpu, i);
-
-			info->tsk_pinned = kcalloc(nr_slots[i], sizeof(int),
-							GFP_KERNEL);
-			if (!info->tsk_pinned) {
-				ret = -ENOMEM;
-				goto err;
-			}
-		}
-	}
+	int ret;
 
 	ret = rhltable_init(&task_bps_ht, &task_bps_ht_params);
 	if (ret)
-		goto err;
+		return ret;
+
+	ret = init_breakpoint_slots();
+	if (ret)
+		return ret;
 
 	constraints_initialized = true;
 
 	perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT);
 
 	return register_die_notifier(&hw_breakpoint_exceptions_nb);
-
-err:
-	for_each_possible_cpu(err_cpu) {
-		for (i = 0; i < TYPE_MAX; i++)
-			kfree(get_bp_info(err_cpu, i)->tsk_pinned);
-		if (err_cpu == cpu)
-			break;
-	}
-
-	return ret;
 }
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 06/14] perf/hw_breakpoint: Optimize constant number of breakpoint slots

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:06 AM Marco Elver <elver@google.com> wrote:
>
> Optimize internal hw_breakpoint state if the architecture's number of
> breakpoint slots is constant. This avoids several kmalloc() calls and
> potentially unnecessary failures if the allocations fail, as well as
> subtly improves code generation and cache locality.
>
> The protocol is that if an architecture defines hw_breakpoint_slots via
> the preprocessor, it must be constant and the same for all types.
>
> Signed-off-by: Marco Elver <elver@google.com>
> Acked-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
>  arch/sh/include/asm/hw_breakpoint.h  |  5 +-
>  arch/x86/include/asm/hw_breakpoint.h |  5 +-
>  kernel/events/hw_breakpoint.c        | 94 ++++++++++++++++++----------
>  3 files changed, 63 insertions(+), 41 deletions(-)
>
> diff --git a/arch/sh/include/asm/hw_breakpoint.h b/arch/sh/include/asm/hw_breakpoint.h
> index 199d17b765f2..361a0f57bdeb 100644
> --- a/arch/sh/include/asm/hw_breakpoint.h
> +++ b/arch/sh/include/asm/hw_breakpoint.h
> @@ -48,10 +48,7 @@ struct pmu;
>  /* Maximum number of UBC channels */
>  #define HBP_NUM                2
>
> -static inline int hw_breakpoint_slots(int type)
> -{
> -       return HBP_NUM;
> -}
> +#define hw_breakpoint_slots(type) (HBP_NUM)
>
>  /* arch/sh/kernel/hw_breakpoint.c */
>  extern int arch_check_bp_in_kernelspace(struct arch_hw_breakpoint *hw);
> diff --git a/arch/x86/include/asm/hw_breakpoint.h b/arch/x86/include/asm/hw_breakpoint.h
> index a1f0e90d0818..0bc931cd0698 100644
> --- a/arch/x86/include/asm/hw_breakpoint.h
> +++ b/arch/x86/include/asm/hw_breakpoint.h
> @@ -44,10 +44,7 @@ struct arch_hw_breakpoint {
>  /* Total number of available HW breakpoint registers */
>  #define HBP_NUM 4
>
> -static inline int hw_breakpoint_slots(int type)
> -{
> -       return HBP_NUM;
> -}
> +#define hw_breakpoint_slots(type) (HBP_NUM)
>
>  struct perf_event_attr;
>  struct perf_event;
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 7df46b276452..9fb66d358d81 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -40,13 +40,16 @@ struct bp_cpuinfo {
>         /* Number of pinned cpu breakpoints in a cpu */
>         unsigned int    cpu_pinned;
>         /* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
> +#ifdef hw_breakpoint_slots
> +       unsigned int    tsk_pinned[hw_breakpoint_slots(0)];
> +#else
>         unsigned int    *tsk_pinned;
> +#endif
>         /* Number of non-pinned cpu/task breakpoints in a cpu */
>         unsigned int    flexible; /* XXX: placeholder, see fetch_this_slot() */
>  };
>
>  static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
> -static int nr_slots[TYPE_MAX] __ro_after_init;
>
>  static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
>  {
> @@ -73,6 +76,54 @@ struct bp_busy_slots {
>  /* Serialize accesses to the above constraints */
>  static DEFINE_MUTEX(nr_bp_mutex);
>
> +#ifdef hw_breakpoint_slots
> +/*
> + * Number of breakpoint slots is constant, and the same for all types.
> + */
> +static_assert(hw_breakpoint_slots(TYPE_INST) == hw_breakpoint_slots(TYPE_DATA));
> +static inline int hw_breakpoint_slots_cached(int type) { return hw_breakpoint_slots(type); }
> +static inline int init_breakpoint_slots(void)          { return 0; }
> +#else
> +/*
> + * Dynamic number of breakpoint slots.
> + */
> +static int __nr_bp_slots[TYPE_MAX] __ro_after_init;
> +
> +static inline int hw_breakpoint_slots_cached(int type)
> +{
> +       return __nr_bp_slots[type];
> +}
> +
> +static __init int init_breakpoint_slots(void)
> +{
> +       int i, cpu, err_cpu;
> +
> +       for (i = 0; i < TYPE_MAX; i++)
> +               __nr_bp_slots[i] = hw_breakpoint_slots(i);
> +
> +       for_each_possible_cpu(cpu) {
> +               for (i = 0; i < TYPE_MAX; i++) {
> +                       struct bp_cpuinfo *info = get_bp_info(cpu, i);
> +
> +                       info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(int), GFP_KERNEL);
> +                       if (!info->tsk_pinned)
> +                               goto err;
> +               }
> +       }
> +
> +       return 0;
> +err:
> +       for_each_possible_cpu(err_cpu) {
> +               for (i = 0; i < TYPE_MAX; i++)
> +                       kfree(get_bp_info(err_cpu, i)->tsk_pinned);
> +               if (err_cpu == cpu)
> +                       break;
> +       }
> +
> +       return -ENOMEM;
> +}
> +#endif
> +
>  __weak int hw_breakpoint_weight(struct perf_event *bp)
>  {
>         return 1;
> @@ -95,7 +146,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>         unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
>         int i;
>
> -       for (i = nr_slots[type] - 1; i >= 0; i--) {
> +       for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
>                 if (tsk_pinned[i] > 0)
>                         return i + 1;
>         }
> @@ -312,7 +363,7 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
>         fetch_this_slot(&slots, weight);
>
>         /* Flexible counters need to keep at least one slot */
> -       if (slots.pinned + (!!slots.flexible) > nr_slots[type])
> +       if (slots.pinned + (!!slots.flexible) > hw_breakpoint_slots_cached(type))
>                 return -ENOSPC;
>
>         ret = arch_reserve_bp_slot(bp);
> @@ -632,7 +683,7 @@ bool hw_breakpoint_is_used(void)
>                         if (info->cpu_pinned)
>                                 return true;
>
> -                       for (int slot = 0; slot < nr_slots[type]; ++slot) {
> +                       for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) {
>                                 if (info->tsk_pinned[slot])
>                                         return true;
>                         }
> @@ -716,42 +767,19 @@ static struct pmu perf_breakpoint = {
>
>  int __init init_hw_breakpoint(void)
>  {
> -       int cpu, err_cpu;
> -       int i, ret;
> -
> -       for (i = 0; i < TYPE_MAX; i++)
> -               nr_slots[i] = hw_breakpoint_slots(i);
> -
> -       for_each_possible_cpu(cpu) {
> -               for (i = 0; i < TYPE_MAX; i++) {
> -                       struct bp_cpuinfo *info = get_bp_info(cpu, i);
> -
> -                       info->tsk_pinned = kcalloc(nr_slots[i], sizeof(int),
> -                                                       GFP_KERNEL);
> -                       if (!info->tsk_pinned) {
> -                               ret = -ENOMEM;
> -                               goto err;
> -                       }
> -               }
> -       }
> +       int ret;
>
>         ret = rhltable_init(&task_bps_ht, &task_bps_ht_params);
>         if (ret)
> -               goto err;
> +               return ret;
> +
> +       ret = init_breakpoint_slots();
> +       if (ret)
> +               return ret;
>
>         constraints_initialized = true;
>
>         perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT);
>
>         return register_die_notifier(&hw_breakpoint_exceptions_nb);
> -
> -err:
> -       for_each_possible_cpu(err_cpu) {
> -               for (i = 0; i < TYPE_MAX; i++)
> -                       kfree(get_bp_info(err_cpu, i)->tsk_pinned);
> -               if (err_cpu == cpu)
> -                       break;
> -       }
> -
> -       return ret;
>  }
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

[PATCH v3 07/14] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable

[Marco Elver]

Due to being a __weak function, hw_breakpoint_weight() will cause the
compiler to always emit a call to it. This generates unnecessarily bad
code (register spills etc.) for no good reason; in fact it appears in
profiles of `perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512`:

    ...
    0.70%  [kernel]       [k] hw_breakpoint_weight
    ...

While a small percentage, no architecture defines its own
hw_breakpoint_weight() nor are there users outside hw_breakpoint.c,
which makes the fact it is currently __weak a poor choice.

Change hw_breakpoint_weight()'s definition to follow a similar protocol
to hw_breakpoint_slots(), such that if <asm/hw_breakpoint.h> defines
hw_breakpoint_weight(), we'll use it instead.

The result is that it is inlined and no longer shows up in profiles.

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
 include/linux/hw_breakpoint.h | 1 -
 kernel/events/hw_breakpoint.c | 4 +++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
index a3fb846705eb..f319bd26b030 100644
--- a/include/linux/hw_breakpoint.h
+++ b/include/linux/hw_breakpoint.h
@@ -80,7 +80,6 @@ extern int dbg_reserve_bp_slot(struct perf_event *bp);
 extern int dbg_release_bp_slot(struct perf_event *bp);
 extern int reserve_bp_slot(struct perf_event *bp);
 extern void release_bp_slot(struct perf_event *bp);
-int hw_breakpoint_weight(struct perf_event *bp);
 int arch_reserve_bp_slot(struct perf_event *bp);
 void arch_release_bp_slot(struct perf_event *bp);
 void arch_unregister_hw_breakpoint(struct perf_event *bp);
diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 9fb66d358d81..9c9bf17666a5 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -124,10 +124,12 @@ static __init int init_breakpoint_slots(void)
 }
 #endif
 
-__weak int hw_breakpoint_weight(struct perf_event *bp)
+#ifndef hw_breakpoint_weight
+static inline int hw_breakpoint_weight(struct perf_event *bp)
 {
 	return 1;
 }
+#endif
 
 static inline enum bp_type_idx find_slot_idx(u64 bp_type)
 {
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 07/14] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:06 AM Marco Elver <elver@google.com> wrote:
>
> Due to being a __weak function, hw_breakpoint_weight() will cause the
> compiler to always emit a call to it. This generates unnecessarily bad
> code (register spills etc.) for no good reason; in fact it appears in
> profiles of `perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512`:
>
>     ...
>     0.70%  [kernel]       [k] hw_breakpoint_weight
>     ...
>
> While a small percentage, no architecture defines its own
> hw_breakpoint_weight() nor are there users outside hw_breakpoint.c,
> which makes the fact it is currently __weak a poor choice.
>
> Change hw_breakpoint_weight()'s definition to follow a similar protocol
> to hw_breakpoint_slots(), such that if <asm/hw_breakpoint.h> defines
> hw_breakpoint_weight(), we'll use it instead.
>
> The result is that it is inlined and no longer shows up in profiles.
>
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
>  include/linux/hw_breakpoint.h | 1 -
>  kernel/events/hw_breakpoint.c | 4 +++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
> index a3fb846705eb..f319bd26b030 100644
> --- a/include/linux/hw_breakpoint.h
> +++ b/include/linux/hw_breakpoint.h
> @@ -80,7 +80,6 @@ extern int dbg_reserve_bp_slot(struct perf_event *bp);
>  extern int dbg_release_bp_slot(struct perf_event *bp);
>  extern int reserve_bp_slot(struct perf_event *bp);
>  extern void release_bp_slot(struct perf_event *bp);
> -int hw_breakpoint_weight(struct perf_event *bp);
>  int arch_reserve_bp_slot(struct perf_event *bp);
>  void arch_release_bp_slot(struct perf_event *bp);
>  void arch_unregister_hw_breakpoint(struct perf_event *bp);
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 9fb66d358d81..9c9bf17666a5 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -124,10 +124,12 @@ static __init int init_breakpoint_slots(void)
>  }
>  #endif
>
> -__weak int hw_breakpoint_weight(struct perf_event *bp)
> +#ifndef hw_breakpoint_weight
> +static inline int hw_breakpoint_weight(struct perf_event *bp)
>  {
>         return 1;
>  }
> +#endif
>
>  static inline enum bp_type_idx find_slot_idx(u64 bp_type)
>  {
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

[PATCH v3 08/14] perf/hw_breakpoint: Remove useless code related to flexible breakpoints

[Marco Elver]

Flexible breakpoints have never been implemented, with
bp_cpuinfo::flexible always being 0. Unfortunately, they still occupy 4
bytes in each bp_cpuinfo and bp_busy_slots, as well as computing the max
flexible count in fetch_bp_busy_slots().

This again causes suboptimal code generation, when we always know that
`!!slots.flexible` will be 0.

Just get rid of the flexible "placeholder" and remove all real code
related to it. Make a note in the comment related to the constraints
algorithm but don't remove them from the algorithm, so that if in future
flexible breakpoints need supporting, it should be trivial to revive
them (along with reverting this change).

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
v2:
* Also remove struct bp_busy_slots, and simplify functions.
---
 kernel/events/hw_breakpoint.c | 57 +++++++++++------------------------
 1 file changed, 17 insertions(+), 40 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 9c9bf17666a5..8b40fca1a063 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -45,8 +45,6 @@ struct bp_cpuinfo {
 #else
 	unsigned int	*tsk_pinned;
 #endif
-	/* Number of non-pinned cpu/task breakpoints in a cpu */
-	unsigned int	flexible; /* XXX: placeholder, see fetch_this_slot() */
 };
 
 static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
@@ -67,12 +65,6 @@ static const struct rhashtable_params task_bps_ht_params = {
 
 static bool constraints_initialized __ro_after_init;
 
-/* Gather the number of total pinned and un-pinned bp in a cpuset */
-struct bp_busy_slots {
-	unsigned int pinned;
-	unsigned int flexible;
-};
-
 /* Serialize accesses to the above constraints */
 static DEFINE_MUTEX(nr_bp_mutex);
 
@@ -190,14 +182,14 @@ static const struct cpumask *cpumask_of_bp(struct perf_event *bp)
 }
 
 /*
- * Report the number of pinned/un-pinned breakpoints we have in
- * a given cpu (cpu > -1) or in all of them (cpu = -1).
+ * Returns the max pinned breakpoint slots in a given
+ * CPU (cpu > -1) or across all of them (cpu = -1).
  */
-static void
-fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp,
-		    enum bp_type_idx type)
+static int
+max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
 {
 	const struct cpumask *cpumask = cpumask_of_bp(bp);
+	int pinned_slots = 0;
 	int cpu;
 
 	for_each_cpu(cpu, cpumask) {
@@ -210,24 +202,10 @@ fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp,
 		else
 			nr += task_bp_pinned(cpu, bp, type);
 
-		if (nr > slots->pinned)
-			slots->pinned = nr;
-
-		nr = info->flexible;
-		if (nr > slots->flexible)
-			slots->flexible = nr;
+		pinned_slots = max(nr, pinned_slots);
 	}
-}
 
-/*
- * For now, continue to consider flexible as pinned, until we can
- * ensure no flexible event can ever be scheduled before a pinned event
- * in a same cpu.
- */
-static void
-fetch_this_slot(struct bp_busy_slots *slots, int weight)
-{
-	slots->pinned += weight;
+	return pinned_slots;
 }
 
 /*
@@ -298,7 +276,12 @@ __weak void arch_unregister_hw_breakpoint(struct perf_event *bp)
 }
 
 /*
- * Constraints to check before allowing this new breakpoint counter:
+ * Constraints to check before allowing this new breakpoint counter.
+ *
+ * Note: Flexible breakpoints are currently unimplemented, but outlined in the
+ * below algorithm for completeness.  The implementation treats flexible as
+ * pinned due to no guarantee that we currently always schedule flexible events
+ * before a pinned event in a same CPU.
  *
  *  == Non-pinned counter == (Considered as pinned for now)
  *
@@ -340,8 +323,8 @@ __weak void arch_unregister_hw_breakpoint(struct perf_event *bp)
  */
 static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
 {
-	struct bp_busy_slots slots = {0};
 	enum bp_type_idx type;
+	int max_pinned_slots;
 	int weight;
 	int ret;
 
@@ -357,15 +340,9 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
 	type = find_slot_idx(bp_type);
 	weight = hw_breakpoint_weight(bp);
 
-	fetch_bp_busy_slots(&slots, bp, type);
-	/*
-	 * Simulate the addition of this breakpoint to the constraints
-	 * and see the result.
-	 */
-	fetch_this_slot(&slots, weight);
-
-	/* Flexible counters need to keep at least one slot */
-	if (slots.pinned + (!!slots.flexible) > hw_breakpoint_slots_cached(type))
+	/* Check if this new breakpoint can be satisfied across all CPUs. */
+	max_pinned_slots = max_bp_pinned_slots(bp, type) + weight;
+	if (max_pinned_slots > hw_breakpoint_slots_cached(type))
 		return -ENOSPC;
 
 	ret = arch_reserve_bp_slot(bp);
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 08/14] perf/hw_breakpoint: Remove useless code related to flexible breakpoints

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:06 AM Marco Elver <elver@google.com> wrote:
>
> Flexible breakpoints have never been implemented, with
> bp_cpuinfo::flexible always being 0. Unfortunately, they still occupy 4
> bytes in each bp_cpuinfo and bp_busy_slots, as well as computing the max
> flexible count in fetch_bp_busy_slots().
>
> This again causes suboptimal code generation, when we always know that
> `!!slots.flexible` will be 0.
>
> Just get rid of the flexible "placeholder" and remove all real code
> related to it. Make a note in the comment related to the constraints
> algorithm but don't remove them from the algorithm, so that if in future
> flexible breakpoints need supporting, it should be trivial to revive
> them (along with reverting this change).
>
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
> v2:
> * Also remove struct bp_busy_slots, and simplify functions.
> ---
>  kernel/events/hw_breakpoint.c | 57 +++++++++++------------------------
>  1 file changed, 17 insertions(+), 40 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 9c9bf17666a5..8b40fca1a063 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -45,8 +45,6 @@ struct bp_cpuinfo {
>  #else
>         unsigned int    *tsk_pinned;
>  #endif
> -       /* Number of non-pinned cpu/task breakpoints in a cpu */
> -       unsigned int    flexible; /* XXX: placeholder, see fetch_this_slot() */
>  };
>
>  static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
> @@ -67,12 +65,6 @@ static const struct rhashtable_params task_bps_ht_params = {
>
>  static bool constraints_initialized __ro_after_init;
>
> -/* Gather the number of total pinned and un-pinned bp in a cpuset */
> -struct bp_busy_slots {
> -       unsigned int pinned;
> -       unsigned int flexible;
> -};
> -
>  /* Serialize accesses to the above constraints */
>  static DEFINE_MUTEX(nr_bp_mutex);
>
> @@ -190,14 +182,14 @@ static const struct cpumask *cpumask_of_bp(struct perf_event *bp)
>  }
>
>  /*
> - * Report the number of pinned/un-pinned breakpoints we have in
> - * a given cpu (cpu > -1) or in all of them (cpu = -1).
> + * Returns the max pinned breakpoint slots in a given
> + * CPU (cpu > -1) or across all of them (cpu = -1).
>   */
> -static void
> -fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp,
> -                   enum bp_type_idx type)
> +static int
> +max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
>  {
>         const struct cpumask *cpumask = cpumask_of_bp(bp);
> +       int pinned_slots = 0;
>         int cpu;
>
>         for_each_cpu(cpu, cpumask) {
> @@ -210,24 +202,10 @@ fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp,
>                 else
>                         nr += task_bp_pinned(cpu, bp, type);
>
> -               if (nr > slots->pinned)
> -                       slots->pinned = nr;
> -
> -               nr = info->flexible;
> -               if (nr > slots->flexible)
> -                       slots->flexible = nr;
> +               pinned_slots = max(nr, pinned_slots);
>         }
> -}
>
> -/*
> - * For now, continue to consider flexible as pinned, until we can
> - * ensure no flexible event can ever be scheduled before a pinned event
> - * in a same cpu.
> - */
> -static void
> -fetch_this_slot(struct bp_busy_slots *slots, int weight)
> -{
> -       slots->pinned += weight;
> +       return pinned_slots;
>  }
>
>  /*
> @@ -298,7 +276,12 @@ __weak void arch_unregister_hw_breakpoint(struct perf_event *bp)
>  }
>
>  /*
> - * Constraints to check before allowing this new breakpoint counter:
> + * Constraints to check before allowing this new breakpoint counter.
> + *
> + * Note: Flexible breakpoints are currently unimplemented, but outlined in the
> + * below algorithm for completeness.  The implementation treats flexible as
> + * pinned due to no guarantee that we currently always schedule flexible events
> + * before a pinned event in a same CPU.
>   *
>   *  == Non-pinned counter == (Considered as pinned for now)
>   *
> @@ -340,8 +323,8 @@ __weak void arch_unregister_hw_breakpoint(struct perf_event *bp)
>   */
>  static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
>  {
> -       struct bp_busy_slots slots = {0};
>         enum bp_type_idx type;
> +       int max_pinned_slots;
>         int weight;
>         int ret;
>
> @@ -357,15 +340,9 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
>         type = find_slot_idx(bp_type);
>         weight = hw_breakpoint_weight(bp);
>
> -       fetch_bp_busy_slots(&slots, bp, type);
> -       /*
> -        * Simulate the addition of this breakpoint to the constraints
> -        * and see the result.
> -        */
> -       fetch_this_slot(&slots, weight);
> -
> -       /* Flexible counters need to keep at least one slot */
> -       if (slots.pinned + (!!slots.flexible) > hw_breakpoint_slots_cached(type))
> +       /* Check if this new breakpoint can be satisfied across all CPUs. */
> +       max_pinned_slots = max_bp_pinned_slots(bp, type) + weight;
> +       if (max_pinned_slots > hw_breakpoint_slots_cached(type))
>                 return -ENOSPC;
>
>         ret = arch_reserve_bp_slot(bp);
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

[PATCH v3 09/14] powerpc/hw_breakpoint: Avoid relying on caller synchronization

[Marco Elver]

Internal data structures (cpu_bps, task_bps) of powerpc's hw_breakpoint
implementation have relied on nr_bp_mutex serializing access to them.

Before overhauling synchronization of kernel/events/hw_breakpoint.c,
introduce 2 spinlocks to synchronize cpu_bps and task_bps respectively,
thus avoiding reliance on callers synchronizing powerpc's hw_breakpoint.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
---
v2:
* New patch.
---
 arch/powerpc/kernel/hw_breakpoint.c | 53 ++++++++++++++++++++++-------
 1 file changed, 40 insertions(+), 13 deletions(-)

diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
index 2669f80b3a49..8db1a15d7acb 100644
--- a/arch/powerpc/kernel/hw_breakpoint.c
+++ b/arch/powerpc/kernel/hw_breakpoint.c
@@ -15,6 +15,7 @@
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/smp.h>
+#include <linux/spinlock.h>
 #include <linux/debugfs.h>
 #include <linux/init.h>
 
@@ -129,7 +130,14 @@ struct breakpoint {
 	bool ptrace_bp;
 };
 
+/*
+ * While kernel/events/hw_breakpoint.c does its own synchronization, we cannot
+ * rely on it safely synchronizing internals here; however, we can rely on it
+ * not requesting more breakpoints than available.
+ */
+static DEFINE_SPINLOCK(cpu_bps_lock);
 static DEFINE_PER_CPU(struct breakpoint *, cpu_bps[HBP_NUM_MAX]);
+static DEFINE_SPINLOCK(task_bps_lock);
 static LIST_HEAD(task_bps);
 
 static struct breakpoint *alloc_breakpoint(struct perf_event *bp)
@@ -174,7 +182,9 @@ static int task_bps_add(struct perf_event *bp)
 	if (IS_ERR(tmp))
 		return PTR_ERR(tmp);
 
+	spin_lock(&task_bps_lock);
 	list_add(&tmp->list, &task_bps);
+	spin_unlock(&task_bps_lock);
 	return 0;
 }
 
@@ -182,6 +192,7 @@ static void task_bps_remove(struct perf_event *bp)
 {
 	struct list_head *pos, *q;
 
+	spin_lock(&task_bps_lock);
 	list_for_each_safe(pos, q, &task_bps) {
 		struct breakpoint *tmp = list_entry(pos, struct breakpoint, list);
 
@@ -191,6 +202,7 @@ static void task_bps_remove(struct perf_event *bp)
 			break;
 		}
 	}
+	spin_unlock(&task_bps_lock);
 }
 
 /*
@@ -200,12 +212,17 @@ static void task_bps_remove(struct perf_event *bp)
 static bool all_task_bps_check(struct perf_event *bp)
 {
 	struct breakpoint *tmp;
+	bool ret = false;
 
+	spin_lock(&task_bps_lock);
 	list_for_each_entry(tmp, &task_bps, list) {
-		if (!can_co_exist(tmp, bp))
-			return true;
+		if (!can_co_exist(tmp, bp)) {
+			ret = true;
+			break;
+		}
 	}
-	return false;
+	spin_unlock(&task_bps_lock);
+	return ret;
 }
 
 /*
@@ -215,13 +232,18 @@ static bool all_task_bps_check(struct perf_event *bp)
 static bool same_task_bps_check(struct perf_event *bp)
 {
 	struct breakpoint *tmp;
+	bool ret = false;
 
+	spin_lock(&task_bps_lock);
 	list_for_each_entry(tmp, &task_bps, list) {
 		if (tmp->bp->hw.target == bp->hw.target &&
-		    !can_co_exist(tmp, bp))
-			return true;
+		    !can_co_exist(tmp, bp)) {
+			ret = true;
+			break;
+		}
 	}
-	return false;
+	spin_unlock(&task_bps_lock);
+	return ret;
 }
 
 static int cpu_bps_add(struct perf_event *bp)
@@ -234,6 +256,7 @@ static int cpu_bps_add(struct perf_event *bp)
 	if (IS_ERR(tmp))
 		return PTR_ERR(tmp);
 
+	spin_lock(&cpu_bps_lock);
 	cpu_bp = per_cpu_ptr(cpu_bps, bp->cpu);
 	for (i = 0; i < nr_wp_slots(); i++) {
 		if (!cpu_bp[i]) {
@@ -241,6 +264,7 @@ static int cpu_bps_add(struct perf_event *bp)
 			break;
 		}
 	}
+	spin_unlock(&cpu_bps_lock);
 	return 0;
 }
 
@@ -249,6 +273,7 @@ static void cpu_bps_remove(struct perf_event *bp)
 	struct breakpoint **cpu_bp;
 	int i = 0;
 
+	spin_lock(&cpu_bps_lock);
 	cpu_bp = per_cpu_ptr(cpu_bps, bp->cpu);
 	for (i = 0; i < nr_wp_slots(); i++) {
 		if (!cpu_bp[i])
@@ -260,19 +285,25 @@ static void cpu_bps_remove(struct perf_event *bp)
 			break;
 		}
 	}
+	spin_unlock(&cpu_bps_lock);
 }
 
 static bool cpu_bps_check(int cpu, struct perf_event *bp)
 {
 	struct breakpoint **cpu_bp;
+	bool ret = false;
 	int i;
 
+	spin_lock(&cpu_bps_lock);
 	cpu_bp = per_cpu_ptr(cpu_bps, cpu);
 	for (i = 0; i < nr_wp_slots(); i++) {
-		if (cpu_bp[i] && !can_co_exist(cpu_bp[i], bp))
-			return true;
+		if (cpu_bp[i] && !can_co_exist(cpu_bp[i], bp)) {
+			ret = true;
+			break;
+		}
 	}
-	return false;
+	spin_unlock(&cpu_bps_lock);
+	return ret;
 }
 
 static bool all_cpu_bps_check(struct perf_event *bp)
@@ -286,10 +317,6 @@ static bool all_cpu_bps_check(struct perf_event *bp)
 	return false;
 }
 
-/*
- * We don't use any locks to serialize accesses to cpu_bps or task_bps
- * because are already inside nr_bp_mutex.
- */
 int arch_reserve_bp_slot(struct perf_event *bp)
 {
 	int ret;
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 09/14] powerpc/hw_breakpoint: Avoid relying on caller synchronization

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:07 AM Marco Elver <elver@google.com> wrote:
>
> Internal data structures (cpu_bps, task_bps) of powerpc's hw_breakpoint
> implementation have relied on nr_bp_mutex serializing access to them.
>
> Before overhauling synchronization of kernel/events/hw_breakpoint.c,
> introduce 2 spinlocks to synchronize cpu_bps and task_bps respectively,
> thus avoiding reliance on callers synchronizing powerpc's hw_breakpoint.
>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Marco Elver <elver@google.com>
> Acked-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
> v2:
> * New patch.
> ---
>  arch/powerpc/kernel/hw_breakpoint.c | 53 ++++++++++++++++++++++-------
>  1 file changed, 40 insertions(+), 13 deletions(-)
>
> diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
> index 2669f80b3a49..8db1a15d7acb 100644
> --- a/arch/powerpc/kernel/hw_breakpoint.c
> +++ b/arch/powerpc/kernel/hw_breakpoint.c
> @@ -15,6 +15,7 @@
>  #include <linux/kernel.h>
>  #include <linux/sched.h>
>  #include <linux/smp.h>
> +#include <linux/spinlock.h>
>  #include <linux/debugfs.h>
>  #include <linux/init.h>
>
> @@ -129,7 +130,14 @@ struct breakpoint {
>         bool ptrace_bp;
>  };
>
> +/*
> + * While kernel/events/hw_breakpoint.c does its own synchronization, we cannot
> + * rely on it safely synchronizing internals here; however, we can rely on it
> + * not requesting more breakpoints than available.
> + */
> +static DEFINE_SPINLOCK(cpu_bps_lock);
>  static DEFINE_PER_CPU(struct breakpoint *, cpu_bps[HBP_NUM_MAX]);
> +static DEFINE_SPINLOCK(task_bps_lock);
>  static LIST_HEAD(task_bps);
>
>  static struct breakpoint *alloc_breakpoint(struct perf_event *bp)
> @@ -174,7 +182,9 @@ static int task_bps_add(struct perf_event *bp)
>         if (IS_ERR(tmp))
>                 return PTR_ERR(tmp);
>
> +       spin_lock(&task_bps_lock);
>         list_add(&tmp->list, &task_bps);
> +       spin_unlock(&task_bps_lock);
>         return 0;
>  }
>
> @@ -182,6 +192,7 @@ static void task_bps_remove(struct perf_event *bp)
>  {
>         struct list_head *pos, *q;
>
> +       spin_lock(&task_bps_lock);
>         list_for_each_safe(pos, q, &task_bps) {
>                 struct breakpoint *tmp = list_entry(pos, struct breakpoint, list);
>
> @@ -191,6 +202,7 @@ static void task_bps_remove(struct perf_event *bp)
>                         break;
>                 }
>         }
> +       spin_unlock(&task_bps_lock);
>  }
>
>  /*
> @@ -200,12 +212,17 @@ static void task_bps_remove(struct perf_event *bp)
>  static bool all_task_bps_check(struct perf_event *bp)
>  {
>         struct breakpoint *tmp;
> +       bool ret = false;
>
> +       spin_lock(&task_bps_lock);
>         list_for_each_entry(tmp, &task_bps, list) {
> -               if (!can_co_exist(tmp, bp))
> -                       return true;
> +               if (!can_co_exist(tmp, bp)) {
> +                       ret = true;
> +                       break;
> +               }
>         }
> -       return false;
> +       spin_unlock(&task_bps_lock);
> +       return ret;
>  }
>
>  /*
> @@ -215,13 +232,18 @@ static bool all_task_bps_check(struct perf_event *bp)
>  static bool same_task_bps_check(struct perf_event *bp)
>  {
>         struct breakpoint *tmp;
> +       bool ret = false;
>
> +       spin_lock(&task_bps_lock);
>         list_for_each_entry(tmp, &task_bps, list) {
>                 if (tmp->bp->hw.target == bp->hw.target &&
> -                   !can_co_exist(tmp, bp))
> -                       return true;
> +                   !can_co_exist(tmp, bp)) {
> +                       ret = true;
> +                       break;
> +               }
>         }
> -       return false;
> +       spin_unlock(&task_bps_lock);
> +       return ret;
>  }
>
>  static int cpu_bps_add(struct perf_event *bp)
> @@ -234,6 +256,7 @@ static int cpu_bps_add(struct perf_event *bp)
>         if (IS_ERR(tmp))
>                 return PTR_ERR(tmp);
>
> +       spin_lock(&cpu_bps_lock);
>         cpu_bp = per_cpu_ptr(cpu_bps, bp->cpu);
>         for (i = 0; i < nr_wp_slots(); i++) {
>                 if (!cpu_bp[i]) {
> @@ -241,6 +264,7 @@ static int cpu_bps_add(struct perf_event *bp)
>                         break;
>                 }
>         }
> +       spin_unlock(&cpu_bps_lock);
>         return 0;
>  }
>
> @@ -249,6 +273,7 @@ static void cpu_bps_remove(struct perf_event *bp)
>         struct breakpoint **cpu_bp;
>         int i = 0;
>
> +       spin_lock(&cpu_bps_lock);
>         cpu_bp = per_cpu_ptr(cpu_bps, bp->cpu);
>         for (i = 0; i < nr_wp_slots(); i++) {
>                 if (!cpu_bp[i])
> @@ -260,19 +285,25 @@ static void cpu_bps_remove(struct perf_event *bp)
>                         break;
>                 }
>         }
> +       spin_unlock(&cpu_bps_lock);
>  }
>
>  static bool cpu_bps_check(int cpu, struct perf_event *bp)
>  {
>         struct breakpoint **cpu_bp;
> +       bool ret = false;
>         int i;
>
> +       spin_lock(&cpu_bps_lock);
>         cpu_bp = per_cpu_ptr(cpu_bps, cpu);
>         for (i = 0; i < nr_wp_slots(); i++) {
> -               if (cpu_bp[i] && !can_co_exist(cpu_bp[i], bp))
> -                       return true;
> +               if (cpu_bp[i] && !can_co_exist(cpu_bp[i], bp)) {
> +                       ret = true;
> +                       break;
> +               }
>         }
> -       return false;
> +       spin_unlock(&cpu_bps_lock);
> +       return ret;
>  }
>
>  static bool all_cpu_bps_check(struct perf_event *bp)
> @@ -286,10 +317,6 @@ static bool all_cpu_bps_check(struct perf_event *bp)
>         return false;
>  }
>
> -/*
> - * We don't use any locks to serialize accesses to cpu_bps or task_bps
> - * because are already inside nr_bp_mutex.
> - */
>  int arch_reserve_bp_slot(struct perf_event *bp)
>  {
>         int ret;
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

[PATCH v3 10/14] locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked()

[Marco Elver]

Implement simple accessors to probe percpu-rwsem's locked state:
percpu_is_write_locked(), percpu_is_read_locked().

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
v2:
* New patch.
---
 include/linux/percpu-rwsem.h  | 6 ++++++
 kernel/locking/percpu-rwsem.c | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index 5fda40f97fe9..36b942b67b7d 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -121,9 +121,15 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
 	preempt_enable();
 }
 
+extern bool percpu_is_read_locked(struct percpu_rw_semaphore *);
 extern void percpu_down_write(struct percpu_rw_semaphore *);
 extern void percpu_up_write(struct percpu_rw_semaphore *);
 
+static inline bool percpu_is_write_locked(struct percpu_rw_semaphore *sem)
+{
+	return atomic_read(&sem->block);
+}
+
 extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
 				const char *, struct lock_class_key *);
 
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 5fe4c5495ba3..213d114fb025 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -192,6 +192,12 @@ EXPORT_SYMBOL_GPL(__percpu_down_read);
 	__sum;								\
 })
 
+bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
+{
+	return per_cpu_sum(*sem->read_count) != 0;
+}
+EXPORT_SYMBOL_GPL(percpu_is_read_locked);
+
 /*
  * Return true if the modular sum of the sem->read_count per-CPU variable is
  * zero.  If this sum is zero, then it is stable due to the fact that if any
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 10/14] locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked()

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:07 AM Marco Elver <elver@google.com> wrote:
>
> Implement simple accessors to probe percpu-rwsem's locked state:
> percpu_is_write_locked(), percpu_is_read_locked().
>
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
> v2:
> * New patch.
> ---
>  include/linux/percpu-rwsem.h  | 6 ++++++
>  kernel/locking/percpu-rwsem.c | 6 ++++++
>  2 files changed, 12 insertions(+)
>
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index 5fda40f97fe9..36b942b67b7d 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -121,9 +121,15 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
>         preempt_enable();
>  }
>
> +extern bool percpu_is_read_locked(struct percpu_rw_semaphore *);
>  extern void percpu_down_write(struct percpu_rw_semaphore *);
>  extern void percpu_up_write(struct percpu_rw_semaphore *);
>
> +static inline bool percpu_is_write_locked(struct percpu_rw_semaphore *sem)
> +{
> +       return atomic_read(&sem->block);
> +}
> +
>  extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
>                                 const char *, struct lock_class_key *);
>
> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
> index 5fe4c5495ba3..213d114fb025 100644
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -192,6 +192,12 @@ EXPORT_SYMBOL_GPL(__percpu_down_read);
>         __sum;                                                          \
>  })
>
> +bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
> +{
> +       return per_cpu_sum(*sem->read_count) != 0;
> +}
> +EXPORT_SYMBOL_GPL(percpu_is_read_locked);
> +
>  /*
>   * Return true if the modular sum of the sem->read_count per-CPU variable is
>   * zero.  If this sum is zero, then it is stable due to the fact that if any
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

Re: [PATCH v3 10/14] locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked()

[Peter Zijlstra]

On Mon, Jul 04, 2022 at 05:05:10PM +0200, Marco Elver wrote:

> +bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
> +{
> +	return per_cpu_sum(*sem->read_count) != 0;
> +}
> +EXPORT_SYMBOL_GPL(percpu_is_read_locked);

I don't think this is correct; read_count can have spurious increments.

If we look at __percpu_down_read_trylock(), it does roughly something
like this:

	this_cpu_inc(*sem->read_count);
	smp_mb();
	if (!sem->block)
		return true;
	this_cpu_dec(*sem->read_count);
	return false;

So percpu_is_read_locked() needs to ensure the read_count is non-zero
*and* that block is not set.

That said; I really dislike the whole _is_locked family with a passion.
Let me try and figure out what you need this for.

Re: [PATCH v3 10/14] locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked()

[Marco Elver]

On Wed, 17 Aug 2022 at 14:48, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Jul 04, 2022 at 05:05:10PM +0200, Marco Elver wrote:
> > +bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
> > +{
> > +     return per_cpu_sum(*sem->read_count) != 0;
> > +}
> > +EXPORT_SYMBOL_GPL(percpu_is_read_locked);
>
> I don't think this is correct; read_count can have spurious increments.
>
> If we look at __percpu_down_read_trylock(), it does roughly something
> like this:
>
>         this_cpu_inc(*sem->read_count);
>         smp_mb();
>         if (!sem->block)
>                 return true;
>         this_cpu_dec(*sem->read_count);
>         return false;
>
> So percpu_is_read_locked() needs to ensure the read_count is non-zero
> *and* that block is not set.

I shall go and fix. v4 incoming (if more comments before that, please shout).

> That said; I really dislike the whole _is_locked family with a passion.
> Let me try and figure out what you need this for.

As in the other email, it's for the dbg_*() functions for kgdb's
benefit (avoiding deadlock if kgdb wants a breakpoint, while we're in
the process of handing out a breakpoint elsewhere and have the locks
taken).

Thanks,
-- Marco

[PATCH v3 11/14] perf/hw_breakpoint: Reduce contention with large number of tasks

[Marco Elver]

While optimizing task_bp_pinned()'s runtime complexity to O(1) on
average helps reduce time spent in the critical section, we still suffer
due to serializing everything via 'nr_bp_mutex'. Indeed, a profile shows
that now contention is the biggest issue:

    95.93%  [kernel]       [k] osq_lock
     0.70%  [kernel]       [k] mutex_spin_on_owner
     0.22%  [kernel]       [k] smp_cfm_core_cond
     0.18%  [kernel]       [k] task_bp_pinned
     0.18%  [kernel]       [k] rhashtable_jhash2
     0.15%  [kernel]       [k] queued_spin_lock_slowpath

when running the breakpoint benchmark with (system with 256 CPUs):

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 0.207 [sec]
 |
 |      108.267188 usecs/op
 |     6929.100000 usecs/op/cpu

The main concern for synchronizing the breakpoint constraints data is
that a consistent snapshot of the per-CPU and per-task data is observed.

The access pattern is as follows:

 1. If the target is a task: the task's pinned breakpoints are counted,
    checked for space, and then appended to; only bp_cpuinfo::cpu_pinned
    is used to check for conflicts with CPU-only breakpoints;
    bp_cpuinfo::tsk_pinned are incremented/decremented, but otherwise
    unused.

 2. If the target is a CPU: bp_cpuinfo::cpu_pinned are counted, along
    with bp_cpuinfo::tsk_pinned; after a successful check, cpu_pinned is
    incremented. No per-task breakpoints are checked.

Since rhltable safely synchronizes insertions/deletions, we can allow
concurrency as follows:

 1. If the target is a task: independent tasks may update and check the
    constraints concurrently, but same-task target calls need to be
    serialized; since bp_cpuinfo::tsk_pinned is only updated, but not
    checked, these modifications can happen concurrently by switching
    tsk_pinned to atomic_t.

 2. If the target is a CPU: access to the per-CPU constraints needs to
    be serialized with other CPU-target and task-target callers (to
    stabilize the bp_cpuinfo::tsk_pinned snapshot).

We can allow the above concurrency by introducing a per-CPU constraints
data reader-writer lock (bp_cpuinfo_sem), and per-task mutexes (reuses
task_struct::perf_event_mutex):

  1. If the target is a task: acquires perf_event_mutex, and acquires
     bp_cpuinfo_sem as a reader. The choice of percpu-rwsem minimizes
     contention in the presence of many read-lock but few write-lock
     acquisitions: we assume many orders of magnitude more task target
     breakpoints creations/destructions than CPU target breakpoints.

  2. If the target is a CPU: acquires bp_cpuinfo_sem as a writer.

With these changes, contention with thousands of tasks is reduced to the
point where waiting on locking no longer dominates the profile:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 0.077 [sec]
 |
 |       40.201563 usecs/op
 |     2572.900000 usecs/op/cpu

    21.54%  [kernel]       [k] task_bp_pinned
    20.18%  [kernel]       [k] rhashtable_jhash2
     6.81%  [kernel]       [k] toggle_bp_slot
     5.47%  [kernel]       [k] queued_spin_lock_slowpath
     3.75%  [kernel]       [k] smp_cfm_core_cond
     3.48%  [kernel]       [k] bcmp

On this particular setup that's a speedup of 2.7x.

We're also getting closer to the theoretical ideal performance through
optimizations in hw_breakpoint.c -- constraints accounting disabled:

 | perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 0.067 [sec]
 |
 |       35.286458 usecs/op
 |     2258.333333 usecs/op/cpu

Which means the current implementation is ~12% slower than the
theoretical ideal.

For reference, performance without any breakpoints:

 | $> bench -r 30 breakpoint thread -b 0 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 0 breakpoints and 64 parallelism
 |      Total time: 0.060 [sec]
 |
 |       31.365625 usecs/op
 |     2007.400000 usecs/op/cpu

On a system with 256 CPUs, the theoretical ideal is only ~12% slower
than no breakpoints at all; the current implementation is ~28% slower.

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
v2:
* Use percpu-rwsem instead of rwlock.
* Use task_struct::perf_event_mutex. See code comment for reasoning.
==> Speedup of 2.7x (vs 2.5x in v1).
---
 kernel/events/hw_breakpoint.c | 161 ++++++++++++++++++++++++++++------
 1 file changed, 133 insertions(+), 28 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 8b40fca1a063..229c6f4fae75 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -19,6 +19,7 @@
 
 #include <linux/hw_breakpoint.h>
 
+#include <linux/atomic.h>
 #include <linux/bug.h>
 #include <linux/cpu.h>
 #include <linux/export.h>
@@ -28,6 +29,7 @@
 #include <linux/kernel.h>
 #include <linux/mutex.h>
 #include <linux/notifier.h>
+#include <linux/percpu-rwsem.h>
 #include <linux/percpu.h>
 #include <linux/rhashtable.h>
 #include <linux/sched.h>
@@ -41,9 +43,9 @@ struct bp_cpuinfo {
 	unsigned int	cpu_pinned;
 	/* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
 #ifdef hw_breakpoint_slots
-	unsigned int	tsk_pinned[hw_breakpoint_slots(0)];
+	atomic_t	tsk_pinned[hw_breakpoint_slots(0)];
 #else
-	unsigned int	*tsk_pinned;
+	atomic_t	*tsk_pinned;
 #endif
 };
 
@@ -65,8 +67,79 @@ static const struct rhashtable_params task_bps_ht_params = {
 
 static bool constraints_initialized __ro_after_init;
 
-/* Serialize accesses to the above constraints */
-static DEFINE_MUTEX(nr_bp_mutex);
+/*
+ * Synchronizes accesses to the per-CPU constraints; the locking rules are:
+ *
+ *  1. Atomic updates to bp_cpuinfo::tsk_pinned only require a held read-lock
+ *     (due to bp_slots_histogram::count being atomic, no update are lost).
+ *
+ *  2. Holding a write-lock is required for computations that require a
+ *     stable snapshot of all bp_cpuinfo::tsk_pinned.
+ *
+ *  3. In all other cases, non-atomic accesses require the appropriately held
+ *     lock (read-lock for read-only accesses; write-lock for reads/writes).
+ */
+DEFINE_STATIC_PERCPU_RWSEM(bp_cpuinfo_sem);
+
+/*
+ * Return mutex to serialize accesses to per-task lists in task_bps_ht. Since
+ * rhltable synchronizes concurrent insertions/deletions, independent tasks may
+ * insert/delete concurrently; therefore, a mutex per task is sufficient.
+ *
+ * Uses task_struct::perf_event_mutex, to avoid extending task_struct with a
+ * hw_breakpoint-only mutex, which may be infrequently used. The caveat here is
+ * that hw_breakpoint may contend with per-task perf event list management. The
+ * assumption is that perf usecases involving hw_breakpoints are very unlikely
+ * to result in unnecessary contention.
+ */
+static inline struct mutex *get_task_bps_mutex(struct perf_event *bp)
+{
+	struct task_struct *tsk = bp->hw.target;
+
+	return tsk ? &tsk->perf_event_mutex : NULL;
+}
+
+static struct mutex *bp_constraints_lock(struct perf_event *bp)
+{
+	struct mutex *tsk_mtx = get_task_bps_mutex(bp);
+
+	if (tsk_mtx) {
+		mutex_lock(tsk_mtx);
+		percpu_down_read(&bp_cpuinfo_sem);
+	} else {
+		percpu_down_write(&bp_cpuinfo_sem);
+	}
+
+	return tsk_mtx;
+}
+
+static void bp_constraints_unlock(struct mutex *tsk_mtx)
+{
+	if (tsk_mtx) {
+		percpu_up_read(&bp_cpuinfo_sem);
+		mutex_unlock(tsk_mtx);
+	} else {
+		percpu_up_write(&bp_cpuinfo_sem);
+	}
+}
+
+static bool bp_constraints_is_locked(struct perf_event *bp)
+{
+	struct mutex *tsk_mtx = get_task_bps_mutex(bp);
+
+	return percpu_is_write_locked(&bp_cpuinfo_sem) ||
+	       (tsk_mtx ? mutex_is_locked(tsk_mtx) :
+			  percpu_is_read_locked(&bp_cpuinfo_sem));
+}
+
+static inline void assert_bp_constraints_lock_held(struct perf_event *bp)
+{
+	struct mutex *tsk_mtx = get_task_bps_mutex(bp);
+
+	if (tsk_mtx)
+		lockdep_assert_held(tsk_mtx);
+	lockdep_assert_held(&bp_cpuinfo_sem);
+}
 
 #ifdef hw_breakpoint_slots
 /*
@@ -97,7 +170,7 @@ static __init int init_breakpoint_slots(void)
 		for (i = 0; i < TYPE_MAX; i++) {
 			struct bp_cpuinfo *info = get_bp_info(cpu, i);
 
-			info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(int), GFP_KERNEL);
+			info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(atomic_t), GFP_KERNEL);
 			if (!info->tsk_pinned)
 				goto err;
 		}
@@ -137,11 +210,19 @@ static inline enum bp_type_idx find_slot_idx(u64 bp_type)
  */
 static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
 {
-	unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
+	atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
 	int i;
 
+	/*
+	 * At this point we want to have acquired the bp_cpuinfo_sem as a
+	 * writer to ensure that there are no concurrent writers in
+	 * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
+	 */
+	lockdep_assert_held_write(&bp_cpuinfo_sem);
+
 	for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
-		if (tsk_pinned[i] > 0)
+		ASSERT_EXCLUSIVE_WRITER(tsk_pinned[i]); /* Catch unexpected writers. */
+		if (atomic_read(&tsk_pinned[i]) > 0)
 			return i + 1;
 	}
 
@@ -158,6 +239,11 @@ static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
 	struct perf_event *iter;
 	int count = 0;
 
+	/*
+	 * We need a stable snapshot of the per-task breakpoint list.
+	 */
+	assert_bp_constraints_lock_held(bp);
+
 	rcu_read_lock();
 	head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params);
 	if (!head)
@@ -214,16 +300,25 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
 static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
 				enum bp_type_idx type, int weight)
 {
-	unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
+	atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
 	int old_idx, new_idx;
 
+	/*
+	 * If bp->hw.target, tsk_pinned is only modified, but not used
+	 * otherwise. We can permit concurrent updates as long as there are no
+	 * other uses: having acquired bp_cpuinfo_sem as a reader allows
+	 * concurrent updates here. Uses of tsk_pinned will require acquiring
+	 * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
+	 */
+	lockdep_assert_held_read(&bp_cpuinfo_sem);
+
 	old_idx = task_bp_pinned(cpu, bp, type) - 1;
 	new_idx = old_idx + weight;
 
 	if (old_idx >= 0)
-		tsk_pinned[old_idx]--;
+		atomic_dec(&tsk_pinned[old_idx]);
 	if (new_idx >= 0)
-		tsk_pinned[new_idx]++;
+		atomic_inc(&tsk_pinned[new_idx]);
 }
 
 /*
@@ -241,6 +336,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
 
 	/* Pinned counter cpu profiling */
 	if (!bp->hw.target) {
+		lockdep_assert_held_write(&bp_cpuinfo_sem);
 		get_bp_info(bp->cpu, type)->cpu_pinned += weight;
 		return 0;
 	}
@@ -249,6 +345,11 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
 	for_each_cpu(cpu, cpumask)
 		toggle_bp_task_slot(bp, cpu, type, weight);
 
+	/*
+	 * Readers want a stable snapshot of the per-task breakpoint list.
+	 */
+	assert_bp_constraints_lock_held(bp);
+
 	if (enable)
 		return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
 	else
@@ -354,14 +455,10 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
 
 int reserve_bp_slot(struct perf_event *bp)
 {
-	int ret;
-
-	mutex_lock(&nr_bp_mutex);
-
-	ret = __reserve_bp_slot(bp, bp->attr.bp_type);
-
-	mutex_unlock(&nr_bp_mutex);
+	struct mutex *mtx = bp_constraints_lock(bp);
+	int ret = __reserve_bp_slot(bp, bp->attr.bp_type);
 
+	bp_constraints_unlock(mtx);
 	return ret;
 }
 
@@ -379,12 +476,11 @@ static void __release_bp_slot(struct perf_event *bp, u64 bp_type)
 
 void release_bp_slot(struct perf_event *bp)
 {
-	mutex_lock(&nr_bp_mutex);
+	struct mutex *mtx = bp_constraints_lock(bp);
 
 	arch_unregister_hw_breakpoint(bp);
 	__release_bp_slot(bp, bp->attr.bp_type);
-
-	mutex_unlock(&nr_bp_mutex);
+	bp_constraints_unlock(mtx);
 }
 
 static int __modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
@@ -411,11 +507,10 @@ static int __modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
 
 static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
 {
-	int ret;
+	struct mutex *mtx = bp_constraints_lock(bp);
+	int ret = __modify_bp_slot(bp, old_type, new_type);
 
-	mutex_lock(&nr_bp_mutex);
-	ret = __modify_bp_slot(bp, old_type, new_type);
-	mutex_unlock(&nr_bp_mutex);
+	bp_constraints_unlock(mtx);
 	return ret;
 }
 
@@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
  */
 int dbg_reserve_bp_slot(struct perf_event *bp)
 {
-	if (mutex_is_locked(&nr_bp_mutex))
+	int ret;
+
+	if (bp_constraints_is_locked(bp))
 		return -1;
 
-	return __reserve_bp_slot(bp, bp->attr.bp_type);
+	/* Locks aren't held; disable lockdep assert checking. */
+	lockdep_off();
+	ret = __reserve_bp_slot(bp, bp->attr.bp_type);
+	lockdep_on();
+
+	return ret;
 }
 
 int dbg_release_bp_slot(struct perf_event *bp)
 {
-	if (mutex_is_locked(&nr_bp_mutex))
+	if (bp_constraints_is_locked(bp))
 		return -1;
 
+	/* Locks aren't held; disable lockdep assert checking. */
+	lockdep_off();
 	__release_bp_slot(bp, bp->attr.bp_type);
+	lockdep_on();
 
 	return 0;
 }
@@ -663,7 +768,7 @@ bool hw_breakpoint_is_used(void)
 				return true;
 
 			for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) {
-				if (info->tsk_pinned[slot])
+				if (atomic_read(&info->tsk_pinned[slot]))
 					return true;
 			}
 		}
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 11/14] perf/hw_breakpoint: Reduce contention with large number of tasks

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:07 AM Marco Elver <elver@google.com> wrote:
>
> While optimizing task_bp_pinned()'s runtime complexity to O(1) on
> average helps reduce time spent in the critical section, we still suffer
> due to serializing everything via 'nr_bp_mutex'. Indeed, a profile shows
> that now contention is the biggest issue:
>
>     95.93%  [kernel]       [k] osq_lock
>      0.70%  [kernel]       [k] mutex_spin_on_owner
>      0.22%  [kernel]       [k] smp_cfm_core_cond
>      0.18%  [kernel]       [k] task_bp_pinned
>      0.18%  [kernel]       [k] rhashtable_jhash2
>      0.15%  [kernel]       [k] queued_spin_lock_slowpath
>
> when running the breakpoint benchmark with (system with 256 CPUs):
>
>  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
>  |      Total time: 0.207 [sec]
>  |
>  |      108.267188 usecs/op
>  |     6929.100000 usecs/op/cpu
>
> The main concern for synchronizing the breakpoint constraints data is
> that a consistent snapshot of the per-CPU and per-task data is observed.
>
> The access pattern is as follows:
>
>  1. If the target is a task: the task's pinned breakpoints are counted,
>     checked for space, and then appended to; only bp_cpuinfo::cpu_pinned
>     is used to check for conflicts with CPU-only breakpoints;
>     bp_cpuinfo::tsk_pinned are incremented/decremented, but otherwise
>     unused.
>
>  2. If the target is a CPU: bp_cpuinfo::cpu_pinned are counted, along
>     with bp_cpuinfo::tsk_pinned; after a successful check, cpu_pinned is
>     incremented. No per-task breakpoints are checked.
>
> Since rhltable safely synchronizes insertions/deletions, we can allow
> concurrency as follows:
>
>  1. If the target is a task: independent tasks may update and check the
>     constraints concurrently, but same-task target calls need to be
>     serialized; since bp_cpuinfo::tsk_pinned is only updated, but not
>     checked, these modifications can happen concurrently by switching
>     tsk_pinned to atomic_t.
>
>  2. If the target is a CPU: access to the per-CPU constraints needs to
>     be serialized with other CPU-target and task-target callers (to
>     stabilize the bp_cpuinfo::tsk_pinned snapshot).
>
> We can allow the above concurrency by introducing a per-CPU constraints
> data reader-writer lock (bp_cpuinfo_sem), and per-task mutexes (reuses
> task_struct::perf_event_mutex):
>
>   1. If the target is a task: acquires perf_event_mutex, and acquires
>      bp_cpuinfo_sem as a reader. The choice of percpu-rwsem minimizes
>      contention in the presence of many read-lock but few write-lock
>      acquisitions: we assume many orders of magnitude more task target
>      breakpoints creations/destructions than CPU target breakpoints.
>
>   2. If the target is a CPU: acquires bp_cpuinfo_sem as a writer.
>
> With these changes, contention with thousands of tasks is reduced to the
> point where waiting on locking no longer dominates the profile:
>
>  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
>  |      Total time: 0.077 [sec]
>  |
>  |       40.201563 usecs/op
>  |     2572.900000 usecs/op/cpu
>
>     21.54%  [kernel]       [k] task_bp_pinned
>     20.18%  [kernel]       [k] rhashtable_jhash2
>      6.81%  [kernel]       [k] toggle_bp_slot
>      5.47%  [kernel]       [k] queued_spin_lock_slowpath
>      3.75%  [kernel]       [k] smp_cfm_core_cond
>      3.48%  [kernel]       [k] bcmp
>
> On this particular setup that's a speedup of 2.7x.
>
> We're also getting closer to the theoretical ideal performance through
> optimizations in hw_breakpoint.c -- constraints accounting disabled:
>
>  | perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
>  |      Total time: 0.067 [sec]
>  |
>  |       35.286458 usecs/op
>  |     2258.333333 usecs/op/cpu
>
> Which means the current implementation is ~12% slower than the
> theoretical ideal.
>
> For reference, performance without any breakpoints:
>
>  | $> bench -r 30 breakpoint thread -b 0 -p 64 -t 64
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 30 threads with 0 breakpoints and 64 parallelism
>  |      Total time: 0.060 [sec]
>  |
>  |       31.365625 usecs/op
>  |     2007.400000 usecs/op/cpu
>
> On a system with 256 CPUs, the theoretical ideal is only ~12% slower
> than no breakpoints at all; the current implementation is ~28% slower.
>
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
> v2:
> * Use percpu-rwsem instead of rwlock.
> * Use task_struct::perf_event_mutex. See code comment for reasoning.
> ==> Speedup of 2.7x (vs 2.5x in v1).
> ---
>  kernel/events/hw_breakpoint.c | 161 ++++++++++++++++++++++++++++------
>  1 file changed, 133 insertions(+), 28 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 8b40fca1a063..229c6f4fae75 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -19,6 +19,7 @@
>
>  #include <linux/hw_breakpoint.h>
>
> +#include <linux/atomic.h>
>  #include <linux/bug.h>
>  #include <linux/cpu.h>
>  #include <linux/export.h>
> @@ -28,6 +29,7 @@
>  #include <linux/kernel.h>
>  #include <linux/mutex.h>
>  #include <linux/notifier.h>
> +#include <linux/percpu-rwsem.h>
>  #include <linux/percpu.h>
>  #include <linux/rhashtable.h>
>  #include <linux/sched.h>
> @@ -41,9 +43,9 @@ struct bp_cpuinfo {
>         unsigned int    cpu_pinned;
>         /* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
>  #ifdef hw_breakpoint_slots
> -       unsigned int    tsk_pinned[hw_breakpoint_slots(0)];
> +       atomic_t        tsk_pinned[hw_breakpoint_slots(0)];
>  #else
> -       unsigned int    *tsk_pinned;
> +       atomic_t        *tsk_pinned;
>  #endif
>  };
>
> @@ -65,8 +67,79 @@ static const struct rhashtable_params task_bps_ht_params = {
>
>  static bool constraints_initialized __ro_after_init;
>
> -/* Serialize accesses to the above constraints */
> -static DEFINE_MUTEX(nr_bp_mutex);
> +/*
> + * Synchronizes accesses to the per-CPU constraints; the locking rules are:
> + *
> + *  1. Atomic updates to bp_cpuinfo::tsk_pinned only require a held read-lock
> + *     (due to bp_slots_histogram::count being atomic, no update are lost).
> + *
> + *  2. Holding a write-lock is required for computations that require a
> + *     stable snapshot of all bp_cpuinfo::tsk_pinned.
> + *
> + *  3. In all other cases, non-atomic accesses require the appropriately held
> + *     lock (read-lock for read-only accesses; write-lock for reads/writes).
> + */
> +DEFINE_STATIC_PERCPU_RWSEM(bp_cpuinfo_sem);
> +
> +/*
> + * Return mutex to serialize accesses to per-task lists in task_bps_ht. Since
> + * rhltable synchronizes concurrent insertions/deletions, independent tasks may
> + * insert/delete concurrently; therefore, a mutex per task is sufficient.
> + *
> + * Uses task_struct::perf_event_mutex, to avoid extending task_struct with a
> + * hw_breakpoint-only mutex, which may be infrequently used. The caveat here is
> + * that hw_breakpoint may contend with per-task perf event list management. The
> + * assumption is that perf usecases involving hw_breakpoints are very unlikely
> + * to result in unnecessary contention.
> + */
> +static inline struct mutex *get_task_bps_mutex(struct perf_event *bp)
> +{
> +       struct task_struct *tsk = bp->hw.target;
> +
> +       return tsk ? &tsk->perf_event_mutex : NULL;
> +}
> +
> +static struct mutex *bp_constraints_lock(struct perf_event *bp)
> +{
> +       struct mutex *tsk_mtx = get_task_bps_mutex(bp);
> +
> +       if (tsk_mtx) {
> +               mutex_lock(tsk_mtx);
> +               percpu_down_read(&bp_cpuinfo_sem);
> +       } else {
> +               percpu_down_write(&bp_cpuinfo_sem);
> +       }
> +
> +       return tsk_mtx;
> +}
> +
> +static void bp_constraints_unlock(struct mutex *tsk_mtx)
> +{
> +       if (tsk_mtx) {
> +               percpu_up_read(&bp_cpuinfo_sem);
> +               mutex_unlock(tsk_mtx);
> +       } else {
> +               percpu_up_write(&bp_cpuinfo_sem);
> +       }
> +}
> +
> +static bool bp_constraints_is_locked(struct perf_event *bp)
> +{
> +       struct mutex *tsk_mtx = get_task_bps_mutex(bp);
> +
> +       return percpu_is_write_locked(&bp_cpuinfo_sem) ||
> +              (tsk_mtx ? mutex_is_locked(tsk_mtx) :
> +                         percpu_is_read_locked(&bp_cpuinfo_sem));
> +}
> +
> +static inline void assert_bp_constraints_lock_held(struct perf_event *bp)
> +{
> +       struct mutex *tsk_mtx = get_task_bps_mutex(bp);
> +
> +       if (tsk_mtx)
> +               lockdep_assert_held(tsk_mtx);
> +       lockdep_assert_held(&bp_cpuinfo_sem);
> +}
>
>  #ifdef hw_breakpoint_slots
>  /*
> @@ -97,7 +170,7 @@ static __init int init_breakpoint_slots(void)
>                 for (i = 0; i < TYPE_MAX; i++) {
>                         struct bp_cpuinfo *info = get_bp_info(cpu, i);
>
> -                       info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(int), GFP_KERNEL);
> +                       info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(atomic_t), GFP_KERNEL);
>                         if (!info->tsk_pinned)
>                                 goto err;
>                 }
> @@ -137,11 +210,19 @@ static inline enum bp_type_idx find_slot_idx(u64 bp_type)
>   */
>  static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>  {
> -       unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
> +       atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
>         int i;
>
> +       /*
> +        * At this point we want to have acquired the bp_cpuinfo_sem as a
> +        * writer to ensure that there are no concurrent writers in
> +        * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
> +        */
> +       lockdep_assert_held_write(&bp_cpuinfo_sem);
> +
>         for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
> -               if (tsk_pinned[i] > 0)
> +               ASSERT_EXCLUSIVE_WRITER(tsk_pinned[i]); /* Catch unexpected writers. */
> +               if (atomic_read(&tsk_pinned[i]) > 0)
>                         return i + 1;
>         }
>
> @@ -158,6 +239,11 @@ static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
>         struct perf_event *iter;
>         int count = 0;
>
> +       /*
> +        * We need a stable snapshot of the per-task breakpoint list.
> +        */
> +       assert_bp_constraints_lock_held(bp);
> +
>         rcu_read_lock();
>         head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params);
>         if (!head)
> @@ -214,16 +300,25 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
>  static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
>                                 enum bp_type_idx type, int weight)
>  {
> -       unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
> +       atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
>         int old_idx, new_idx;
>
> +       /*
> +        * If bp->hw.target, tsk_pinned is only modified, but not used
> +        * otherwise. We can permit concurrent updates as long as there are no
> +        * other uses: having acquired bp_cpuinfo_sem as a reader allows
> +        * concurrent updates here. Uses of tsk_pinned will require acquiring
> +        * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
> +        */
> +       lockdep_assert_held_read(&bp_cpuinfo_sem);
> +
>         old_idx = task_bp_pinned(cpu, bp, type) - 1;
>         new_idx = old_idx + weight;
>
>         if (old_idx >= 0)
> -               tsk_pinned[old_idx]--;
> +               atomic_dec(&tsk_pinned[old_idx]);
>         if (new_idx >= 0)
> -               tsk_pinned[new_idx]++;
> +               atomic_inc(&tsk_pinned[new_idx]);
>  }
>
>  /*
> @@ -241,6 +336,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>
>         /* Pinned counter cpu profiling */
>         if (!bp->hw.target) {
> +               lockdep_assert_held_write(&bp_cpuinfo_sem);
>                 get_bp_info(bp->cpu, type)->cpu_pinned += weight;
>                 return 0;
>         }
> @@ -249,6 +345,11 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>         for_each_cpu(cpu, cpumask)
>                 toggle_bp_task_slot(bp, cpu, type, weight);
>
> +       /*
> +        * Readers want a stable snapshot of the per-task breakpoint list.
> +        */
> +       assert_bp_constraints_lock_held(bp);
> +
>         if (enable)
>                 return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
>         else
> @@ -354,14 +455,10 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
>
>  int reserve_bp_slot(struct perf_event *bp)
>  {
> -       int ret;
> -
> -       mutex_lock(&nr_bp_mutex);
> -
> -       ret = __reserve_bp_slot(bp, bp->attr.bp_type);
> -
> -       mutex_unlock(&nr_bp_mutex);
> +       struct mutex *mtx = bp_constraints_lock(bp);
> +       int ret = __reserve_bp_slot(bp, bp->attr.bp_type);
>
> +       bp_constraints_unlock(mtx);
>         return ret;
>  }
>
> @@ -379,12 +476,11 @@ static void __release_bp_slot(struct perf_event *bp, u64 bp_type)
>
>  void release_bp_slot(struct perf_event *bp)
>  {
> -       mutex_lock(&nr_bp_mutex);
> +       struct mutex *mtx = bp_constraints_lock(bp);
>
>         arch_unregister_hw_breakpoint(bp);
>         __release_bp_slot(bp, bp->attr.bp_type);
> -
> -       mutex_unlock(&nr_bp_mutex);
> +       bp_constraints_unlock(mtx);
>  }
>
>  static int __modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
> @@ -411,11 +507,10 @@ static int __modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
>
>  static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
>  {
> -       int ret;
> +       struct mutex *mtx = bp_constraints_lock(bp);
> +       int ret = __modify_bp_slot(bp, old_type, new_type);
>
> -       mutex_lock(&nr_bp_mutex);
> -       ret = __modify_bp_slot(bp, old_type, new_type);
> -       mutex_unlock(&nr_bp_mutex);
> +       bp_constraints_unlock(mtx);
>         return ret;
>  }
>
> @@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
>   */
>  int dbg_reserve_bp_slot(struct perf_event *bp)
>  {
> -       if (mutex_is_locked(&nr_bp_mutex))
> +       int ret;
> +
> +       if (bp_constraints_is_locked(bp))
>                 return -1;
>
> -       return __reserve_bp_slot(bp, bp->attr.bp_type);
> +       /* Locks aren't held; disable lockdep assert checking. */
> +       lockdep_off();
> +       ret = __reserve_bp_slot(bp, bp->attr.bp_type);
> +       lockdep_on();
> +
> +       return ret;
>  }
>
>  int dbg_release_bp_slot(struct perf_event *bp)
>  {
> -       if (mutex_is_locked(&nr_bp_mutex))
> +       if (bp_constraints_is_locked(bp))
>                 return -1;
>
> +       /* Locks aren't held; disable lockdep assert checking. */
> +       lockdep_off();
>         __release_bp_slot(bp, bp->attr.bp_type);
> +       lockdep_on();
>
>         return 0;
>  }
> @@ -663,7 +768,7 @@ bool hw_breakpoint_is_used(void)
>                                 return true;
>
>                         for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) {
> -                               if (info->tsk_pinned[slot])
> +                               if (atomic_read(&info->tsk_pinned[slot]))
>                                         return true;
>                         }
>                 }
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

Re: [PATCH v3 11/14] perf/hw_breakpoint: Reduce contention with large number of tasks

[Peter Zijlstra]

On Mon, Jul 04, 2022 at 05:05:11PM +0200, Marco Elver wrote:
> +static bool bp_constraints_is_locked(struct perf_event *bp)
> +{
> +	struct mutex *tsk_mtx = get_task_bps_mutex(bp);
> +
> +	return percpu_is_write_locked(&bp_cpuinfo_sem) ||
> +	       (tsk_mtx ? mutex_is_locked(tsk_mtx) :
> +			  percpu_is_read_locked(&bp_cpuinfo_sem));
> +}

> @@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
>   */
>  int dbg_reserve_bp_slot(struct perf_event *bp)
>  {
> -	if (mutex_is_locked(&nr_bp_mutex))
> +	int ret;
> +
> +	if (bp_constraints_is_locked(bp))
>  		return -1;
>  
> -	return __reserve_bp_slot(bp, bp->attr.bp_type);
> +	/* Locks aren't held; disable lockdep assert checking. */
> +	lockdep_off();
> +	ret = __reserve_bp_slot(bp, bp->attr.bp_type);
> +	lockdep_on();
> +
> +	return ret;
>  }
>  
>  int dbg_release_bp_slot(struct perf_event *bp)
>  {
> -	if (mutex_is_locked(&nr_bp_mutex))
> +	if (bp_constraints_is_locked(bp))
>  		return -1;
>  
> +	/* Locks aren't held; disable lockdep assert checking. */
> +	lockdep_off();
>  	__release_bp_slot(bp, bp->attr.bp_type);
> +	lockdep_on();
>  
>  	return 0;
>  }

Urggghhhh... this is horrible crap. That is, the current code is that
and this makes it worse :/

Re: [PATCH v3 11/14] perf/hw_breakpoint: Reduce contention with large number of tasks

[Marco Elver]

On Wed, 17 Aug 2022 at 15:03, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Jul 04, 2022 at 05:05:11PM +0200, Marco Elver wrote:
> > +static bool bp_constraints_is_locked(struct perf_event *bp)
> > +{
> > +     struct mutex *tsk_mtx = get_task_bps_mutex(bp);
> > +
> > +     return percpu_is_write_locked(&bp_cpuinfo_sem) ||
> > +            (tsk_mtx ? mutex_is_locked(tsk_mtx) :
> > +                       percpu_is_read_locked(&bp_cpuinfo_sem));
> > +}
>
> > @@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
> >   */
> >  int dbg_reserve_bp_slot(struct perf_event *bp)
> >  {
> > -     if (mutex_is_locked(&nr_bp_mutex))
> > +     int ret;
> > +
> > +     if (bp_constraints_is_locked(bp))
> >               return -1;
> >
> > -     return __reserve_bp_slot(bp, bp->attr.bp_type);
> > +     /* Locks aren't held; disable lockdep assert checking. */
> > +     lockdep_off();
> > +     ret = __reserve_bp_slot(bp, bp->attr.bp_type);
> > +     lockdep_on();
> > +
> > +     return ret;
> >  }
> >
> >  int dbg_release_bp_slot(struct perf_event *bp)
> >  {
> > -     if (mutex_is_locked(&nr_bp_mutex))
> > +     if (bp_constraints_is_locked(bp))
> >               return -1;
> >
> > +     /* Locks aren't held; disable lockdep assert checking. */
> > +     lockdep_off();
> >       __release_bp_slot(bp, bp->attr.bp_type);
> > +     lockdep_on();
> >
> >       return 0;
> >  }
>
> Urggghhhh... this is horrible crap. That is, the current code is that
> and this makes it worse :/

Heh, yes and when I looked at it I really wanted to see if it can
change. But from what I can tell, when the kernel debugger is being
attached, the kernel does stop everything it does and we need the
horrible thing above to not deadlock. And these dbg_ functions are not
normally used, so I decided to leave it as-is. Suggestions?

Re: [PATCH v3 11/14] perf/hw_breakpoint: Reduce contention with large number of tasks

[Peter Zijlstra]

On Wed, Aug 17, 2022 at 03:14:54PM +0200, Marco Elver wrote:
> On Wed, 17 Aug 2022 at 15:03, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, Jul 04, 2022 at 05:05:11PM +0200, Marco Elver wrote:
> > > +static bool bp_constraints_is_locked(struct perf_event *bp)
> > > +{
> > > +     struct mutex *tsk_mtx = get_task_bps_mutex(bp);
> > > +
> > > +     return percpu_is_write_locked(&bp_cpuinfo_sem) ||
> > > +            (tsk_mtx ? mutex_is_locked(tsk_mtx) :
> > > +                       percpu_is_read_locked(&bp_cpuinfo_sem));
> > > +}
> >
> > > @@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
> > >   */
> > >  int dbg_reserve_bp_slot(struct perf_event *bp)
> > >  {
> > > -     if (mutex_is_locked(&nr_bp_mutex))
> > > +     int ret;
> > > +
> > > +     if (bp_constraints_is_locked(bp))
> > >               return -1;
> > >
> > > -     return __reserve_bp_slot(bp, bp->attr.bp_type);
> > > +     /* Locks aren't held; disable lockdep assert checking. */
> > > +     lockdep_off();
> > > +     ret = __reserve_bp_slot(bp, bp->attr.bp_type);
> > > +     lockdep_on();
> > > +
> > > +     return ret;
> > >  }
> > >
> > >  int dbg_release_bp_slot(struct perf_event *bp)
> > >  {
> > > -     if (mutex_is_locked(&nr_bp_mutex))
> > > +     if (bp_constraints_is_locked(bp))
> > >               return -1;
> > >
> > > +     /* Locks aren't held; disable lockdep assert checking. */
> > > +     lockdep_off();
> > >       __release_bp_slot(bp, bp->attr.bp_type);
> > > +     lockdep_on();
> > >
> > >       return 0;
> > >  }
> >
> > Urggghhhh... this is horrible crap. That is, the current code is that
> > and this makes it worse :/
> 
> Heh, yes and when I looked at it I really wanted to see if it can
> change. But from what I can tell, when the kernel debugger is being
> attached, the kernel does stop everything it does and we need the
> horrible thing above to not deadlock. And these dbg_ functions are not
> normally used, so I decided to leave it as-is. Suggestions?

What context is this ran in? NMI should already have lockdep disabled.

Re: [PATCH v3 11/14] perf/hw_breakpoint: Reduce contention with large number of tasks

[Marco Elver]

On Mon, 29 Aug 2022 at 10:38, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Aug 17, 2022 at 03:14:54PM +0200, Marco Elver wrote:
> > On Wed, 17 Aug 2022 at 15:03, Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Mon, Jul 04, 2022 at 05:05:11PM +0200, Marco Elver wrote:
> > > > +static bool bp_constraints_is_locked(struct perf_event *bp)
> > > > +{
> > > > +     struct mutex *tsk_mtx = get_task_bps_mutex(bp);
> > > > +
> > > > +     return percpu_is_write_locked(&bp_cpuinfo_sem) ||
> > > > +            (tsk_mtx ? mutex_is_locked(tsk_mtx) :
> > > > +                       percpu_is_read_locked(&bp_cpuinfo_sem));
> > > > +}
> > >
> > > > @@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
> > > >   */
> > > >  int dbg_reserve_bp_slot(struct perf_event *bp)
> > > >  {
> > > > -     if (mutex_is_locked(&nr_bp_mutex))
> > > > +     int ret;
> > > > +
> > > > +     if (bp_constraints_is_locked(bp))
> > > >               return -1;
> > > >
> > > > -     return __reserve_bp_slot(bp, bp->attr.bp_type);
> > > > +     /* Locks aren't held; disable lockdep assert checking. */
> > > > +     lockdep_off();
> > > > +     ret = __reserve_bp_slot(bp, bp->attr.bp_type);
> > > > +     lockdep_on();
> > > > +
> > > > +     return ret;
> > > >  }
> > > >
> > > >  int dbg_release_bp_slot(struct perf_event *bp)
> > > >  {
> > > > -     if (mutex_is_locked(&nr_bp_mutex))
> > > > +     if (bp_constraints_is_locked(bp))
> > > >               return -1;
> > > >
> > > > +     /* Locks aren't held; disable lockdep assert checking. */
> > > > +     lockdep_off();
> > > >       __release_bp_slot(bp, bp->attr.bp_type);
> > > > +     lockdep_on();
> > > >
> > > >       return 0;
> > > >  }
> > >
> > > Urggghhhh... this is horrible crap. That is, the current code is that
> > > and this makes it worse :/
> >
> > Heh, yes and when I looked at it I really wanted to see if it can
> > change. But from what I can tell, when the kernel debugger is being
> > attached, the kernel does stop everything it does and we need the
> > horrible thing above to not deadlock. And these dbg_ functions are not
> > normally used, so I decided to leave it as-is. Suggestions?
>
> What context is this ran in? NMI should already have lockdep disabled.

kgdb can enter via kgdb_nmicall*() but also via
kgdb_handle_exception(), which isn't for NMI.

[PATCH v3 12/14] perf/hw_breakpoint: Introduce bp_slots_histogram

[Marco Elver]

Factor out the existing `atomic_t count[N]` into its own struct called
'bp_slots_histogram', to generalize and make its intent clearer in
preparation of reusing elsewhere. The basic idea of bucketing "total
uses of N slots" resembles a histogram, so calling it such seems most
intuitive.

No functional change.

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
v3:
* Also warn in bp_slots_histogram_add() if count goes below 0.

v2:
* New patch.
---
 kernel/events/hw_breakpoint.c | 96 +++++++++++++++++++++++------------
 1 file changed, 63 insertions(+), 33 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 229c6f4fae75..03ebecf048c0 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -36,19 +36,27 @@
 #include <linux/slab.h>
 
 /*
- * Constraints data
+ * Datastructure to track the total uses of N slots across tasks or CPUs;
+ * bp_slots_histogram::count[N] is the number of assigned N+1 breakpoint slots.
  */
-struct bp_cpuinfo {
-	/* Number of pinned cpu breakpoints in a cpu */
-	unsigned int	cpu_pinned;
-	/* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
+struct bp_slots_histogram {
 #ifdef hw_breakpoint_slots
-	atomic_t	tsk_pinned[hw_breakpoint_slots(0)];
+	atomic_t count[hw_breakpoint_slots(0)];
 #else
-	atomic_t	*tsk_pinned;
+	atomic_t *count;
 #endif
 };
 
+/*
+ * Per-CPU constraints data.
+ */
+struct bp_cpuinfo {
+	/* Number of pinned CPU breakpoints in a CPU. */
+	unsigned int			cpu_pinned;
+	/* Histogram of pinned task breakpoints in a CPU. */
+	struct bp_slots_histogram	tsk_pinned;
+};
+
 static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
 
 static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
@@ -159,6 +167,18 @@ static inline int hw_breakpoint_slots_cached(int type)
 	return __nr_bp_slots[type];
 }
 
+static __init bool
+bp_slots_histogram_alloc(struct bp_slots_histogram *hist, enum bp_type_idx type)
+{
+	hist->count = kcalloc(hw_breakpoint_slots_cached(type), sizeof(*hist->count), GFP_KERNEL);
+	return hist->count;
+}
+
+static __init void bp_slots_histogram_free(struct bp_slots_histogram *hist)
+{
+	kfree(hist->count);
+}
+
 static __init int init_breakpoint_slots(void)
 {
 	int i, cpu, err_cpu;
@@ -170,8 +190,7 @@ static __init int init_breakpoint_slots(void)
 		for (i = 0; i < TYPE_MAX; i++) {
 			struct bp_cpuinfo *info = get_bp_info(cpu, i);
 
-			info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(atomic_t), GFP_KERNEL);
-			if (!info->tsk_pinned)
+			if (!bp_slots_histogram_alloc(&info->tsk_pinned, i))
 				goto err;
 		}
 	}
@@ -180,7 +199,7 @@ static __init int init_breakpoint_slots(void)
 err:
 	for_each_possible_cpu(err_cpu) {
 		for (i = 0; i < TYPE_MAX; i++)
-			kfree(get_bp_info(err_cpu, i)->tsk_pinned);
+			bp_slots_histogram_free(&get_bp_info(err_cpu, i)->tsk_pinned);
 		if (err_cpu == cpu)
 			break;
 	}
@@ -189,6 +208,34 @@ static __init int init_breakpoint_slots(void)
 }
 #endif
 
+static inline void
+bp_slots_histogram_add(struct bp_slots_histogram *hist, int old, int val)
+{
+	const int old_idx = old - 1;
+	const int new_idx = old_idx + val;
+
+	if (old_idx >= 0)
+		WARN_ON(atomic_dec_return_relaxed(&hist->count[old_idx]) < 0);
+	if (new_idx >= 0)
+		WARN_ON(atomic_inc_return_relaxed(&hist->count[new_idx]) < 0);
+}
+
+static int
+bp_slots_histogram_max(struct bp_slots_histogram *hist, enum bp_type_idx type)
+{
+	for (int i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
+		const int count = atomic_read(&hist->count[i]);
+
+		/* Catch unexpected writers; we want a stable snapshot. */
+		ASSERT_EXCLUSIVE_WRITER(hist->count[i]);
+		if (count > 0)
+			return i + 1;
+		WARN(count < 0, "inconsistent breakpoint slots histogram");
+	}
+
+	return 0;
+}
+
 #ifndef hw_breakpoint_weight
 static inline int hw_breakpoint_weight(struct perf_event *bp)
 {
@@ -205,13 +252,11 @@ static inline enum bp_type_idx find_slot_idx(u64 bp_type)
 }
 
 /*
- * Report the maximum number of pinned breakpoints a task
- * have in this cpu
+ * Return the maximum number of pinned breakpoints a task has in this CPU.
  */
 static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
 {
-	atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
-	int i;
+	struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
 
 	/*
 	 * At this point we want to have acquired the bp_cpuinfo_sem as a
@@ -219,14 +264,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
 	 * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
 	 */
 	lockdep_assert_held_write(&bp_cpuinfo_sem);
-
-	for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
-		ASSERT_EXCLUSIVE_WRITER(tsk_pinned[i]); /* Catch unexpected writers. */
-		if (atomic_read(&tsk_pinned[i]) > 0)
-			return i + 1;
-	}
-
-	return 0;
+	return bp_slots_histogram_max(tsk_pinned, type);
 }
 
 /*
@@ -300,8 +338,7 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
 static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
 				enum bp_type_idx type, int weight)
 {
-	atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
-	int old_idx, new_idx;
+	struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
 
 	/*
 	 * If bp->hw.target, tsk_pinned is only modified, but not used
@@ -311,14 +348,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
 	 * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
 	 */
 	lockdep_assert_held_read(&bp_cpuinfo_sem);
-
-	old_idx = task_bp_pinned(cpu, bp, type) - 1;
-	new_idx = old_idx + weight;
-
-	if (old_idx >= 0)
-		atomic_dec(&tsk_pinned[old_idx]);
-	if (new_idx >= 0)
-		atomic_inc(&tsk_pinned[new_idx]);
+	bp_slots_histogram_add(tsk_pinned, task_bp_pinned(cpu, bp, type), weight);
 }
 
 /*
@@ -768,7 +798,7 @@ bool hw_breakpoint_is_used(void)
 				return true;
 
 			for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) {
-				if (atomic_read(&info->tsk_pinned[slot]))
+				if (atomic_read(&info->tsk_pinned.count[slot]))
 					return true;
 			}
 		}
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 12/14] perf/hw_breakpoint: Introduce bp_slots_histogram

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:07 AM Marco Elver <elver@google.com> wrote:
>
> Factor out the existing `atomic_t count[N]` into its own struct called
> 'bp_slots_histogram', to generalize and make its intent clearer in
> preparation of reusing elsewhere. The basic idea of bucketing "total
> uses of N slots" resembles a histogram, so calling it such seems most
> intuitive.
>
> No functional change.
>
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
> v3:
> * Also warn in bp_slots_histogram_add() if count goes below 0.
>
> v2:
> * New patch.
> ---
>  kernel/events/hw_breakpoint.c | 96 +++++++++++++++++++++++------------
>  1 file changed, 63 insertions(+), 33 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 229c6f4fae75..03ebecf048c0 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -36,19 +36,27 @@
>  #include <linux/slab.h>
>
>  /*
> - * Constraints data
> + * Datastructure to track the total uses of N slots across tasks or CPUs;
> + * bp_slots_histogram::count[N] is the number of assigned N+1 breakpoint slots.
>   */
> -struct bp_cpuinfo {
> -       /* Number of pinned cpu breakpoints in a cpu */
> -       unsigned int    cpu_pinned;
> -       /* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
> +struct bp_slots_histogram {
>  #ifdef hw_breakpoint_slots
> -       atomic_t        tsk_pinned[hw_breakpoint_slots(0)];
> +       atomic_t count[hw_breakpoint_slots(0)];
>  #else
> -       atomic_t        *tsk_pinned;
> +       atomic_t *count;
>  #endif
>  };
>
> +/*
> + * Per-CPU constraints data.
> + */
> +struct bp_cpuinfo {
> +       /* Number of pinned CPU breakpoints in a CPU. */
> +       unsigned int                    cpu_pinned;
> +       /* Histogram of pinned task breakpoints in a CPU. */
> +       struct bp_slots_histogram       tsk_pinned;
> +};
> +
>  static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
>
>  static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
> @@ -159,6 +167,18 @@ static inline int hw_breakpoint_slots_cached(int type)
>         return __nr_bp_slots[type];
>  }
>
> +static __init bool
> +bp_slots_histogram_alloc(struct bp_slots_histogram *hist, enum bp_type_idx type)
> +{
> +       hist->count = kcalloc(hw_breakpoint_slots_cached(type), sizeof(*hist->count), GFP_KERNEL);
> +       return hist->count;
> +}
> +
> +static __init void bp_slots_histogram_free(struct bp_slots_histogram *hist)
> +{
> +       kfree(hist->count);
> +}
> +
>  static __init int init_breakpoint_slots(void)
>  {
>         int i, cpu, err_cpu;
> @@ -170,8 +190,7 @@ static __init int init_breakpoint_slots(void)
>                 for (i = 0; i < TYPE_MAX; i++) {
>                         struct bp_cpuinfo *info = get_bp_info(cpu, i);
>
> -                       info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(atomic_t), GFP_KERNEL);
> -                       if (!info->tsk_pinned)
> +                       if (!bp_slots_histogram_alloc(&info->tsk_pinned, i))
>                                 goto err;
>                 }
>         }
> @@ -180,7 +199,7 @@ static __init int init_breakpoint_slots(void)
>  err:
>         for_each_possible_cpu(err_cpu) {
>                 for (i = 0; i < TYPE_MAX; i++)
> -                       kfree(get_bp_info(err_cpu, i)->tsk_pinned);
> +                       bp_slots_histogram_free(&get_bp_info(err_cpu, i)->tsk_pinned);
>                 if (err_cpu == cpu)
>                         break;
>         }
> @@ -189,6 +208,34 @@ static __init int init_breakpoint_slots(void)
>  }
>  #endif
>
> +static inline void
> +bp_slots_histogram_add(struct bp_slots_histogram *hist, int old, int val)
> +{
> +       const int old_idx = old - 1;
> +       const int new_idx = old_idx + val;
> +
> +       if (old_idx >= 0)
> +               WARN_ON(atomic_dec_return_relaxed(&hist->count[old_idx]) < 0);
> +       if (new_idx >= 0)
> +               WARN_ON(atomic_inc_return_relaxed(&hist->count[new_idx]) < 0);
> +}
> +
> +static int
> +bp_slots_histogram_max(struct bp_slots_histogram *hist, enum bp_type_idx type)
> +{
> +       for (int i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
> +               const int count = atomic_read(&hist->count[i]);
> +
> +               /* Catch unexpected writers; we want a stable snapshot. */
> +               ASSERT_EXCLUSIVE_WRITER(hist->count[i]);
> +               if (count > 0)
> +                       return i + 1;
> +               WARN(count < 0, "inconsistent breakpoint slots histogram");
> +       }
> +
> +       return 0;
> +}
> +
>  #ifndef hw_breakpoint_weight
>  static inline int hw_breakpoint_weight(struct perf_event *bp)
>  {
> @@ -205,13 +252,11 @@ static inline enum bp_type_idx find_slot_idx(u64 bp_type)
>  }
>
>  /*
> - * Report the maximum number of pinned breakpoints a task
> - * have in this cpu
> + * Return the maximum number of pinned breakpoints a task has in this CPU.
>   */
>  static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>  {
> -       atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
> -       int i;
> +       struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
>
>         /*
>          * At this point we want to have acquired the bp_cpuinfo_sem as a
> @@ -219,14 +264,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>          * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
>          */
>         lockdep_assert_held_write(&bp_cpuinfo_sem);
> -
> -       for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
> -               ASSERT_EXCLUSIVE_WRITER(tsk_pinned[i]); /* Catch unexpected writers. */
> -               if (atomic_read(&tsk_pinned[i]) > 0)
> -                       return i + 1;
> -       }
> -
> -       return 0;
> +       return bp_slots_histogram_max(tsk_pinned, type);
>  }
>
>  /*
> @@ -300,8 +338,7 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
>  static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
>                                 enum bp_type_idx type, int weight)
>  {
> -       atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
> -       int old_idx, new_idx;
> +       struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
>
>         /*
>          * If bp->hw.target, tsk_pinned is only modified, but not used
> @@ -311,14 +348,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
>          * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
>          */
>         lockdep_assert_held_read(&bp_cpuinfo_sem);
> -
> -       old_idx = task_bp_pinned(cpu, bp, type) - 1;
> -       new_idx = old_idx + weight;
> -
> -       if (old_idx >= 0)
> -               atomic_dec(&tsk_pinned[old_idx]);
> -       if (new_idx >= 0)
> -               atomic_inc(&tsk_pinned[new_idx]);
> +       bp_slots_histogram_add(tsk_pinned, task_bp_pinned(cpu, bp, type), weight);
>  }
>
>  /*
> @@ -768,7 +798,7 @@ bool hw_breakpoint_is_used(void)
>                                 return true;
>
>                         for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) {
> -                               if (atomic_read(&info->tsk_pinned[slot]))
> +                               if (atomic_read(&info->tsk_pinned.count[slot]))
>                                         return true;
>                         }
>                 }
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

[PATCH v3 13/14] perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets

[Marco Elver]

Running the perf benchmark with (note: more aggressive parameters vs.
preceding changes, but same 256 CPUs host):

 | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
 |      Total time: 1.989 [sec]
 |
 |       38.854160 usecs/op
 |     4973.332500 usecs/op/cpu

    20.43%  [kernel]       [k] queued_spin_lock_slowpath
    18.75%  [kernel]       [k] osq_lock
    16.98%  [kernel]       [k] rhashtable_jhash2
     8.34%  [kernel]       [k] task_bp_pinned
     4.23%  [kernel]       [k] smp_cfm_core_cond
     3.65%  [kernel]       [k] bcmp
     2.83%  [kernel]       [k] toggle_bp_slot
     1.87%  [kernel]       [k] find_next_bit
     1.49%  [kernel]       [k] __reserve_bp_slot

We can see that a majority of the time is now spent hashing task
pointers to index into task_bps_ht in task_bp_pinned().

Obtaining the max_bp_pinned_slots() for CPU-independent task targets
currently is O(#cpus), and calls task_bp_pinned() for each CPU, even if
the result of task_bp_pinned() is CPU-independent.

The loop in max_bp_pinned_slots() wants to compute the maximum slots
across all CPUs. If task_bp_pinned() is CPU-independent, we can do so by
obtaining the max slots across all CPUs and adding task_bp_pinned().

To do so in O(1), use a bp_slots_histogram for CPU-pinned slots.

After this optimization:

 | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
 |      Total time: 1.930 [sec]
 |
 |       37.697832 usecs/op
 |     4825.322500 usecs/op/cpu

    19.13%  [kernel]       [k] queued_spin_lock_slowpath
    18.21%  [kernel]       [k] rhashtable_jhash2
    15.46%  [kernel]       [k] osq_lock
     6.27%  [kernel]       [k] toggle_bp_slot
     5.91%  [kernel]       [k] task_bp_pinned
     5.05%  [kernel]       [k] smp_cfm_core_cond
     1.78%  [kernel]       [k] update_sg_lb_stats
     1.36%  [kernel]       [k] llist_reverse_order
     1.34%  [kernel]       [k] find_next_bit
     1.19%  [kernel]       [k] bcmp

Suggesting that time spent in task_bp_pinned() has been reduced.
However, we're still hashing too much, which will be addressed in the
subsequent change.

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
v3:
* Update hw_breakpoint_is_used() to include global cpu_pinned.

v2:
* New patch.
---
 kernel/events/hw_breakpoint.c | 57 ++++++++++++++++++++++++++++++++---
 1 file changed, 53 insertions(+), 4 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 03ebecf048c0..a489f31fe147 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -64,6 +64,9 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
 	return per_cpu_ptr(bp_cpuinfo + type, cpu);
 }
 
+/* Number of pinned CPU breakpoints globally. */
+static struct bp_slots_histogram cpu_pinned[TYPE_MAX];
+
 /* Keep track of the breakpoints attached to tasks */
 static struct rhltable task_bps_ht;
 static const struct rhashtable_params task_bps_ht_params = {
@@ -194,6 +197,10 @@ static __init int init_breakpoint_slots(void)
 				goto err;
 		}
 	}
+	for (i = 0; i < TYPE_MAX; i++) {
+		if (!bp_slots_histogram_alloc(&cpu_pinned[i], i))
+			goto err;
+	}
 
 	return 0;
 err:
@@ -203,6 +210,8 @@ static __init int init_breakpoint_slots(void)
 		if (err_cpu == cpu)
 			break;
 	}
+	for (i = 0; i < TYPE_MAX; i++)
+		bp_slots_histogram_free(&cpu_pinned[i]);
 
 	return -ENOMEM;
 }
@@ -270,6 +279,9 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
 /*
  * Count the number of breakpoints of the same type and same task.
  * The given event must be not on the list.
+ *
+ * If @cpu is -1, but the result of task_bp_pinned() is not CPU-independent,
+ * returns a negative value.
  */
 static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
 {
@@ -288,9 +300,18 @@ static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
 		goto out;
 
 	rhl_for_each_entry_rcu(iter, pos, head, hw.bp_list) {
-		if (find_slot_idx(iter->attr.bp_type) == type &&
-		    (iter->cpu < 0 || cpu == iter->cpu))
-			count += hw_breakpoint_weight(iter);
+		if (find_slot_idx(iter->attr.bp_type) != type)
+			continue;
+
+		if (iter->cpu >= 0) {
+			if (cpu == -1) {
+				count = -1;
+				goto out;
+			} else if (cpu != iter->cpu)
+				continue;
+		}
+
+		count += hw_breakpoint_weight(iter);
 	}
 
 out:
@@ -316,6 +337,19 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
 	int pinned_slots = 0;
 	int cpu;
 
+	if (bp->hw.target && bp->cpu < 0) {
+		int max_pinned = task_bp_pinned(-1, bp, type);
+
+		if (max_pinned >= 0) {
+			/*
+			 * Fast path: task_bp_pinned() is CPU-independent and
+			 * returns the same value for any CPU.
+			 */
+			max_pinned += bp_slots_histogram_max(&cpu_pinned[type], type);
+			return max_pinned;
+		}
+	}
+
 	for_each_cpu(cpu, cpumask) {
 		struct bp_cpuinfo *info = get_bp_info(cpu, type);
 		int nr;
@@ -366,8 +400,11 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
 
 	/* Pinned counter cpu profiling */
 	if (!bp->hw.target) {
+		struct bp_cpuinfo *info = get_bp_info(bp->cpu, type);
+
 		lockdep_assert_held_write(&bp_cpuinfo_sem);
-		get_bp_info(bp->cpu, type)->cpu_pinned += weight;
+		bp_slots_histogram_add(&cpu_pinned[type], info->cpu_pinned, weight);
+		info->cpu_pinned += weight;
 		return 0;
 	}
 
@@ -804,6 +841,18 @@ bool hw_breakpoint_is_used(void)
 		}
 	}
 
+	for (int type = 0; type < TYPE_MAX; ++type) {
+		for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) {
+			/*
+			 * Warn, because if there are CPU pinned counters,
+			 * should never get here; bp_cpuinfo::cpu_pinned should
+			 * be consistent with the global cpu_pinned histogram.
+			 */
+			if (WARN_ON(atomic_read(&cpu_pinned[type].count[slot])))
+				return true;
+		}
+	}
+
 	return false;
 }
 
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 13/14] perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:07 AM Marco Elver <elver@google.com> wrote:
>
> Running the perf benchmark with (note: more aggressive parameters vs.
> preceding changes, but same 256 CPUs host):
>
>  | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
>  |      Total time: 1.989 [sec]
>  |
>  |       38.854160 usecs/op
>  |     4973.332500 usecs/op/cpu
>
>     20.43%  [kernel]       [k] queued_spin_lock_slowpath
>     18.75%  [kernel]       [k] osq_lock
>     16.98%  [kernel]       [k] rhashtable_jhash2
>      8.34%  [kernel]       [k] task_bp_pinned
>      4.23%  [kernel]       [k] smp_cfm_core_cond
>      3.65%  [kernel]       [k] bcmp
>      2.83%  [kernel]       [k] toggle_bp_slot
>      1.87%  [kernel]       [k] find_next_bit
>      1.49%  [kernel]       [k] __reserve_bp_slot
>
> We can see that a majority of the time is now spent hashing task
> pointers to index into task_bps_ht in task_bp_pinned().
>
> Obtaining the max_bp_pinned_slots() for CPU-independent task targets
> currently is O(#cpus), and calls task_bp_pinned() for each CPU, even if
> the result of task_bp_pinned() is CPU-independent.
>
> The loop in max_bp_pinned_slots() wants to compute the maximum slots
> across all CPUs. If task_bp_pinned() is CPU-independent, we can do so by
> obtaining the max slots across all CPUs and adding task_bp_pinned().
>
> To do so in O(1), use a bp_slots_histogram for CPU-pinned slots.
>
> After this optimization:
>
>  | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
>  |      Total time: 1.930 [sec]
>  |
>  |       37.697832 usecs/op
>  |     4825.322500 usecs/op/cpu
>
>     19.13%  [kernel]       [k] queued_spin_lock_slowpath
>     18.21%  [kernel]       [k] rhashtable_jhash2
>     15.46%  [kernel]       [k] osq_lock
>      6.27%  [kernel]       [k] toggle_bp_slot
>      5.91%  [kernel]       [k] task_bp_pinned
>      5.05%  [kernel]       [k] smp_cfm_core_cond
>      1.78%  [kernel]       [k] update_sg_lb_stats
>      1.36%  [kernel]       [k] llist_reverse_order
>      1.34%  [kernel]       [k] find_next_bit
>      1.19%  [kernel]       [k] bcmp
>
> Suggesting that time spent in task_bp_pinned() has been reduced.
> However, we're still hashing too much, which will be addressed in the
> subsequent change.
>
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
> v3:
> * Update hw_breakpoint_is_used() to include global cpu_pinned.
>
> v2:
> * New patch.
> ---
>  kernel/events/hw_breakpoint.c | 57 ++++++++++++++++++++++++++++++++---
>  1 file changed, 53 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 03ebecf048c0..a489f31fe147 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -64,6 +64,9 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
>         return per_cpu_ptr(bp_cpuinfo + type, cpu);
>  }
>
> +/* Number of pinned CPU breakpoints globally. */
> +static struct bp_slots_histogram cpu_pinned[TYPE_MAX];
> +
>  /* Keep track of the breakpoints attached to tasks */
>  static struct rhltable task_bps_ht;
>  static const struct rhashtable_params task_bps_ht_params = {
> @@ -194,6 +197,10 @@ static __init int init_breakpoint_slots(void)
>                                 goto err;
>                 }
>         }
> +       for (i = 0; i < TYPE_MAX; i++) {
> +               if (!bp_slots_histogram_alloc(&cpu_pinned[i], i))
> +                       goto err;
> +       }
>
>         return 0;
>  err:
> @@ -203,6 +210,8 @@ static __init int init_breakpoint_slots(void)
>                 if (err_cpu == cpu)
>                         break;
>         }
> +       for (i = 0; i < TYPE_MAX; i++)
> +               bp_slots_histogram_free(&cpu_pinned[i]);
>
>         return -ENOMEM;
>  }
> @@ -270,6 +279,9 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>  /*
>   * Count the number of breakpoints of the same type and same task.
>   * The given event must be not on the list.
> + *
> + * If @cpu is -1, but the result of task_bp_pinned() is not CPU-independent,
> + * returns a negative value.
>   */
>  static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
>  {
> @@ -288,9 +300,18 @@ static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
>                 goto out;
>
>         rhl_for_each_entry_rcu(iter, pos, head, hw.bp_list) {
> -               if (find_slot_idx(iter->attr.bp_type) == type &&
> -                   (iter->cpu < 0 || cpu == iter->cpu))
> -                       count += hw_breakpoint_weight(iter);
> +               if (find_slot_idx(iter->attr.bp_type) != type)
> +                       continue;
> +
> +               if (iter->cpu >= 0) {
> +                       if (cpu == -1) {
> +                               count = -1;
> +                               goto out;
> +                       } else if (cpu != iter->cpu)
> +                               continue;
> +               }
> +
> +               count += hw_breakpoint_weight(iter);
>         }
>
>  out:
> @@ -316,6 +337,19 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
>         int pinned_slots = 0;
>         int cpu;
>
> +       if (bp->hw.target && bp->cpu < 0) {
> +               int max_pinned = task_bp_pinned(-1, bp, type);
> +
> +               if (max_pinned >= 0) {
> +                       /*
> +                        * Fast path: task_bp_pinned() is CPU-independent and
> +                        * returns the same value for any CPU.
> +                        */
> +                       max_pinned += bp_slots_histogram_max(&cpu_pinned[type], type);
> +                       return max_pinned;
> +               }
> +       }
> +
>         for_each_cpu(cpu, cpumask) {
>                 struct bp_cpuinfo *info = get_bp_info(cpu, type);
>                 int nr;
> @@ -366,8 +400,11 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>
>         /* Pinned counter cpu profiling */
>         if (!bp->hw.target) {
> +               struct bp_cpuinfo *info = get_bp_info(bp->cpu, type);
> +
>                 lockdep_assert_held_write(&bp_cpuinfo_sem);
> -               get_bp_info(bp->cpu, type)->cpu_pinned += weight;
> +               bp_slots_histogram_add(&cpu_pinned[type], info->cpu_pinned, weight);
> +               info->cpu_pinned += weight;
>                 return 0;
>         }
>
> @@ -804,6 +841,18 @@ bool hw_breakpoint_is_used(void)
>                 }
>         }
>
> +       for (int type = 0; type < TYPE_MAX; ++type) {
> +               for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) {
> +                       /*
> +                        * Warn, because if there are CPU pinned counters,
> +                        * should never get here; bp_cpuinfo::cpu_pinned should
> +                        * be consistent with the global cpu_pinned histogram.
> +                        */
> +                       if (WARN_ON(atomic_read(&cpu_pinned[type].count[slot])))
> +                               return true;
> +               }
> +       }
> +
>         return false;
>  }
>
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

[PATCH v3 14/14] perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets

[Marco Elver]

We can still see that a majority of the time is spent hashing task pointers:

    ...
    16.98%  [kernel]       [k] rhashtable_jhash2
    ...

Doing the bookkeeping in toggle_bp_slots() is currently O(#cpus),
calling task_bp_pinned() for each CPU, even if task_bp_pinned() is
CPU-independent. The reason for this is to update the per-CPU
'tsk_pinned' histogram.

To optimize the CPU-independent case to O(1), keep a separate
CPU-independent 'tsk_pinned_all' histogram.

The major source of complexity are transitions between "all
CPU-independent task breakpoints" and "mixed CPU-independent and
CPU-dependent task breakpoints". The code comments list all cases that
require handling.

After this optimization:

 | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
 |      Total time: 1.758 [sec]
 |
 |       34.336621 usecs/op
 |     4395.087500 usecs/op/cpu

    38.08%  [kernel]       [k] queued_spin_lock_slowpath
    10.81%  [kernel]       [k] smp_cfm_core_cond
     3.01%  [kernel]       [k] update_sg_lb_stats
     2.58%  [kernel]       [k] osq_lock
     2.57%  [kernel]       [k] llist_reverse_order
     1.45%  [kernel]       [k] find_next_bit
     1.21%  [kernel]       [k] flush_tlb_func_common
     1.01%  [kernel]       [k] arch_install_hw_breakpoint

Showing that the time spent hashing keys has become insignificant.

With the given benchmark parameters, that's an improvement of 12%
compared with the old O(#cpus) version.

And finally, using the less aggressive parameters from the preceding
changes, we now observe:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 0.067 [sec]
 |
 |       35.292187 usecs/op
 |     2258.700000 usecs/op/cpu

Which is an improvement of 12% compared to without the histogram
optimizations (baseline is 40 usecs/op). This is now on par with the
theoretical ideal (constraints disabled), and only 12% slower than no
breakpoints at all.

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
v3:
* Fix typo "5 cases" -> "4 cases".
* Update hw_breakpoint_is_used() to check tsk_pinned_all.

v2:
* New patch.
---
 kernel/events/hw_breakpoint.c | 155 +++++++++++++++++++++++++++-------
 1 file changed, 124 insertions(+), 31 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index a489f31fe147..7ef0e98d31e2 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -66,6 +66,8 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
 
 /* Number of pinned CPU breakpoints globally. */
 static struct bp_slots_histogram cpu_pinned[TYPE_MAX];
+/* Number of pinned CPU-independent task breakpoints. */
+static struct bp_slots_histogram tsk_pinned_all[TYPE_MAX];
 
 /* Keep track of the breakpoints attached to tasks */
 static struct rhltable task_bps_ht;
@@ -200,6 +202,8 @@ static __init int init_breakpoint_slots(void)
 	for (i = 0; i < TYPE_MAX; i++) {
 		if (!bp_slots_histogram_alloc(&cpu_pinned[i], i))
 			goto err;
+		if (!bp_slots_histogram_alloc(&tsk_pinned_all[i], i))
+			goto err;
 	}
 
 	return 0;
@@ -210,8 +214,10 @@ static __init int init_breakpoint_slots(void)
 		if (err_cpu == cpu)
 			break;
 	}
-	for (i = 0; i < TYPE_MAX; i++)
+	for (i = 0; i < TYPE_MAX; i++) {
 		bp_slots_histogram_free(&cpu_pinned[i]);
+		bp_slots_histogram_free(&tsk_pinned_all[i]);
+	}
 
 	return -ENOMEM;
 }
@@ -245,6 +251,26 @@ bp_slots_histogram_max(struct bp_slots_histogram *hist, enum bp_type_idx type)
 	return 0;
 }
 
+static int
+bp_slots_histogram_max_merge(struct bp_slots_histogram *hist1, struct bp_slots_histogram *hist2,
+			     enum bp_type_idx type)
+{
+	for (int i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
+		const int count1 = atomic_read(&hist1->count[i]);
+		const int count2 = atomic_read(&hist2->count[i]);
+
+		/* Catch unexpected writers; we want a stable snapshot. */
+		ASSERT_EXCLUSIVE_WRITER(hist1->count[i]);
+		ASSERT_EXCLUSIVE_WRITER(hist2->count[i]);
+		if (count1 + count2 > 0)
+			return i + 1;
+		WARN(count1 < 0, "inconsistent breakpoint slots histogram");
+		WARN(count2 < 0, "inconsistent breakpoint slots histogram");
+	}
+
+	return 0;
+}
+
 #ifndef hw_breakpoint_weight
 static inline int hw_breakpoint_weight(struct perf_event *bp)
 {
@@ -273,7 +299,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
 	 * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
 	 */
 	lockdep_assert_held_write(&bp_cpuinfo_sem);
-	return bp_slots_histogram_max(tsk_pinned, type);
+	return bp_slots_histogram_max_merge(tsk_pinned, &tsk_pinned_all[type], type);
 }
 
 /*
@@ -366,40 +392,22 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
 	return pinned_slots;
 }
 
-/*
- * Add a pinned breakpoint for the given task in our constraint table
- */
-static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
-				enum bp_type_idx type, int weight)
-{
-	struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
-
-	/*
-	 * If bp->hw.target, tsk_pinned is only modified, but not used
-	 * otherwise. We can permit concurrent updates as long as there are no
-	 * other uses: having acquired bp_cpuinfo_sem as a reader allows
-	 * concurrent updates here. Uses of tsk_pinned will require acquiring
-	 * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
-	 */
-	lockdep_assert_held_read(&bp_cpuinfo_sem);
-	bp_slots_histogram_add(tsk_pinned, task_bp_pinned(cpu, bp, type), weight);
-}
-
 /*
  * Add/remove the given breakpoint in our constraint table
  */
 static int
-toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
-	       int weight)
+toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, int weight)
 {
-	const struct cpumask *cpumask = cpumask_of_bp(bp);
-	int cpu;
+	int cpu, next_tsk_pinned;
 
 	if (!enable)
 		weight = -weight;
 
-	/* Pinned counter cpu profiling */
 	if (!bp->hw.target) {
+		/*
+		 * Update the pinned CPU slots, in per-CPU bp_cpuinfo and in the
+		 * global histogram.
+		 */
 		struct bp_cpuinfo *info = get_bp_info(bp->cpu, type);
 
 		lockdep_assert_held_write(&bp_cpuinfo_sem);
@@ -408,9 +416,91 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
 		return 0;
 	}
 
-	/* Pinned counter task profiling */
-	for_each_cpu(cpu, cpumask)
-		toggle_bp_task_slot(bp, cpu, type, weight);
+	/*
+	 * If bp->hw.target, tsk_pinned is only modified, but not used
+	 * otherwise. We can permit concurrent updates as long as there are no
+	 * other uses: having acquired bp_cpuinfo_sem as a reader allows
+	 * concurrent updates here. Uses of tsk_pinned will require acquiring
+	 * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
+	 */
+	lockdep_assert_held_read(&bp_cpuinfo_sem);
+
+	/*
+	 * Update the pinned task slots, in per-CPU bp_cpuinfo and in the global
+	 * histogram. We need to take care of 4 cases:
+	 *
+	 *  1. This breakpoint targets all CPUs (cpu < 0), and there may only
+	 *     exist other task breakpoints targeting all CPUs. In this case we
+	 *     can simply update the global slots histogram.
+	 *
+	 *  2. This breakpoint targets a specific CPU (cpu >= 0), but there may
+	 *     only exist other task breakpoints targeting all CPUs.
+	 *
+	 *     a. On enable: remove the existing breakpoints from the global
+	 *        slots histogram and use the per-CPU histogram.
+	 *
+	 *     b. On disable: re-insert the existing breakpoints into the global
+	 *        slots histogram and remove from per-CPU histogram.
+	 *
+	 *  3. Some other existing task breakpoints target specific CPUs. Only
+	 *     update the per-CPU slots histogram.
+	 */
+
+	if (!enable) {
+		/*
+		 * Remove before updating histograms so we can determine if this
+		 * was the last task breakpoint for a specific CPU.
+		 */
+		int ret = rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
+
+		if (ret)
+			return ret;
+	}
+	/*
+	 * Note: If !enable, next_tsk_pinned will not count the to-be-removed breakpoint.
+	 */
+	next_tsk_pinned = task_bp_pinned(-1, bp, type);
+
+	if (next_tsk_pinned >= 0) {
+		if (bp->cpu < 0) { /* Case 1: fast path */
+			if (!enable)
+				next_tsk_pinned += hw_breakpoint_weight(bp);
+			bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned, weight);
+		} else if (enable) { /* Case 2.a: slow path */
+			/* Add existing to per-CPU histograms. */
+			for_each_possible_cpu(cpu) {
+				bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
+						       0, next_tsk_pinned);
+			}
+			/* Add this first CPU-pinned task breakpoint. */
+			bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
+					       next_tsk_pinned, weight);
+			/* Rebalance global task pinned histogram. */
+			bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned,
+					       -next_tsk_pinned);
+		} else { /* Case 2.b: slow path */
+			/* Remove this last CPU-pinned task breakpoint. */
+			bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
+					       next_tsk_pinned + hw_breakpoint_weight(bp), weight);
+			/* Remove all from per-CPU histograms. */
+			for_each_possible_cpu(cpu) {
+				bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
+						       next_tsk_pinned, -next_tsk_pinned);
+			}
+			/* Rebalance global task pinned histogram. */
+			bp_slots_histogram_add(&tsk_pinned_all[type], 0, next_tsk_pinned);
+		}
+	} else { /* Case 3: slow path */
+		const struct cpumask *cpumask = cpumask_of_bp(bp);
+
+		for_each_cpu(cpu, cpumask) {
+			next_tsk_pinned = task_bp_pinned(cpu, bp, type);
+			if (!enable)
+				next_tsk_pinned += hw_breakpoint_weight(bp);
+			bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
+					       next_tsk_pinned, weight);
+		}
+	}
 
 	/*
 	 * Readers want a stable snapshot of the per-task breakpoint list.
@@ -419,8 +509,8 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
 
 	if (enable)
 		return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
-	else
-		return rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
+
+	return 0;
 }
 
 __weak int arch_reserve_bp_slot(struct perf_event *bp)
@@ -850,6 +940,9 @@ bool hw_breakpoint_is_used(void)
 			 */
 			if (WARN_ON(atomic_read(&cpu_pinned[type].count[slot])))
 				return true;
+
+			if (atomic_read(&tsk_pinned_all[type].count[slot]))
+				return true;
 		}
 	}
 
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH v3 14/14] perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets

[Ian Rogers]

On Mon, Jul 4, 2022 at 8:07 AM Marco Elver <elver@google.com> wrote:
>
> We can still see that a majority of the time is spent hashing task pointers:
>
>     ...
>     16.98%  [kernel]       [k] rhashtable_jhash2
>     ...
>
> Doing the bookkeeping in toggle_bp_slots() is currently O(#cpus),
> calling task_bp_pinned() for each CPU, even if task_bp_pinned() is
> CPU-independent. The reason for this is to update the per-CPU
> 'tsk_pinned' histogram.
>
> To optimize the CPU-independent case to O(1), keep a separate
> CPU-independent 'tsk_pinned_all' histogram.
>
> The major source of complexity are transitions between "all
> CPU-independent task breakpoints" and "mixed CPU-independent and
> CPU-dependent task breakpoints". The code comments list all cases that
> require handling.
>
> After this optimization:
>
>  | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
>  |      Total time: 1.758 [sec]
>  |
>  |       34.336621 usecs/op
>  |     4395.087500 usecs/op/cpu
>
>     38.08%  [kernel]       [k] queued_spin_lock_slowpath
>     10.81%  [kernel]       [k] smp_cfm_core_cond
>      3.01%  [kernel]       [k] update_sg_lb_stats
>      2.58%  [kernel]       [k] osq_lock
>      2.57%  [kernel]       [k] llist_reverse_order
>      1.45%  [kernel]       [k] find_next_bit
>      1.21%  [kernel]       [k] flush_tlb_func_common
>      1.01%  [kernel]       [k] arch_install_hw_breakpoint
>
> Showing that the time spent hashing keys has become insignificant.
>
> With the given benchmark parameters, that's an improvement of 12%
> compared with the old O(#cpus) version.
>
> And finally, using the less aggressive parameters from the preceding
> changes, we now observe:
>
>  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
>  |      Total time: 0.067 [sec]
>  |
>  |       35.292187 usecs/op
>  |     2258.700000 usecs/op/cpu
>
> Which is an improvement of 12% compared to without the histogram
> optimizations (baseline is 40 usecs/op). This is now on par with the
> theoretical ideal (constraints disabled), and only 12% slower than no
> breakpoints at all.
>
> Signed-off-by: Marco Elver <elver@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
> v3:
> * Fix typo "5 cases" -> "4 cases".
> * Update hw_breakpoint_is_used() to check tsk_pinned_all.
>
> v2:
> * New patch.
> ---
>  kernel/events/hw_breakpoint.c | 155 +++++++++++++++++++++++++++-------
>  1 file changed, 124 insertions(+), 31 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index a489f31fe147..7ef0e98d31e2 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -66,6 +66,8 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
>
>  /* Number of pinned CPU breakpoints globally. */
>  static struct bp_slots_histogram cpu_pinned[TYPE_MAX];
> +/* Number of pinned CPU-independent task breakpoints. */
> +static struct bp_slots_histogram tsk_pinned_all[TYPE_MAX];
>
>  /* Keep track of the breakpoints attached to tasks */
>  static struct rhltable task_bps_ht;
> @@ -200,6 +202,8 @@ static __init int init_breakpoint_slots(void)
>         for (i = 0; i < TYPE_MAX; i++) {
>                 if (!bp_slots_histogram_alloc(&cpu_pinned[i], i))
>                         goto err;
> +               if (!bp_slots_histogram_alloc(&tsk_pinned_all[i], i))
> +                       goto err;
>         }
>
>         return 0;
> @@ -210,8 +214,10 @@ static __init int init_breakpoint_slots(void)
>                 if (err_cpu == cpu)
>                         break;
>         }
> -       for (i = 0; i < TYPE_MAX; i++)
> +       for (i = 0; i < TYPE_MAX; i++) {
>                 bp_slots_histogram_free(&cpu_pinned[i]);
> +               bp_slots_histogram_free(&tsk_pinned_all[i]);
> +       }
>
>         return -ENOMEM;
>  }
> @@ -245,6 +251,26 @@ bp_slots_histogram_max(struct bp_slots_histogram *hist, enum bp_type_idx type)
>         return 0;
>  }
>
> +static int
> +bp_slots_histogram_max_merge(struct bp_slots_histogram *hist1, struct bp_slots_histogram *hist2,
> +                            enum bp_type_idx type)
> +{
> +       for (int i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
> +               const int count1 = atomic_read(&hist1->count[i]);
> +               const int count2 = atomic_read(&hist2->count[i]);
> +
> +               /* Catch unexpected writers; we want a stable snapshot. */
> +               ASSERT_EXCLUSIVE_WRITER(hist1->count[i]);
> +               ASSERT_EXCLUSIVE_WRITER(hist2->count[i]);
> +               if (count1 + count2 > 0)
> +                       return i + 1;
> +               WARN(count1 < 0, "inconsistent breakpoint slots histogram");
> +               WARN(count2 < 0, "inconsistent breakpoint slots histogram");
> +       }
> +
> +       return 0;
> +}
> +
>  #ifndef hw_breakpoint_weight
>  static inline int hw_breakpoint_weight(struct perf_event *bp)
>  {
> @@ -273,7 +299,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>          * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
>          */
>         lockdep_assert_held_write(&bp_cpuinfo_sem);
> -       return bp_slots_histogram_max(tsk_pinned, type);
> +       return bp_slots_histogram_max_merge(tsk_pinned, &tsk_pinned_all[type], type);
>  }
>
>  /*
> @@ -366,40 +392,22 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
>         return pinned_slots;
>  }
>
> -/*
> - * Add a pinned breakpoint for the given task in our constraint table
> - */
> -static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
> -                               enum bp_type_idx type, int weight)
> -{
> -       struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
> -
> -       /*
> -        * If bp->hw.target, tsk_pinned is only modified, but not used
> -        * otherwise. We can permit concurrent updates as long as there are no
> -        * other uses: having acquired bp_cpuinfo_sem as a reader allows
> -        * concurrent updates here. Uses of tsk_pinned will require acquiring
> -        * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
> -        */
> -       lockdep_assert_held_read(&bp_cpuinfo_sem);
> -       bp_slots_histogram_add(tsk_pinned, task_bp_pinned(cpu, bp, type), weight);
> -}
> -
>  /*
>   * Add/remove the given breakpoint in our constraint table
>   */
>  static int
> -toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
> -              int weight)
> +toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, int weight)
>  {
> -       const struct cpumask *cpumask = cpumask_of_bp(bp);
> -       int cpu;
> +       int cpu, next_tsk_pinned;
>
>         if (!enable)
>                 weight = -weight;
>
> -       /* Pinned counter cpu profiling */
>         if (!bp->hw.target) {
> +               /*
> +                * Update the pinned CPU slots, in per-CPU bp_cpuinfo and in the
> +                * global histogram.
> +                */
>                 struct bp_cpuinfo *info = get_bp_info(bp->cpu, type);
>
>                 lockdep_assert_held_write(&bp_cpuinfo_sem);
> @@ -408,9 +416,91 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>                 return 0;
>         }
>
> -       /* Pinned counter task profiling */
> -       for_each_cpu(cpu, cpumask)
> -               toggle_bp_task_slot(bp, cpu, type, weight);
> +       /*
> +        * If bp->hw.target, tsk_pinned is only modified, but not used
> +        * otherwise. We can permit concurrent updates as long as there are no
> +        * other uses: having acquired bp_cpuinfo_sem as a reader allows
> +        * concurrent updates here. Uses of tsk_pinned will require acquiring
> +        * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
> +        */
> +       lockdep_assert_held_read(&bp_cpuinfo_sem);
> +
> +       /*
> +        * Update the pinned task slots, in per-CPU bp_cpuinfo and in the global
> +        * histogram. We need to take care of 4 cases:
> +        *
> +        *  1. This breakpoint targets all CPUs (cpu < 0), and there may only
> +        *     exist other task breakpoints targeting all CPUs. In this case we
> +        *     can simply update the global slots histogram.
> +        *
> +        *  2. This breakpoint targets a specific CPU (cpu >= 0), but there may
> +        *     only exist other task breakpoints targeting all CPUs.
> +        *
> +        *     a. On enable: remove the existing breakpoints from the global
> +        *        slots histogram and use the per-CPU histogram.
> +        *
> +        *     b. On disable: re-insert the existing breakpoints into the global
> +        *        slots histogram and remove from per-CPU histogram.
> +        *
> +        *  3. Some other existing task breakpoints target specific CPUs. Only
> +        *     update the per-CPU slots histogram.
> +        */
> +
> +       if (!enable) {
> +               /*
> +                * Remove before updating histograms so we can determine if this
> +                * was the last task breakpoint for a specific CPU.
> +                */
> +               int ret = rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
> +
> +               if (ret)
> +                       return ret;
> +       }
> +       /*
> +        * Note: If !enable, next_tsk_pinned will not count the to-be-removed breakpoint.
> +        */
> +       next_tsk_pinned = task_bp_pinned(-1, bp, type);
> +
> +       if (next_tsk_pinned >= 0) {
> +               if (bp->cpu < 0) { /* Case 1: fast path */
> +                       if (!enable)
> +                               next_tsk_pinned += hw_breakpoint_weight(bp);
> +                       bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned, weight);
> +               } else if (enable) { /* Case 2.a: slow path */
> +                       /* Add existing to per-CPU histograms. */
> +                       for_each_possible_cpu(cpu) {
> +                               bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
> +                                                      0, next_tsk_pinned);
> +                       }
> +                       /* Add this first CPU-pinned task breakpoint. */
> +                       bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
> +                                              next_tsk_pinned, weight);
> +                       /* Rebalance global task pinned histogram. */
> +                       bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned,
> +                                              -next_tsk_pinned);
> +               } else { /* Case 2.b: slow path */
> +                       /* Remove this last CPU-pinned task breakpoint. */
> +                       bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
> +                                              next_tsk_pinned + hw_breakpoint_weight(bp), weight);
> +                       /* Remove all from per-CPU histograms. */
> +                       for_each_possible_cpu(cpu) {
> +                               bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
> +                                                      next_tsk_pinned, -next_tsk_pinned);
> +                       }
> +                       /* Rebalance global task pinned histogram. */
> +                       bp_slots_histogram_add(&tsk_pinned_all[type], 0, next_tsk_pinned);
> +               }
> +       } else { /* Case 3: slow path */
> +               const struct cpumask *cpumask = cpumask_of_bp(bp);
> +
> +               for_each_cpu(cpu, cpumask) {
> +                       next_tsk_pinned = task_bp_pinned(cpu, bp, type);
> +                       if (!enable)
> +                               next_tsk_pinned += hw_breakpoint_weight(bp);
> +                       bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
> +                                              next_tsk_pinned, weight);
> +               }
> +       }
>
>         /*
>          * Readers want a stable snapshot of the per-task breakpoint list.
> @@ -419,8 +509,8 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>
>         if (enable)
>                 return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
> -       else
> -               return rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
> +
> +       return 0;
>  }
>
>  __weak int arch_reserve_bp_slot(struct perf_event *bp)
> @@ -850,6 +940,9 @@ bool hw_breakpoint_is_used(void)
>                          */
>                         if (WARN_ON(atomic_read(&cpu_pinned[type].count[slot])))
>                                 return true;
> +
> +                       if (atomic_read(&tsk_pinned_all[type].count[slot]))
> +                               return true;
>                 }
>         }
>
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

Re: [PATCH v3 00/14] perf/hw_breakpoint: Optimize for thousands of tasks

[Marco Elver]

On Mon, 4 Jul 2022 at 17:05, Marco Elver <elver@google.com> wrote:
>
> The hw_breakpoint subsystem's code has seen little change in over 10
> years. In that time, systems with >100s of CPUs have become common,
> along with improvements to the perf subsystem: using breakpoints on
> thousands of concurrent tasks should be a supported usecase.
[...]
> Marco Elver (14):
>   perf/hw_breakpoint: Add KUnit test for constraints accounting
>   perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test
>   perf/hw_breakpoint: Clean up headers
>   perf/hw_breakpoint: Optimize list of per-task breakpoints
>   perf/hw_breakpoint: Mark data __ro_after_init
>   perf/hw_breakpoint: Optimize constant number of breakpoint slots
>   perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable
>   perf/hw_breakpoint: Remove useless code related to flexible
>     breakpoints
>   powerpc/hw_breakpoint: Avoid relying on caller synchronization
>   locking/percpu-rwsem: Add percpu_is_write_locked() and
>     percpu_is_read_locked()
>   perf/hw_breakpoint: Reduce contention with large number of tasks
>   perf/hw_breakpoint: Introduce bp_slots_histogram
>   perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent
>     task targets
>   perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task
>     targets
[...]

This is ready from our side, and given the silence, assume it's ready
to pick up and/or have a maintainer take a look. Since this is mostly
kernel/events, would -tip/perf/core be appropriate?

Thanks,
-- Marco

Re: [PATCH v3 00/14] perf/hw_breakpoint: Optimize for thousands of tasks

[Ian Rogers]

On Tue, Jul 12, 2022 at 6:41 AM Marco Elver <elver@google.com> wrote:
>
> On Mon, 4 Jul 2022 at 17:05, Marco Elver <elver@google.com> wrote:
> >
> > The hw_breakpoint subsystem's code has seen little change in over 10
> > years. In that time, systems with >100s of CPUs have become common,
> > along with improvements to the perf subsystem: using breakpoints on
> > thousands of concurrent tasks should be a supported usecase.
> [...]
> > Marco Elver (14):
> >   perf/hw_breakpoint: Add KUnit test for constraints accounting
> >   perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test
> >   perf/hw_breakpoint: Clean up headers
> >   perf/hw_breakpoint: Optimize list of per-task breakpoints
> >   perf/hw_breakpoint: Mark data __ro_after_init
> >   perf/hw_breakpoint: Optimize constant number of breakpoint slots
> >   perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable
> >   perf/hw_breakpoint: Remove useless code related to flexible
> >     breakpoints
> >   powerpc/hw_breakpoint: Avoid relying on caller synchronization
> >   locking/percpu-rwsem: Add percpu_is_write_locked() and
> >     percpu_is_read_locked()
> >   perf/hw_breakpoint: Reduce contention with large number of tasks
> >   perf/hw_breakpoint: Introduce bp_slots_histogram
> >   perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent
> >     task targets
> >   perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task
> >     targets
> [...]
>
> This is ready from our side, and given the silence, assume it's ready
> to pick up and/or have a maintainer take a look. Since this is mostly
> kernel/events, would -tip/perf/core be appropriate?

These are awesome improvements, I've added my acked-by to every
change. I hope we can pull these changes, as you say, into tip.git
perf/core and get them into 5.20.

Thanks,
Ian

> Thanks,
> -- Marco

Re: [PATCH v3 00/14] perf/hw_breakpoint: Optimize for thousands of tasks

[Marco Elver]

On Wed, 20 Jul 2022 at 17:47, Ian Rogers <irogers@google.com> wrote:
> On Tue, Jul 12, 2022 at 6:41 AM Marco Elver <elver@google.com> wrote:
> > On Mon, 4 Jul 2022 at 17:05, Marco Elver <elver@google.com> wrote:
> > > The hw_breakpoint subsystem's code has seen little change in over 10
> > > years. In that time, systems with >100s of CPUs have become common,
> > > along with improvements to the perf subsystem: using breakpoints on
> > > thousands of concurrent tasks should be a supported usecase.
> > [...]
> > > Marco Elver (14):
> > >   perf/hw_breakpoint: Add KUnit test for constraints accounting
> > >   perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test
> > >   perf/hw_breakpoint: Clean up headers
> > >   perf/hw_breakpoint: Optimize list of per-task breakpoints
> > >   perf/hw_breakpoint: Mark data __ro_after_init
> > >   perf/hw_breakpoint: Optimize constant number of breakpoint slots
> > >   perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable
> > >   perf/hw_breakpoint: Remove useless code related to flexible
> > >     breakpoints
> > >   powerpc/hw_breakpoint: Avoid relying on caller synchronization
> > >   locking/percpu-rwsem: Add percpu_is_write_locked() and
> > >     percpu_is_read_locked()
> > >   perf/hw_breakpoint: Reduce contention with large number of tasks
> > >   perf/hw_breakpoint: Introduce bp_slots_histogram
> > >   perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent
> > >     task targets
> > >   perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task
> > >     targets
> > [...]
> >
> > This is ready from our side, and given the silence, assume it's ready
> > to pick up and/or have a maintainer take a look. Since this is mostly
> > kernel/events, would -tip/perf/core be appropriate?
>
> These are awesome improvements, I've added my acked-by to every
> change. I hope we can pull these changes, as you say, into tip.git
> perf/core and get them into 5.20.

These still apply cleanly to 6.0-rc1 and the test passes, but let me
know if I shall send a rebased version.

Thanks
-- Marco