Re: [PATCH v2 01/13] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Marco Elver <elver@google.com>]

On Tue, 28 Jun 2022 at 14:53, Dmitry Vyukov <dvyukov@google.com> wrote:
>
>  On Tue, 28 Jun 2022 at 11:59, 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>
> > ---
> > v2:
> > * New patch.
> > ---
> >  kernel/events/Makefile             |   1 +
> >  kernel/events/hw_breakpoint_test.c | 321 +++++++++++++++++++++++++++++
> >  lib/Kconfig.debug                  |  10 +
> >  3 files changed, 332 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..747a0249a606
> > --- /dev/null
> > +++ b/kernel/events/hw_breakpoint_test.c
> > @@ -0,0 +1,321 @@
> > +// 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;
>
> Why is this function needed? Is hw_breakpoint_slots() very slow?

It seems non-trivial on some architectures (e.g.
arch/arm64/kernel/hw_breakpoint.c). Also the reason why
hw_breakpoint.c itself caches it, so I decided to follow the same
because it's called very often in the tests.

> > +
> > +       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_other_cpu(void)
> > +{
> > +       int cpu;
> > +
> > +       for_each_online_cpu(cpu) {
> > +               if (cpu != raw_smp_processor_id())
>
> Are we guaranteed to not be rescheduled in the middle of a test?
> If not, can't get_other_cpu() return the same CPU that was returned by
> raw_smp_processor_id() earlier in the test?

Yes, good point. I think I'll change it to just not use
raw_smp_processor_id() and instead have get_test_cpu(int num) and it
tries to find the 'num' online CPU. In the tests I'll just use CPU
#num 0 and 1.