[PATCH v4 bpf-next 0/3] Add bpf_perf_prog_read_branches() helper

[PATCH v4 bpf-next 0/3] Add bpf_perf_prog_read_branches() helper

[Daniel Xu]

Branch records are a CPU feature that can be configured to record
certain branches that are taken during code execution. This data is
particularly interesting for profile guided optimizations. perf has had
branch record support for a while but the data collection can be a bit
coarse grained.

We (Facebook) have seen in experiments that associating metadata with
branch records can improve results (after postprocessing). We generally
use bpf_probe_read_*() to get metadata out of userspace. That's why bpf
support for branch records is useful.

Aside from this particular use case, having branch data available to bpf
progs can be useful to get stack traces out of userspace applications
that omit frame pointers.

Changes in v4:
- Add BPF_F_GET_BR_SIZE flag
- Return -ENOENT on unsupported architectures
- Only accept initialized memory in helper
- Check buffer size is multiple of sizeof(struct perf_branch_entry)
- Use bpf skeleton in selftest
- Add commit messages
- Spelling and formatting

Changes in v3:
- Document filling unused buffer with zero
- Formatting fixes
- Rebase

Changes in v2:
- Change to a bpf helper instead of context access
- Avoid mentioning Intel specific things

Daniel Xu (3):
  bpf: Add bpf_perf_prog_read_branches() helper
  tools/bpf: Sync uapi header bpf.h
  selftests/bpf: add bpf_perf_prog_read_branches() selftest

 include/uapi/linux/bpf.h                      |  25 +++-
 kernel/trace/bpf_trace.c                      |  41 +++++++
 tools/include/uapi/linux/bpf.h                |  25 +++-
 .../selftests/bpf/prog_tests/perf_branches.c  | 112 ++++++++++++++++++
 .../selftests/bpf/progs/test_perf_branches.c  |  74 ++++++++++++
 5 files changed, 275 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/perf_branches.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_perf_branches.c

-- 
2.21.1

[PATCH v4 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches() helper

[Daniel Xu]

Branch records are a CPU feature that can be configured to record
certain branches that are taken during code execution. This data is
particularly interesting for profile guided optimizations. perf has had
branch record support for a while but the data collection can be a bit
coarse grained.

We (Facebook) have seen in experiments that associating metadata with
branch records can improve results (after postprocessing). We generally
use bpf_probe_read_*() to get metadata out of userspace. That's why bpf
support for branch records is useful.

Aside from this particular use case, having branch data available to bpf
progs can be useful to get stack traces out of userspace applications
that omit frame pointers.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 include/uapi/linux/bpf.h | 25 +++++++++++++++++++++++-
 kernel/trace/bpf_trace.c | 41 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index f1d74a2bd234..39bfba0091dc 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2892,6 +2892,25 @@ union bpf_attr {
  *		Obtain the 64bit jiffies
  *	Return
  *		The 64 bit jiffies
+ *
+ * int bpf_perf_prog_read_branches(struct bpf_perf_event_data *ctx, void *buf, u32 buf_size, u64 flags)
+ *	Description
+ *		For an eBPF program attached to a perf event, retrieve the
+ *		branch records (struct perf_branch_entry) associated to *ctx*
+ *		and store it in	the buffer pointed by *buf* up to size
+ *		*buf_size* bytes.
+ *
+ *		The *flags* can be set to **BPF_F_GET_BR_SIZE** to instead
+ *		return the number of bytes required to store all the branch
+ *		entries. If this flag is set, *buf* may be NULL.
+ *	Return
+ *		On success, number of bytes written to *buf*. On error, a
+ *		negative value.
+ *
+ *		**-EINVAL** if arguments invalid or **buf_size** not a multiple
+ *		of sizeof(struct perf_branch_entry).
+ *
+ *		**-ENOENT** if architecture does not support branch records.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3012,7 +3031,8 @@ union bpf_attr {
 	FN(probe_read_kernel_str),	\
 	FN(tcp_send_ack),		\
 	FN(send_signal_thread),		\
-	FN(jiffies64),
+	FN(jiffies64),			\
+	FN(perf_prog_read_branches),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
@@ -3091,6 +3111,9 @@ enum bpf_func_id {
 /* BPF_FUNC_sk_storage_get flags */
 #define BPF_SK_STORAGE_GET_F_CREATE	(1ULL << 0)
 
+/* BPF_FUNC_perf_prog_read_branches flags. */
+#define BPF_F_GET_BR_SIZE		(1ULL << 0)
+
 /* Mode for BPF_FUNC_skb_adjust_room helper. */
 enum bpf_adj_room_mode {
 	BPF_ADJ_ROOM_NET,
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 19e793aa441a..2f48fc85d793 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1028,6 +1028,45 @@ static const struct bpf_func_proto bpf_perf_prog_read_value_proto = {
          .arg3_type      = ARG_CONST_SIZE,
 };
 
+BPF_CALL_4(bpf_perf_prog_read_branches, struct bpf_perf_event_data_kern *, ctx,
+	   void *, buf, u32, size, u64, flags)
+{
+	struct perf_branch_stack *br_stack = ctx->data->br_stack;
+	u32 br_entry_size = sizeof(struct perf_branch_entry);
+	u32 to_copy;
+
+#ifndef CONFIG_X86
+	return -ENOENT;
+#endif
+
+	if (unlikely(flags & ~BPF_F_GET_BR_SIZE))
+		return -EINVAL;
+
+	if (unlikely(!br_stack))
+		return -EINVAL;
+
+	if (flags & BPF_F_GET_BR_SIZE)
+		return br_stack->nr * br_entry_size;
+
+	if (!buf || (size % br_entry_size != 0))
+		return -EINVAL;
+
+	to_copy = min_t(u32, br_stack->nr * br_entry_size, size);
+	memcpy(buf, br_stack->entries, to_copy);
+
+	return to_copy;
+}
+
+static const struct bpf_func_proto bpf_perf_prog_read_branches_proto = {
+	.func           = bpf_perf_prog_read_branches,
+	.gpl_only       = true,
+	.ret_type       = RET_INTEGER,
+	.arg1_type      = ARG_PTR_TO_CTX,
+	.arg2_type      = ARG_PTR_TO_MEM_OR_NULL,
+	.arg3_type      = ARG_CONST_SIZE_OR_ZERO,
+	.arg4_type      = ARG_ANYTHING,
+};
+
 static const struct bpf_func_proto *
 pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
@@ -1040,6 +1079,8 @@ pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_get_stack_proto_tp;
 	case BPF_FUNC_perf_prog_read_value:
 		return &bpf_perf_prog_read_value_proto;
+	case BPF_FUNC_perf_prog_read_branches:
+		return &bpf_perf_prog_read_branches_proto;
 	default:
 		return tracing_func_proto(func_id, prog);
 	}
-- 
2.21.1

[PATCH v4 bpf-next 2/3] tools/bpf: Sync uapi header bpf.h

[Daniel Xu]

Sync the header file in a separate commit to help with external sync.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 tools/include/uapi/linux/bpf.h | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index f1d74a2bd234..39bfba0091dc 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -2892,6 +2892,25 @@ union bpf_attr {
  *		Obtain the 64bit jiffies
  *	Return
  *		The 64 bit jiffies
+ *
+ * int bpf_perf_prog_read_branches(struct bpf_perf_event_data *ctx, void *buf, u32 buf_size, u64 flags)
+ *	Description
+ *		For an eBPF program attached to a perf event, retrieve the
+ *		branch records (struct perf_branch_entry) associated to *ctx*
+ *		and store it in	the buffer pointed by *buf* up to size
+ *		*buf_size* bytes.
+ *
+ *		The *flags* can be set to **BPF_F_GET_BR_SIZE** to instead
+ *		return the number of bytes required to store all the branch
+ *		entries. If this flag is set, *buf* may be NULL.
+ *	Return
+ *		On success, number of bytes written to *buf*. On error, a
+ *		negative value.
+ *
+ *		**-EINVAL** if arguments invalid or **buf_size** not a multiple
+ *		of sizeof(struct perf_branch_entry).
+ *
+ *		**-ENOENT** if architecture does not support branch records.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3012,7 +3031,8 @@ union bpf_attr {
 	FN(probe_read_kernel_str),	\
 	FN(tcp_send_ack),		\
 	FN(send_signal_thread),		\
-	FN(jiffies64),
+	FN(jiffies64),			\
+	FN(perf_prog_read_branches),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
@@ -3091,6 +3111,9 @@ enum bpf_func_id {
 /* BPF_FUNC_sk_storage_get flags */
 #define BPF_SK_STORAGE_GET_F_CREATE	(1ULL << 0)
 
+/* BPF_FUNC_perf_prog_read_branches flags. */
+#define BPF_F_GET_BR_SIZE		(1ULL << 0)
+
 /* Mode for BPF_FUNC_skb_adjust_room helper. */
 enum bpf_adj_room_mode {
 	BPF_ADJ_ROOM_NET,
-- 
2.21.1

[PATCH v4 bpf-next 3/3] selftests/bpf: add bpf_perf_prog_read_branches() selftest

[Daniel Xu]

Add a selftest to test:

* default bpf_perf_prog_read_branches() behavior
* BPF_F_GET_BR_SIZE flag behavior
* using helper to write to stack
* using helper to write to map

Tested by running:

    # ./test_progs -t perf_branches
    #27 perf_branches:OK
    Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 .../selftests/bpf/prog_tests/perf_branches.c  | 112 ++++++++++++++++++
 .../selftests/bpf/progs/test_perf_branches.c  |  74 ++++++++++++
 2 files changed, 186 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/perf_branches.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_perf_branches.c

diff --git a/tools/testing/selftests/bpf/prog_tests/perf_branches.c b/tools/testing/selftests/bpf/prog_tests/perf_branches.c
new file mode 100644
index 000000000000..54a982a6c513
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/perf_branches.c
@@ -0,0 +1,112 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <pthread.h>
+#include <sched.h>
+#include <sys/socket.h>
+#include <test_progs.h>
+#include "bpf/libbpf_internal.h"
+#include "test_perf_branches.skel.h"
+
+struct output {
+	int required_size;
+	int written_stack;
+	int written_map;
+};
+
+static void on_sample(void *ctx, int cpu, void *data, __u32 size)
+{
+	int pbe_size = sizeof(struct perf_branch_entry);
+	int required_size = ((struct output *)data)->required_size;
+	int written_stack = ((struct output *)data)->written_stack;
+	int written_map = ((struct output *)data)->written_map;
+	int duration = 0;
+
+	/*
+	 * It's hard to validate the contents of the branch entries b/c it
+	 * would require some kind of disassembler and also encoding the
+	 * valid jump instructions for supported architectures. So just check
+	 * the easy stuff for now.
+	 */
+	CHECK(required_size <= 0, "read_branches_size", "err %d\n", required_size);
+	CHECK(written_stack < 0, "read_branches_stack", "err %d\n", written_stack);
+	CHECK(written_stack % pbe_size != 0, "read_branches_stack",
+	      "stack bytes written=%d not multiple of struct size=%d\n",
+	      written_stack, pbe_size);
+	CHECK(written_map < 0, "read_branches_map", "err %d\n", written_map);
+	CHECK(written_map % pbe_size != 0, "read_branches_map",
+	      "map bytes written=%d not multiple of struct size=%d\n",
+	      written_map, pbe_size);
+	CHECK(written_map < written_stack, "read_branches_size",
+	      "written_map=%d < written_stack=%d\n", written_map, written_stack);
+
+	*(int *)ctx = 1;
+}
+
+void test_perf_branches(void)
+{
+	int err, i, pfd = -1, duration = 0, ok = 0;
+	struct perf_buffer_opts pb_opts = {};
+	struct perf_event_attr attr = {};
+	struct perf_buffer *pb;
+	struct bpf_link *link;
+	volatile int j = 0;
+	cpu_set_t cpu_set;
+
+
+	struct test_perf_branches *skel;
+	skel = test_perf_branches__open_and_load();
+	if (CHECK(!skel, "test_perf_branches_load",
+		  "perf_branches skeleton failed\n"))
+		goto out_destroy;
+
+	/* create perf event */
+	attr.size = sizeof(attr);
+	attr.type = PERF_TYPE_HARDWARE;
+	attr.config = PERF_COUNT_HW_CPU_CYCLES;
+	attr.freq = 1;
+	attr.sample_freq = 4000;
+	attr.sample_type = PERF_SAMPLE_BRANCH_STACK;
+	attr.branch_sample_type = PERF_SAMPLE_BRANCH_USER | PERF_SAMPLE_BRANCH_ANY;
+	pfd = syscall(__NR_perf_event_open, &attr, -1, 0, -1, PERF_FLAG_FD_CLOEXEC);
+	if (CHECK(pfd < 0, "perf_event_open", "err %d\n", pfd))
+		goto out_destroy;
+
+	/* attach perf_event */
+	link = bpf_program__attach_perf_event(skel->progs.perf_branches, pfd);
+	if (CHECK(IS_ERR(link), "attach_perf_event", "err %ld\n", PTR_ERR(link)))
+		goto out_close_perf;
+
+	/* set up perf buffer */
+	pb_opts.sample_cb = on_sample;
+	pb_opts.ctx = &ok;
+	pb = perf_buffer__new(bpf_map__fd(skel->maps.perf_buf_map), 1, &pb_opts);
+	if (CHECK(IS_ERR(pb), "perf_buf__new", "err %ld\n", PTR_ERR(pb)))
+		goto out_detach;
+
+	/* generate some branches on cpu 0 */
+	CPU_ZERO(&cpu_set);
+	CPU_SET(0, &cpu_set);
+	err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set);
+	if (CHECK(err, "set_affinity", "cpu #0, err %d\n", err))
+		goto out_free_pb;
+	/* spin the loop for a while (random high number) */
+	for (i = 0; i < 1000000; ++i)
+		++j;
+
+	/* read perf buffer */
+	err = perf_buffer__poll(pb, 500);
+	if (CHECK(err < 0, "perf_buffer__poll", "err %d\n", err))
+		goto out_free_pb;
+
+	if (CHECK(!ok, "ok", "not ok\n"))
+		goto out_free_pb;
+
+out_free_pb:
+	perf_buffer__free(pb);
+out_detach:
+	bpf_link__destroy(link);
+out_close_perf:
+	close(pfd);
+out_destroy:
+	test_perf_branches__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_perf_branches.c b/tools/testing/selftests/bpf/progs/test_perf_branches.c
new file mode 100644
index 000000000000..6811ad5839e7
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_perf_branches.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2019 Facebook
+
+#include <stddef.h>
+#include <linux/ptrace.h>
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_trace_helpers.h"
+
+struct fake_perf_branch_entry {
+	__u64 _a;
+	__u64 _b;
+	__u64 _c;
+};
+
+struct output {
+	int required_size;
+	int written_stack;
+	int written_map;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
+	__uint(key_size, sizeof(int));
+	__uint(value_size, sizeof(int));
+} perf_buf_map SEC(".maps");
+
+typedef struct fake_perf_branch_entry fpbe_t[30];
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, fpbe_t);
+} scratch_map SEC(".maps");
+
+SEC("perf_event")
+int perf_branches(void *ctx)
+{
+	struct fake_perf_branch_entry entries[4] = {0};
+	struct output output = {0};
+	__u32 key = 0, *value;
+
+	/* write to stack */
+	output.written_stack =
+		bpf_perf_prog_read_branches(ctx, entries,
+					    sizeof(entries), 0);
+	/* ignore spurious events */
+	if (!output.written_stack)
+		return 1;
+
+	/* get required size */
+	output.required_size =
+		bpf_perf_prog_read_branches(ctx, NULL, 0, BPF_F_GET_BR_SIZE);
+
+	/* write to map */
+	value = bpf_map_lookup_elem(&scratch_map, &key);
+	if (value)
+		output.written_map =
+			bpf_perf_prog_read_branches(ctx,
+						    value,
+						    30 * sizeof(struct fake_perf_branch_entry),
+						    0);
+
+	/* ignore spurious events */
+	if (!output.written_map)
+		return 1;
+
+	bpf_perf_event_output(ctx, &perf_buf_map, BPF_F_CURRENT_CPU,
+			      &output, sizeof(output));
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.21.1

Re: [PATCH v4 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches() helper

[Alexei Starovoitov]

On Fri, Jan 24, 2020 at 01:17:03PM -0800, Daniel Xu wrote:
> + *
> + * int bpf_perf_prog_read_branches(struct bpf_perf_event_data *ctx, void *buf, u32 buf_size, u64 flags)
> + *	Description
> + *		For an eBPF program attached to a perf event, retrieve the
> + *		branch records (struct perf_branch_entry) associated to *ctx*
> + *		and store it in	the buffer pointed by *buf* up to size
> + *		*buf_size* bytes.
> + *
> + *		The *flags* can be set to **BPF_F_GET_BR_SIZE** to instead
> + *		return the number of bytes required to store all the branch
> + *		entries. If this flag is set, *buf* may be NULL.
> + *	Return
> + *		On success, number of bytes written to *buf*. On error, a
> + *		negative value.
> + *
> + *		**-EINVAL** if arguments invalid or **buf_size** not a multiple
> + *		of sizeof(struct perf_branch_entry).
> + *
> + *		**-ENOENT** if architecture does not support branch records.

the patches look good, but I'm struggling to decode the names.
perf_prog_read_branches... perf_prog means that it's for bpf prog that is of
'perf' type? I don't think any other helper has such prefix.
read branches... branches? they are branch records or branch entries.
How about bpf_read_branch_records() ?

I think BPF_F_GET_BR_SIZE is too cryptic.
How about BPF_F_GET_BRANCH_RECORDS_SIZE ?

Re: [PATCH v4 bpf-next 2/3] tools/bpf: Sync uapi header bpf.h

[Alexei Starovoitov]

On Fri, Jan 24, 2020 at 01:17:04PM -0800, Daniel Xu wrote:
> Sync the header file in a separate commit to help with external sync.
> 
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---
>  tools/include/uapi/linux/bpf.h | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)

fyi we don't have to split tools/ update into separate patch.
Feel free to squash into patch 3 or keep it as-is.