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SubjectRe: [PATCH v6 2/4] perf: support build BPF skeletons with perf
Em Tue, Dec 29, 2020 at 04:01:41PM +0900, Namhyung Kim escreveu:
> On Tue, Dec 29, 2020 at 2:41 AM Song Liu <> wrote:
> > BPF programs are useful in perf to profile BPF programs. BPF skeleton is

> I'm having difficulties understanding the first sentence - looks like a
> recursion. :) So do you want to use two (or more) BPF programs?

Yeah, we use perf to perf perf, so we need to use bpf with perf to perf
bpf :-)

Look at tools/perf/util/bpf_skel/bpf_prog_profiler.bpf.c, the BPF
skeleton used to create the in-kernel scaffold to profile BPF programs.

It uses two BPF programs (fentry/XXX and fexit/XXX) and some a
PERF_EVENT_ARRAY map and an array to diff counters read at exit from
counters read at exit of the profiled BPF programs and then accumulate
those diffs in another PERCPU_ARRAY.

This all ends up composing a "BPF PMU" that is what the userspace perf
tooling will read (from "accum_readings" BPF map) and 'perf stat' will
consume as if reading from an "old style perf counter" :-)

Song, did I get it right? :-)

For convenience, it is below:

- Arnaldo

[acme@five perf]$ cat tools/perf/util/bpf_skel/bpf_prog_profiler.bpf.c
// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
// Copyright (c) 2020 Facebook
#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>

/* map of perf event fds, num_cpu * num_metric entries */
struct {
__uint(key_size, sizeof(__u32));
__uint(value_size, sizeof(int));
} events SEC(".maps");

/* readings at fentry */
struct {
__uint(key_size, sizeof(__u32));
__uint(value_size, sizeof(struct bpf_perf_event_value));
__uint(max_entries, 1);
} fentry_readings SEC(".maps");

/* accumulated readings */
struct {
__uint(key_size, sizeof(__u32));
__uint(value_size, sizeof(struct bpf_perf_event_value));
__uint(max_entries, 1);
} accum_readings SEC(".maps");

const volatile __u32 num_cpu = 1;

int BPF_PROG(fentry_XXX)
__u32 key = bpf_get_smp_processor_id();
struct bpf_perf_event_value *ptr;
__u32 zero = 0;
long err;

/* look up before reading, to reduce error */
ptr = bpf_map_lookup_elem(&fentry_readings, &zero);
if (!ptr)
return 0;

err = bpf_perf_event_read_value(&events, key, ptr, sizeof(*ptr));
if (err)
return 0;

return 0;

static inline void
fexit_update_maps(struct bpf_perf_event_value *after)
struct bpf_perf_event_value *before, diff, *accum;
__u32 zero = 0;

before = bpf_map_lookup_elem(&fentry_readings, &zero);
/* only account samples with a valid fentry_reading */
if (before && before->counter) {
struct bpf_perf_event_value *accum;

diff.counter = after->counter - before->counter;
diff.enabled = after->enabled - before->enabled;
diff.running = after->running - before->running;

accum = bpf_map_lookup_elem(&accum_readings, &zero);
if (accum) {
accum->counter += diff.counter;
accum->enabled += diff.enabled;
accum->running += diff.running;

int BPF_PROG(fexit_XXX)
struct bpf_perf_event_value reading;
__u32 cpu = bpf_get_smp_processor_id();
__u32 one = 1, zero = 0;
int err;

/* read all events before updating the maps, to reduce error */
err = bpf_perf_event_read_value(&events, cpu, &reading, sizeof(reading));
if (err)
return 0;

return 0;

char LICENSE[] SEC("license") = "Dual BSD/GPL";
[acme@five perf]$

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