linux-kernel - Re: [PATCH v6 2/4] perf: support build BPF skeletons with perf

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Date:   Tue, 29 Dec 2020 17:14:12 +0000
From:   Song Liu <songliubraving@...com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
CC:     Namhyung Kim <namhyung@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ingo Molnar" <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>, Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v6 2/4] perf: support build BPF skeletons with perf



> On Dec 29, 2020, at 3:48 AM, Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> 
> Em Tue, Dec 29, 2020 at 04:01:41PM +0900, Namhyung Kim escreveu:
>> On Tue, Dec 29, 2020 at 2:41 AM Song Liu <songliubraving@...com> 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? :-)

Thanks Arnaldo! I don't think anyone can explain it better. :-)

Song

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