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Date:   Mon, 9 Mar 2020 15:53:41 -0700
From:   Yonghong Song <yhs@...com>
To:     Song Liu <songliubraving@...com>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>
CC:     <quentin@...valent.com>, <kernel-team@...com>, <ast@...nel.org>,
        <daniel@...earbox.net>, <arnaldo.melo@...il.com>,
        <jolsa@...nel.org>
Subject: Re: [PATCH v6 bpf-next 1/4] bpftool: introduce "prog profile" command



On 3/9/20 10:32 AM, Song Liu wrote:
> With fentry/fexit programs, it is possible to profile BPF program with
> hardware counters. Introduce bpftool "prog profile", which measures key
> metrics of a BPF program.
> 
> bpftool prog profile command creates per-cpu perf events. Then it attaches
> fentry/fexit programs to the target BPF program. The fentry program saves
> perf event value to a map. The fexit program reads the perf event again,
> and calculates the difference, which is the instructions/cycles used by
> the target program.
> 
> Example input and output:
> 
>    ./bpftool prog profile id 337 duration 3 cycles instructions llc_misses
> 
>          4228 run_cnt
>       3403698 cycles                                              (84.08%)
>       3525294 instructions   #  1.04 insn per cycle               (84.05%)
>            13 llc_misses     #  3.69 LLC misses per million isns  (83.50%)
> 
> This command measures cycles and instructions for BPF program with id
> 337 for 3 seconds. The program has triggered 4228 times. The rest of the
> output is similar to perf-stat. In this example, the counters were only
> counting ~84% of the time because of time multiplexing of perf counters.
> 
> Note that, this approach measures cycles and instructions in very small
> increments. So the fentry/fexit programs introduce noticeable errors to
> the measurement results.
> 
> The fentry/fexit programs are generated with BPF skeletons. Therefore, we
> build bpftool twice. The first time _bpftool is built without skeletons.
> Then, _bpftool is used to generate the skeletons. The second time, bpftool
> is built with skeletons.
> 
> Reviewed-by: Quentin Monnet <quentin@...valent.com>
> Signed-off-by: Song Liu <songliubraving@...com>

Acked-by: Yonghong Song <yhs@...com>

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