netdev - Re: [PATCH v2 bpf-next 1/2] bpftool: introduce "prog profile" command

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Message-ID: <D919D5EE-2447-4B2F-B343-9F24B69FAB26@fb.com>
Date:   Mon, 2 Mar 2020 16:52:15 +0000
From:   Song Liu <songliubraving@...com>
To:     Quentin Monnet <quentin@...valent.com>, Yonghong Song <yhs@...com>
CC:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        "Kernel Team" <Kernel-team@...com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "arnaldo.melo@...il.com" <arnaldo.melo@...il.com>,
        "jolsa@...nel.org" <jolsa@...nel.org>
Subject: Re: [PATCH v2 bpf-next 1/2] bpftool: introduce "prog profile" command



> On Mar 2, 2020, at 5:36 AM, Quentin Monnet <quentin@...valent.com> wrote:
> 
> Hi Song,
> 
> Thanks for this work! Some remarks (mostly nitpicks) inline.
> 
> 2020-02-28 15:40 UTC-0800 ~ Song Liu <songliubraving@...com>
>> 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 3 id 337 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.
>> 
>> Signed-off-by: Song Liu <songliubraving@...com>
>> ---

Thanks Yonghong and Quentin! I will fix these and send v3. 

Song