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

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Date:   Sun, 1 Mar 2020 22:37:41 +0000
From:   Song Liu <songliubraving@...com>
To:     Yonghong Song <yhs@...com>
CC:     Networking <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <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 Feb 29, 2020, at 8:14 PM, Yonghong Song <yhs@...com> wrote:
> 
> 
> 
> On 2/28/20 3:40 PM, 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 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%)
> 
> if run_cnt is 0, the following is the result:
> 
> -bash-4.4$ sudo ./bpftool prog profile 3 id 52 cycles instructions llc_misses
> 
>                 0 run_cnt
>                 0 cycles 
>                 0 instructions        #     -nan insn per cycle 
>                 0 llc_misses          #     -nan LLC misses per million isns
> 
> -nan is a little bit crypto for user output. maybe just says
>  unknown insns per cycle
> in the comment?
> 
> We can still display "0 cycles" etc. just to make output uniform.

Good catch! Will fix this in next version. 

Song