Re: [PATCH -next v3 1/2] perf stat: Support inherit events during fork() for bperf

Song Liu
2024-10-09T17:18:56Z

On Sun, Sep 15, 2024 at 6:53 PM Tengda Wu <wutengda@huaweicloud.com> wrote:
>
> bperf has a nice ability to share PMUs, but it still does not support
> inherit events during fork(), resulting in some deviations in its stat
> results compared with perf.
>
> perf stat result:
> $ ./perf stat -e cycles,instructions -- ./perf test -w sqrtloop
>
> Performance counter stats for './perf test -w sqrtloop':
>
> 2,316,038,116 cycles
> 2,859,350,725 instructions
>
> 1.009603637 seconds time elapsed
>
> 1.004196000 seconds user
> 0.003950000 seconds sys
>
> bperf stat result:
> $ ./perf stat --bpf-counters -e cycles,instructions -- \
> ./perf test -w sqrtloop
>
> Performance counter stats for './perf test -w sqrtloop':
>
> 18,762,093 cycles
> 23,487,766 instructions
>
> 1.008913769 seconds time elapsed
>
> 1.003248000 seconds user
> 0.004069000 seconds sys
>
> In order to support event inheritance, two new bpf programs are added
> to monitor the fork and exit of tasks respectively. When a task is
> created, add it to the filter map to enable counting, and reuse the
> `accum_key` of its parent task to count together with the parent task.
> When a task exits, remove it from the filter map to disable counting.
>
> After support:
> $ ./perf stat --bpf-counters -e cycles,instructions -- \
> ./perf test -w sqrtloop
>
> Performance counter stats for './perf test -w sqrtloop':
>
> 2,316,252,189 cycles
> 2,859,946,547 instructions
>
> 1.009422314 seconds time elapsed
>
> 1.003597000 seconds user
> 0.004270000 seconds sys
>
> Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>

The solution looks good to me. Question on the UI: do we always
want the inherit behavior from PID and TGID monitoring? If not,
maybe we should add a flag for it. (I think we do need the flag).

One nitpick below.

Thanks,
Song

[...]
>
> +struct bperf_filter_value {
> + __u32 accum_key;
> + __u8 exited;
> +};
nit:
Can we use a special value of accum_key to replace exited==1
case?

> +
> #endif /* __BPERF_STAT_U_H */
> --
> 2.34.1
>