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

[Tengda Wu]

On 2024/10/10 8:31, Namhyung Kim wrote:
> On Wed, Oct 09, 2024 at 10:18:44AM -0700, Song Liu wrote:
>> On Sun, Sep 15, 2024 at 6:53 PM Tengda Wu <wutengda@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
>>
>> 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).
> 
> I think it should depend on the value of attr.inherit.  Maybe we can
> disable the autoload for !inherit.
> 

Got it. The attr.inherit flag(related to --no-inherit in perf command)
is suitable for controlling inherit behavior. I will fix it. Thanks!

>>
>> 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?
> 
> I'm not sure.  I guess it still needs to use the accum_key to save the
> final value when exited = 1.

In theory, it is possible. The accum_key is currently only used to index value
in accum_readings map, so if the task is not being counted, the accum_key can
be set to an special value.

Due to accum_key is of u32 type, there are two special values to choose from: 0
or max_entries+1. I think the latter, max_entries+1, may be more suitable because
it can avoid memory waste in the accum_readings map and does not require too
many changes to bpf_counter.

Thanks for your kindly review!
Tengda

> 
> Thanks,
> Namhyung
> 
>>
>>> +
>>>  #endif /* __BPERF_STAT_U_H */
>>> --
>>> 2.34.1
>>>