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SubjectRe: [RFC 2/2] perf: Sharing PMU counters across compatible events
Date
On May 28, 2018, at 4:24 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, May 04, 2018 at 04:11:02PM -0700, Song Liu wrote:
>> On the critical paths, perf_events are added to/removed from the
>> active_dup list of the perf_event. The first event added to the list
>> will be the master event, and the only event that runs pmu->add().
>> Later events will all refer to this master for read().
>>
>> cpuctx -> perf_event_dup -> master
>> ^ -> active_dup (list)
>> | ^ ^
>> perf_event /| ----------/ |
>> | |
>> perf_event / -------------/
>>
>
>> +static void add_event_to_dup_event_list(struct perf_event *event,
>> + struct perf_cpu_context *cpuctx)
>> +{
>> + int i;
>> +
>> + for (i = 0; i < cpuctx->dup_event_count; ++i)
>> + if (memcmp(&event->attr,
>> + &cpuctx->dup_event_list[i].first->attr,
>> + sizeof(event->attr)) == 0) {
>> + event->dup_id = i;
>> + return;
>> + }
>
> (style nit: this needs {})
>
> So we merge events when the attr's are an exact match; which includes
> sampling and all those fancy things, right?

I think we will need better analysis on which events could share the
same PMU. I will refine it in the next version.

>
> I think this scheme causes phase shifts in the samples when we combine
> two otherwise identical events. Because while they have the same
> sampling interval, they might not have the same effective runtime and
> thus have a different 'remainder' for the current sample interval.
>
> This could add up to a significant sample skew for unlucky
> circumstances. On average I think it works out, but if you're always
> landing on a shorter interval, the effective sample rate can go up
> significantly.

Maybe we can somehow shift the reminder here? Let me think more about
this. Thanks for the feedback!

Thanks,
Song

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