Re: [PATCH v8] perf: Sharing PMU counters across compatible events

Song Liu
2019-12-12T15:49:57Z

> On Dec 12, 2019, at 7:44 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Dec 12, 2019 at 03:37:05PM +0000, Song Liu wrote:
>>
>>
>>> On Dec 12, 2019, at 5:49 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>
>>> On Fri, Dec 06, 2019 at 04:24:47PM -0800, Song Liu wrote:
>>>
>>>> @@ -750,6 +752,16 @@ struct perf_event {
>>>> void *security;
>>>> #endif
>>>> struct list_head sb_list;
>>>> +
>>>> + /* for PMU sharing */
>>>> + struct perf_event *dup_master;
>>>> + /* check event_sync_dup_count() for the use of dup_base_* */
>>>> + u64 dup_base_count;
>>>> + u64 dup_base_child_count;
>>>> + /* when this event is master, read from master*count */
>>>> + local64_t master_count;
>>>> + atomic64_t master_child_count;
>>>> + int dup_active_count;
>>>> #endif /* CONFIG_PERF_EVENTS */
>>>> };
>>>
>>>> +/* PMU sharing aware version of event->pmu->add() */
>>>> +static int event_pmu_add(struct perf_event *event,
>>>> + struct perf_event_context *ctx)
>>>> +{
>>>> + struct perf_event *master;
>>>> + int ret;
>>>> +
>>>> + /* no sharing, just do event->pmu->add() */
>>>> + if (!event->dup_master)
>>>> + return event->pmu->add(event, PERF_EF_START);
>>>
>>> Possibly we should look at the location of perf_event::dup_master to be
>>> in a hot cacheline. Because I'm thinking you just added a guaranteed
>>> miss here.
>>
>> I am not quite sure the best location for these. How about:
>>
>> diff --git i/include/linux/perf_event.h w/include/linux/perf_event.h
>> index 7d49f9251621..218cc7f75775 100644
>> --- i/include/linux/perf_event.h
>> +++ w/include/linux/perf_event.h
>> @@ -643,6 +643,16 @@ struct perf_event {
>> local64_t count;
>> atomic64_t child_count;
>>
>> + /* for PMU sharing */
>> + struct perf_event *dup_master;
>> + /* check event_sync_dup_count() for the use of dup_base_* */
>> + u64 dup_base_count;
>> + u64 dup_base_child_count;
>> + /* when this event is master, read from master*count */
>> + local64_t master_count;
>> + atomic64_t master_child_count;
>> + int dup_active_count;
>> +
>> /*
>> * These are the total time in nanoseconds that the event
>> * has been enabled (i.e. eligible to run, and the task has
>
> Ah, no. Only put dup_master somewhere hot. The rest is not that
> important. For instance, you can put it right next to event->pmu,
> because that's going to be used right next to it, right?

I see. Will fix in that way.

Thanks,
Song