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

Song Liu
2019-12-12T18:01:58Z

Hi Peter,

> On Dec 12, 2019, at 8:00 AM, Song Liu <songliubraving@fb.com> wrote:
>
>
>
>> On Dec 12, 2019, at 7:45 AM, Song Liu <songliubraving@fb.com> wrote:
>>
>>
>>
>>> On Dec 12, 2019, at 7:39 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>
>>> On Fri, Dec 06, 2019 at 04:24:47PM -0800, Song Liu wrote:
>>>
>>>> @@ -2174,6 +2410,14 @@ __perf_remove_from_context(struct perf_event *event,
>>>> update_cgrp_time_from_cpuctx(cpuctx);
>>>> }
>>>>
>>>> + if (event->dup_master == event) {
>>>> + if (ctx->is_active)
>>>> + ctx_resched(cpuctx, cpuctx->task_ctx,
>>>> + get_event_type(event), NULL, event);
>>>> + else
>>>> + perf_event_remove_dup(event, ctx);
>>>> + }
>>>> +
>>>> event_sched_out(event, cpuctx, ctx);
>>>> if (flags & DETACH_GROUP)
>>>> perf_group_detach(event);
>>>> @@ -2241,6 +2485,14 @@ static void __perf_event_disable(struct perf_event *event,
>>>> update_cgrp_time_from_event(event);
>>>> }
>>>>
>>>> + if (event->dup_master == event) {
>>>> + if (ctx->is_active)
>>>> + ctx_resched(cpuctx, cpuctx->task_ctx,
>>>> + get_event_type(event), NULL, event);
>>>> + else
>>>> + perf_event_remove_dup(event, ctx);
>>>> + }
>>>> +
>>>> if (event == event->group_leader)
>>>> group_sched_out(event, cpuctx, ctx);
>>>> else
>>>
>>>> @@ -2544,7 +2793,9 @@ static void perf_event_sched_in(struct perf_cpu_context *cpuctx,
>>>> */
>>>> static void ctx_resched(struct perf_cpu_context *cpuctx,
>>>> struct perf_event_context *task_ctx,
>>>> - enum event_type_t event_type)
>>>> + enum event_type_t event_type,
>>>> + struct perf_event *event_add_dup,
>>>> + struct perf_event *event_del_dup)
>>>> {
>>>> enum event_type_t ctx_event_type;
>>>> bool cpu_event = !!(event_type & EVENT_CPU);
>>>> @@ -2574,6 +2825,18 @@ static void ctx_resched(struct perf_cpu_context *cpuctx,
>>>> else if (ctx_event_type & EVENT_PINNED)
>>>> cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
>>>>
>>>> + if (event_add_dup) {
>>>> + if (event_add_dup->ctx->is_active)
>>>> + ctx_sched_out(event_add_dup->ctx, cpuctx, EVENT_ALL);
>>>> + perf_event_setup_dup(event_add_dup, event_add_dup->ctx);
>>>> + }
>>>> +
>>>> + if (event_del_dup) {
>>>> + if (event_del_dup->ctx->is_active)
>>>> + ctx_sched_out(event_del_dup->ctx, cpuctx, EVENT_ALL);
>>>> + perf_event_remove_dup(event_del_dup, event_del_dup->ctx);
>>>> + }
>>>> +
>>>> perf_event_sched_in(cpuctx, task_ctx, current);
>>>> perf_pmu_enable(cpuctx->ctx.pmu);
>>>> }
>>>
>>> Yuck!
>>>
>>> Why do you do a full reschedule when you take out a master?
>>
>> If there is active slave using this master, we need to schedule out
>> them before removing the master.
>>
>> We can improve the check though. We only need to do it if the master
>> is in state PERF_EVENT_STATE_ENABLED.
>>
>> Or we can add a different function to only schedule out slaves.
>
> It is tricky to only schedule out slaves, because the slave could be in
> a group. If we don't reschedule all events, we need to make sure that
> "swapping master" always succeed.

What would you suggest for this one? Maybe we can keep this as-is and
optimize later?

Thanks,
Song