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

Song Liu
2019-12-12T16:00:54Z

> 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.

Song