linux-kernel - Re: [PATCH v2] perf/core: Optimize event reschedule for a PMU

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Message-ID: <CAM9d7ciSJXFC4Wdioy2PxZNCd+6-+kcA6+qqJs-BvQ3yAgQmAg@mail.gmail.com>
Date: Tue, 6 Aug 2024 12:29:01 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Liang, Kan" <kan.liang@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Ravi Bangoria <ravi.bangoria@....com>, Stephane Eranian <eranian@...gle.com>, 
	Ian Rogers <irogers@...gle.com>, Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v2] perf/core: Optimize event reschedule for a PMU

On Tue, Aug 6, 2024 at 1:08 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Aug 06, 2024 at 09:56:30AM +0200, Peter Zijlstra wrote:
>
> > Does this help? What would be an easy reproducer?

Yep, it helps!  I guess you can reproduce it easily with:

  $ sudo perf stat -a sleep 1

> >
> > ---
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index c67fc43fe877..4a04611333d9 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -179,23 +179,27 @@ static void perf_ctx_lock(struct perf_cpu_context *cpuctx,
> >       }
> >  }
> >
> > +static inline void __perf_ctx_unlock(struct perf_event_context *ctx)
> > +{
> > +     /*
> > +      * If ctx_sched_in() didn't again set any ALL flags, clean up
> > +      * after ctx_sched_out() by clearing is_active.
> > +      */
> > +     if (ctx->is_active & EVENT_FROZEN) {
> > +             if (!(ctx->is_active & EVENT_ALL))
> > +                     ctx->is_active = 0;
> > +             else
> > +                     ctx->is_active &= ~EVENT_FROZEN;
> > +     }
> > +     raw_spin_unlock(&ctx->lock);
> > +}
> > +
> >  static void perf_ctx_unlock(struct perf_cpu_context *cpuctx,
> >                           struct perf_event_context *ctx)
> >  {
> > -     if (ctx) {
> > -             /*
> > -              * If ctx_sched_in() didn't again set any ALL flags, clean up
> > -              * after ctx_sched_out() by clearing is_active.
> > -              */
> > -             if (ctx->is_active & EVENT_FROZEN) {
> > -                     if (!(ctx->is_active & EVENT_ALL))
> > -                             ctx->is_active = 0;
> > -                     else
> > -                             ctx->is_active &= ~EVENT_FROZEN;
> > -             }
> > -             raw_spin_unlock(&ctx->lock);
> > -     }
> > -     raw_spin_unlock(&cpuctx->ctx.lock);
> > +     if (ctx)
> > +             __perf_ctx_unlock(ctx);
> > +     __perf_ctx_unlock(&cpuctx->ctx.lock);
>
> Obviously that wants to be just: &cpuctx->ctx :-)

Sure.  And I think we need this for perf_ctx_lock() too.

Thanks,
Namhyung

>
> >  }
> >
> >  #define TASK_TOMBSTONE ((void *)-1L)