Re: [PATCH V2 3/3] perf: Optimize sched_task() in a context switch

[Namhyung Kim]

Hi Peter and Kan,

How can we move this forward?

Thanks,
Namhyung

On Fri, Dec 4, 2020 at 4:14 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> Hi Peter,
>
> On Wed, Dec 2, 2020 at 2:29 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, Nov 30, 2020 at 11:38:42AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> > > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > >
> > > Some calls to sched_task() in a context switch can be avoided. For
> > > example, large PEBS only requires flushing the buffer in context switch
> > > out. The current code still invokes the sched_task() for large PEBS in
> > > context switch in.
> >
> > I still hate this one, how's something like this then?
> > Which I still don't really like.. but at least its simpler.
> >
> > (completely untested, may contain spurious edits, might ICE the
> > compiler and set your pets on fire if it doesn't)
>
> I've tested this version... and it worked well besides the optimization.. :)
>
> [SNIP]
> > +static void context_sched_task(struct perf_cpu_context *cpuctx,
> > +                              struct perf_event_context *ctx,
> > +                              bool sched_in)
> > +{
> > +       struct pmu *pmu = ctx->pmu;
> > +
> > +       if (cpuctx->sched_cb_dir[sched_in] && pmu->sched_task)
> > +               pmu->sched_task(ctx, false);
>
> applied: s/false/sched_in/
>
>
> > +}
> > +
> >  static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
> >                                          struct task_struct *next)
> >  {
> > @@ -3424,9 +3433,7 @@ static void perf_event_context_sched_out
> >                         WRITE_ONCE(next_ctx->task, task);
> >
> >                         perf_pmu_disable(pmu);
> > -
> > -                       if (cpuctx->sched_cb_usage && pmu->sched_task)
> > -                               pmu->sched_task(ctx, false);
> > +                       context_sched_task(cpuctx, ctx, false);
> >
> >                         /*
> >                          * PMU specific parts of task perf context can require
> > @@ -3465,8 +3472,7 @@ static void perf_event_context_sched_out
> >                 raw_spin_lock(&ctx->lock);
> >                 perf_pmu_disable(pmu);
> >
> > -               if (cpuctx->sched_cb_usage && pmu->sched_task)
> > -                       pmu->sched_task(ctx, false);
> > +               context_sched_task(cpuctx, ctx, false);
> >                 task_ctx_sched_out(cpuctx, ctx, EVENT_ALL);
> >
> >                 perf_pmu_enable(pmu);
>
> [SNIP]
> > @@ -3563,8 +3582,7 @@ void __perf_event_task_sched_out(struct
> >  {
> >         int ctxn;
> >
> > -       if (__this_cpu_read(perf_sched_cb_usage))
> > -               perf_pmu_sched_task(task, next, false);
> > +       perf_pmu_sched_task(task, next, false);
>
> I think the reason is this change.  It now calls perf_pmu_sched_task()
> without checking the counter.  And this is for per-cpu events.
>
> >
> >         if (atomic_read(&nr_switch_events))
> >                 perf_event_switch(task, next, false);
> > @@ -3828,8 +3846,7 @@ static void perf_event_context_sched_in(
> >                 cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
> >         perf_event_sched_in(cpuctx, ctx, task);
> >
> > -       if (cpuctx->sched_cb_usage && pmu->sched_task)
> > -               pmu->sched_task(cpuctx->task_ctx, true);
> > +       context_sched_task(cpuctx, ctx, true);
> >
> >         perf_pmu_enable(pmu);
> >
> > @@ -3875,8 +3892,7 @@ void __perf_event_task_sched_in(struct t
> >         if (atomic_read(&nr_switch_events))
> >                 perf_event_switch(task, prev, true);
> >
> > -       if (__this_cpu_read(perf_sched_cb_usage))
> > -               perf_pmu_sched_task(prev, task, true);
> > +       perf_pmu_sched_task(prev, task, true);
>
> Ditto.
>
> >  }
> >
> >  static u64 perf_calculate_period(struct perf_event *event, u64 nsec, u64 count)
>
> So I made a change like below.. and it could bring the optimization back.
>
> Thanks,
> Namhyung
>
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 9107e7c3ccfb..a30243a9fab5 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3528,6 +3528,9 @@ static void __perf_pmu_sched_task(struct
> perf_cpu_context *cpuctx, bool sched_in
>  {
>         struct pmu *pmu;
>
> +       if (!cpuctx->sched_cb_dir[sched_in])
> +               return;
> +
>         pmu = cpuctx->ctx.pmu; /* software PMUs will not have sched_task */
>
>         if (WARN_ON_ONCE(!pmu->sched_task))