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

[Namhyung Kim <namhyung@kernel.org>]

Hi Peter,

On Wed, Dec 2, 2020 at 2:29 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Nov 30, 2020 at 11:38:42AM -0800, kan.liang@linux.intel.com wrote:
> > From: Kan Liang <kan.liang@linux.intel.com>
> >
> > 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))