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Date:   Fri, 14 Jun 2019 12:10:08 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     Ian Rogers <irogers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] perf cgroups: Don't rotate events for cgroups unnecessarily

On Thu, Jun 13, 2019 at 9:13 AM Liang, Kan <kan.liang@...ux.intel.com> wrote:
>
>
>
> On 6/1/2019 4:27 AM, Ian Rogers wrote:
> > Currently perf_rotate_context assumes that if the context's nr_events !=
> > nr_active a rotation is necessary for perf event multiplexing. With
> > cgroups, nr_events is the total count of events for all cgroups and
> > nr_active will not include events in a cgroup other than the current
> > task's. This makes rotation appear necessary for cgroups when it is not.
> >
> > Add a perf_event_context flag that is set when rotation is necessary.
> > Clear the flag during sched_out and set it when a flexible sched_in
> > fails due to resources.
> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> >   include/linux/perf_event.h |  5 +++++
> >   kernel/events/core.c       | 42 +++++++++++++++++++++++---------------
> >   2 files changed, 30 insertions(+), 17 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 15a82ff0aefe..7ab6c251aa3d 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -747,6 +747,11 @@ struct perf_event_context {
> >       int                             nr_stat;
> >       int                             nr_freq;
> >       int                             rotate_disable;
> > +     /*
> > +      * Set when nr_events != nr_active, except tolerant to events not
> > +      * needing to be active due to scheduling constraints, such as cgroups.
> > +      */
> > +     int                             rotate_necessary;
>
> It looks like the rotate_necessary is only useful for cgroup and cpuctx.
> Why not move it to struct perf_cpu_context and under #ifdef
> CONFIG_CGROUP_PERF?
> And rename it cgrp_rotate_necessary?
>
I am not sure I see the point here. What I'd like to see is a uniform
signal for rotation needed in per-task, per-cpu or per-cgroup modes.
Ian's patch does that. It does make it a lot more efficient in cgroup
mode, by avoiding unnecessary rotations, and does not alter/improve
on any of the other two modes.

> Thanks,
> Kan
>
> >       refcount_t                      refcount;
> >       struct task_struct              *task;
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index abbd4b3b96c2..41ae424b9f1d 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -2952,6 +2952,12 @@ static void ctx_sched_out(struct perf_event_context *ctx,
> >       if (!ctx->nr_active || !(is_active & EVENT_ALL))
> >               return;
> >
> > +     /*
> > +      * If we had been multiplexing, no rotations are necessary now no events
> > +      * are active.
> > +      */
> > +     ctx->rotate_necessary = 0;
> > +
> >       perf_pmu_disable(ctx->pmu);
> >       if (is_active & EVENT_PINNED) {
> >               list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list)
> > @@ -3325,6 +3331,15 @@ static int flexible_sched_in(struct perf_event *event, void *data)
> >                       sid->can_add_hw = 0;
> >       }
> >
> > +     /*
> > +      * If the group wasn't scheduled then set that multiplexing is necessary
> > +      * for the context. Note, this won't be set if the event wasn't
> > +      * scheduled due to event_filter_match failing due to the earlier
> > +      * return.
> > +      */
> > +     if (event->state == PERF_EVENT_STATE_INACTIVE)
> > +             sid->ctx->rotate_necessary = 1;
> > +
> >       return 0;
> >   }
> >
> > @@ -3690,24 +3705,17 @@ ctx_first_active(struct perf_event_context *ctx)
> >   static bool perf_rotate_context(struct perf_cpu_context *cpuctx)
> >   {
> >       struct perf_event *cpu_event = NULL, *task_event = NULL;
> > -     bool cpu_rotate = false, task_rotate = false;
> > -     struct perf_event_context *ctx = NULL;
> > +     struct perf_event_context *task_ctx = NULL;
> > +     int cpu_rotate, task_rotate;
> >
> >       /*
> >        * Since we run this from IRQ context, nobody can install new
> >        * events, thus the event count values are stable.
> >        */
> >
> > -     if (cpuctx->ctx.nr_events) {
> > -             if (cpuctx->ctx.nr_events != cpuctx->ctx.nr_active)
> > -                     cpu_rotate = true;
> > -     }
> > -
> > -     ctx = cpuctx->task_ctx;
> > -     if (ctx && ctx->nr_events) {
> > -             if (ctx->nr_events != ctx->nr_active)
> > -                     task_rotate = true;
> > -     }
> > +     cpu_rotate = cpuctx->ctx.rotate_necessary;
> > +     task_ctx = cpuctx->task_ctx;
> > +     task_rotate = task_ctx ? task_ctx->rotate_necessary : 0;
> >
> >       if (!(cpu_rotate || task_rotate))
> >               return false;
> > @@ -3716,7 +3724,7 @@ static bool perf_rotate_context(struct perf_cpu_context *cpuctx)
> >       perf_pmu_disable(cpuctx->ctx.pmu);
> >
> >       if (task_rotate)
> > -             task_event = ctx_first_active(ctx);
> > +             task_event = ctx_first_active(task_ctx);
> >       if (cpu_rotate)
> >               cpu_event = ctx_first_active(&cpuctx->ctx);
> >
> > @@ -3724,17 +3732,17 @@ static bool perf_rotate_context(struct perf_cpu_context *cpuctx)
> >        * As per the order given at ctx_resched() first 'pop' task flexible
> >        * and then, if needed CPU flexible.
> >        */
> > -     if (task_event || (ctx && cpu_event))
> > -             ctx_sched_out(ctx, cpuctx, EVENT_FLEXIBLE);
> > +     if (task_event || (task_ctx && cpu_event))
> > +             ctx_sched_out(task_ctx, cpuctx, EVENT_FLEXIBLE);
> >       if (cpu_event)
> >               cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
> >
> >       if (task_event)
> > -             rotate_ctx(ctx, task_event);
> > +             rotate_ctx(task_ctx, task_event);
> >       if (cpu_event)
> >               rotate_ctx(&cpuctx->ctx, cpu_event);
> >
> > -     perf_event_sched_in(cpuctx, ctx, current);
> > +     perf_event_sched_in(cpuctx, task_ctx, current);
> >
> >       perf_pmu_enable(cpuctx->ctx.pmu);
> >       perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
> >

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