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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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