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Date: Fri, 14 Jun 2019 17:43:24 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Stephane Eranian <eranian@...gle.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 6/14/2019 3:10 PM, Stephane Eranian wrote:
> 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.
I just thought if it is only used by cgroup, it may be better to move it
under #ifdef CONFIG_CGROUP_PERF. For users who don't care about cgroup,
it may save some space.
But if it's designed as a uniform signal, this is OK.
Thanks,
Kan
>
>> 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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