linux-kernel - Re: [RFC 1/2] perf/core: Enable sched

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Date:   Fri, 6 Nov 2020 00:45:02 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>, Gabriel Marin <gmx@...gle.com>
Subject: Re: [RFC 1/2] perf/core: Enable sched_task callbacks if PMU has it

Hello,

On Thu, Nov 5, 2020 at 11:47 PM Liang, Kan <kan.liang@...ux.intel.com> wrote:
>
>
>
> On 11/2/2020 9:52 AM, Namhyung Kim wrote:
> > If an event associated with a PMU which has a sched_task callback,
> > it should be called regardless of cpu/task context.  For example,
>
>
> I don't think it's necessary. We should call it when we have to.
> Otherwise, it just waste cycles.
> Shouldn't the patch 2 be enough?

I'm not sure, without this patch __perf_event_task_sched_in/out
cannot be called for per-cpu events (w/o cgroups)  IMHO.
And I could not find any other place to check the
perf_sched_cb_usages.

Thanks
Namhyung


>
> > a per-cpu event might enable large PEBS buffers so it needs to flush
> > the buffer whenever task scheduling happens. >
> > The underlying PMU may or may not require this for the given event,
> > but it will be handled in the pmu::sched_task() callback anyway.
> >
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> >   kernel/events/core.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index b458ed3dc81b..aaa0155c4142 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -4696,6 +4696,8 @@ static void unaccount_event(struct perf_event *event)
> >               dec = true;
> >       if (has_branch_stack(event))
> >               dec = true;
> > +     if (event->pmu->sched_task)
> > +             dec = true;
> >       if (event->attr.ksymbol)
> >               atomic_dec(&nr_ksymbol_events);
> >       if (event->attr.bpf_event)
> > @@ -11225,6 +11227,8 @@ static void account_event(struct perf_event *event)
> >               inc = true;
> >       if (is_cgroup_event(event))
> >               inc = true;
> > +     if (event->pmu->sched_task)
> > +             inc = true;
> >       if (event->attr.ksymbol)
> >               atomic_inc(&nr_ksymbol_events);
> >       if (event->attr.bpf_event)
> >