linux-kernel - Re: [PATCH] perf cgroups: Don't rotate events for cgroups unnecessarily

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Date:   Mon, 24 Jun 2019 09:55:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     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>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf cgroups: Don't rotate events for cgroups
 unnecessarily

On Fri, Jun 21, 2019 at 11:01:29AM -0700, Ian Rogers wrote:
> On Fri, Jun 21, 2019 at 1:24 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Sat, Jun 01, 2019 at 01:27:22AM -0700, Ian Rogers wrote:
> > > @@ -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;
> > >  }
> >
> > That looked odd; which had me look harder at this function which
> > resulted in the below. Should we not terminate the context interation
> > the moment one flexible thingy fails to schedule?
> 
> If we knew all the events were hardware events then this would be
> true, as there may be software events that always schedule then the
> continued iteration is necessary.

But this is the 'old' code, where this is guaranteed by the context.
That is, if this is a hardware context; there wil only be software
events due to them being in a group with hardware events.

If this is a software group, then we'll never fail to schedule and we'll
not get in this branch to begin with.

Or am I now confused for having been staring at two different code-bases
at the same time?