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

[Peter Zijlstra]

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@xxxxxxxxxxxxx> 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?