Re: [PATCH v2] perf/core: Optimize event reschedule for a PMU

[Namhyung Kim]

On Mon, Aug 5, 2024 at 7:58 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Aug 05, 2024 at 11:20:58AM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 02, 2024 at 02:30:19PM -0400, Liang, Kan wrote:
> > > > @@ -2792,7 +2833,14 @@ static int  __perf_install_in_context(void *info)
> > > >   if (reprogram) {
> > > >           ctx_sched_out(ctx, EVENT_TIME);
> > > >           add_event_to_ctx(event, ctx);
> > > > -         ctx_resched(cpuctx, task_ctx, get_event_type(event));
> > > > +         if (ctx->nr_events == 1) {
> > > > +                 /* The first event needs to set ctx->is_active. */
> > > > +                 ctx_resched(cpuctx, task_ctx, NULL, get_event_type(event));
> > > > +         } else {
> > > > +                 ctx_resched(cpuctx, task_ctx, event->pmu_ctx->pmu,
> > > > +                             get_event_type(event));
> > > > +                 ctx_sched_in(ctx, EVENT_TIME);
> > >
> > > The changelog doesn't mention the time difference much. As my
> > > understanding, the time is shared among PMUs in the same ctx.
> > > When perf does ctx_resched(), the time is deducted.
> > > There is no problem to stop and restart the global time when perf
> > > re-schedule all PMUs.
> > > But if only one PMU is re-scheduled while others are still running, it
> > > may be a problem to stop and restart the global time. Other PMUs will be
> > > impacted.
> >
> > So afaict, since we hold ctx->lock, nobody can observe EVENT_TIME was
> > cleared for a little while.
> >
> > So the point was to make all the various ctx_sched_out() calls have the
> > same timestamp. It does this by clearing EVENT_TIME first. Then the
> > first ctx_sched_in() will set it again, and later ctx_sched_in() won't
> > touch time.
> >
> > That leaves a little hole, because the time between
> > ctx_sched_out(EVENT_TIME) and the first ctx_sched_in() gets lost.
> >
> > This isn't typically a problem, but not very nice. Let me go find an
> > alternative solution for this. The simple update I did saturday is
> > broken as per the perf test.
>
> OK, took a little longer than I would have liked, nor is it entirely
> pretty, but it seems to pass 'perf test'.
>
> Please look at: queue.git perf/resched
>
> I'll try and post it all tomorrow.

Thanks for doing this.  But some of my tests are still failing.
I'm seeing some system-wide events are not counted.
Let me take a deeper look at it.

Thanks,
Namhyung