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

[Namhyung Kim]

On Fri, Aug 2, 2024 at 12:12 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Fri, Aug 02, 2024 at 08:50:23PM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 02, 2024 at 08:43:50PM +0200, Peter Zijlstra wrote:
> > > On Fri, Aug 02, 2024 at 08:38:41PM +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);
> > >
> > > Clearly I should read better...
> > >
> > > > > >               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 yeah, this stops ctx time but not all PMUs.
> >
> > But isn't this already the case? We don't have perf_ctx_disable() here
> > currently.
> >
> > Bah, this heat is melting my brain.
>
> I think all it wants is to update time and ensure the added event and
> the resched all use the same time, which could be done differently.
>
> But I'll have to continue staring at this later.

Right, probably we can move time tracking to pmu_ctx.

thanks,
Namhyung