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

[Peter Zijlstra <peterz@infradead.org>]

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.