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

[Peter Zijlstra <peterz@infradead.org>]

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.