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

[Peter Zijlstra <peterz@infradead.org>]

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.