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

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Message-ID: <20240802185023.GB12673@noisy.programming.kicks-ass.net>
Date: Fri, 2 Aug 2024 20:50:23 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Liang, Kan" <kan.liang@...ux.intel.com>
Cc: Namhyung Kim <namhyung@...nel.org>, Ingo Molnar <mingo@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ravi Bangoria <ravi.bangoria@....com>,
	Stephane Eranian <eranian@...gle.com>,
	Ian Rogers <irogers@...gle.com>, Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v2] perf/core: Optimize event reschedule for a PMU

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.