linux-kernel - Re: [PATCH V8 01/12] perf/core: Add aux_pause, aux_resume, aux_start

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Message-ID: <20240711115123.GK4587@noisy.programming.kicks-ass.net>
Date: Thu, 11 Jul 2024 13:51:23 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: Ingo Molnar <mingo@...hat.com>, Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Thomas Richter <tmricht@...ux.ibm.com>,
	Hendrik Brueckner <brueckner@...ux.ibm.com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Mike Leach <mike.leach@...aro.org>,
	James Clark <james.clark@....com>, coresight@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org,
	Yicong Yang <yangyicong@...ilicon.com>,
	Jonathan Cameron <jonathan.cameron@...wei.com>,
	Will Deacon <will@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>, Andi Kleen <ak@...ux.intel.com>,
	linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH V8 01/12] perf/core: Add aux_pause, aux_resume,
 aux_start_paused

On Thu, Jul 11, 2024 at 01:42:33PM +0300, Adrian Hunter wrote:
> On 1/07/24 13:26, Peter Zijlstra wrote:
> > On Fri, Jun 28, 2024 at 09:51:00AM +0300, Adrian Hunter wrote:
> > 
> >> Add aux_paused to struct perf_event for AUX area events to keep track of
> >> the "paused" state. aux_paused is initialized to aux_start_paused.
> > 	
> >> @@ -798,6 +810,9 @@ struct perf_event {
> >>  	/* for aux_output events */
> >>  	struct perf_event		*aux_event;
> >>  
> >> +	/* for AUX area events */
> >> +	unsigned int			aux_paused;
> >> +
> >>  	void (*destroy)(struct perf_event *);
> >>  	struct rcu_head			rcu_head;
> >>  
> > 
> > Should this not be part of struct hw_perf_event for whatever hw event
> > implements this AUX stuff?
> > 
> > In fact, I would expect PERF_HES_PAUSED or something to go in
> > perf_event::hw::state.
> 
> Sorry for the slow reply due to vacation.
> 
> There doesn't seem to be anything preventing pause/resume from
> "racing" (PMI vs task context) with changes in the aux event state,
> so aux_paused must be separate from perf_event::hw::state.

So normally, context switching perf would be under pmu_disable(), which
should ensure no PMI happens while we're reprogramming the PMU.

That said, I think for the aux case we have perf_event_stop() that can
race vs PMI, so yeah, bummer.

Sticking it in hw_perf_event would be good though. Also perhaps add a
comment about exactly why this cannot be in ::state.