linux-kernel - Re: [PATCH] perf: sched out groups atomically

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Message-ID: <20160808121246.GD6879@twins.programming.kicks-ass.net>
Date:	Mon, 8 Aug 2016 14:12:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: Re: [PATCH] perf: sched out groups atomically

On Tue, Jul 26, 2016 at 06:12:21PM +0100, Mark Rutland wrote:
> Groups of events are supposed to be scheduled atomically, such that it
> is possible to derive meaningful ratios between their values.
> 
> We take great pains to achieve this when scheduling event groups to a
> PMU in group_sched_in(), calling {start,commit}_txn() (which fall back
> to perf_pmu_{disable,enable}() if necessary) to provide this guarantee.
> However we don't mirror this in group_sched_out(), and in some cases
> events will not be scheduled out atomically.
> 
> For example, if we disable an event group with PERF_EVENT_IOC_DISABLE,
> we'll cross-call __perf_event_disable() for the group leader, and will
> call group_sched_out() without having first disabled the relevant PMU.
> We will disable/enable the PMU around each pmu->del() call, but between
> each call the PMU will be enabled and events may count.
> 
> Avoid this by explicitly disabling and enabling the PMU around event
> removal in group_sched_out(), mirroring what we do in group_sched_in().

Thanks!