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Date: Mon, 8 Aug 2016 14:12:46 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>
Subject: Re: [PATCH] perf: sched out groups atomically
Message-ID: <20160808121246.GD6879@twins.programming.kicks-ass.net>
References: <1469553141-28314-1-git-send-email-mark.rutland@arm.com>
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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!