linux-kernel - [17/70] ARM: 6989/1: perf: do not start the PMU when no events are present

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Date:	Mon, 01 Aug 2011 16:18:32 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Will Deacon <will.deacon@....com>,
	Russell King <rmk+kernel@....linux.org.uk>
Subject: [17/70] ARM: 6989/1: perf: do not start the PMU when no events are present

2.6.39-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Will Deacon <will.deacon@....com>

commit f4f38430c94c38187db73a2cf3892cc8b12a2713 upstream.

armpmu_enable can be called in situations where no events are present
(for example, from the event rotation tick after a profiled task has
exited). In this case, we currently start the PMU anyway which may
leave it active inevitably without any events being monitored.

This patch adds a simple check to the enabling code so that we avoid
starting the PMU when no events are present.

Reported-by: Ashwin Chaugle <ashwinc@...eaurora.org>
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 arch/arm/kernel/perf_event.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/arch/arm/kernel/perf_event.c
+++ b/arch/arm/kernel/perf_event.c
@@ -588,7 +588,7 @@ static int armpmu_event_init(struct perf
 static void armpmu_enable(struct pmu *pmu)
 {
 	/* Enable all of the perf events on hardware. */
-	int idx;
+	int idx, enabled = 0;
 	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
 
 	if (!armpmu)
@@ -601,9 +601,11 @@ static void armpmu_enable(struct pmu *pm
 			continue;
 
 		armpmu->enable(&event->hw, idx);
+		enabled = 1;
 	}
 
-	armpmu->start();
+	if (enabled)
+		armpmu->start();
 }
 
 static void armpmu_disable(struct pmu *pmu)


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