linux-kernel - [PATCH] [46/50] ARM: 6989/1: perf: do not start the PMU when no events are

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Date:	Thu, 28 Jul 2011 16:44:51 -0700 (PDT)
From:	Andi Kleen <andi@...stfloor.org>
To:	will.deacon@....com, rmk+kernel@....linux.org.uk,
	ak@...ux.intel.com, linux-kernel@...r.kernel.org,
	stable@...nel.org, tim.bird@...sony.com
Subject: [PATCH] [46/50] ARM: 6989/1: perf: do not start the PMU when no events are

2.6.35-longterm review patch.  If anyone has any objections, please let me know.

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

[ upstream commit f4f38430c94c38187db73a2cf3892cc8b12a2713 ]
 present

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.

Cc: <stable@...nel.org>
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: Andi Kleen <ak@...ux.intel.com>

Index: linux-2.6.35.y/arch/arm/kernel/perf_event.c
===================================================================
--- linux-2.6.35.y.orig/arch/arm/kernel/perf_event.c
+++ linux-2.6.35.y/arch/arm/kernel/perf_event.c
@@ -531,7 +531,7 @@ void
 hw_perf_enable(void)
 {
 	/* 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)
@@ -544,9 +544,11 @@ hw_perf_enable(void)
 			continue;
 
 		armpmu->enable(&event->hw, idx);
+		enabled = 1;
 	}
 
-	armpmu->start();
+	if (enabled)
+		armpmu->start();
 }
 
 void
--
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