linux-kernel - [ 02/72] powerpc/perf: power_pmu_start restores incorrect values, breaking frequency events

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Message-Id: <20120228010429.568794329@linuxfoundation.org>
Date:	Mon, 27 Feb 2012 17:04:31 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Anton Blanchard <anton@...ba.org>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: [ 02/72] powerpc/perf: power_pmu_start restores incorrect values, breaking frequency events

3.2-stable review patch.  If anyone has any objections, please let me know.

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

From: Anton Blanchard <anton@...ba.org>

commit 9a45a9407c69d068500923480884661e2b9cc421 upstream.

perf on POWER stopped working after commit e050e3f0a71b (perf: Fix
broken interrupt rate throttling). That patch exposed a bug in
the POWER perf_events code.

Since the PMCs count upwards and take an exception when the top bit
is set, we want to write 0x80000000 - left in power_pmu_start. We were
instead programming in left which effectively disables the counter
until we eventually hit 0x80000000. This could take seconds or longer.

With the patch applied I get the expected number of samples:

          SAMPLE events:       9948

Signed-off-by: Anton Blanchard <anton@...ba.org>
Acked-by: Paul Mackerras <paulus@...ba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/powerpc/kernel/perf_event.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/arch/powerpc/kernel/perf_event.c
+++ b/arch/powerpc/kernel/perf_event.c
@@ -865,6 +865,7 @@ static void power_pmu_start(struct perf_
 {
 	unsigned long flags;
 	s64 left;
+	unsigned long val;
 
 	if (!event->hw.idx || !event->hw.sample_period)
 		return;
@@ -880,7 +881,12 @@ static void power_pmu_start(struct perf_
 
 	event->hw.state = 0;
 	left = local64_read(&event->hw.period_left);
-	write_pmc(event->hw.idx, left);
+
+	val = 0;
+	if (left < 0x80000000L)
+		val = 0x80000000L - left;
+
+	write_pmc(event->hw.idx, val);
 
 	perf_event_update_userpage(event);
 	perf_pmu_enable(event->pmu);


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