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From: tip-bot for Anton Blanchard <>
Subject: [tip:perf/urgent] powerpc: perf: Fix frequency calculation for overflowing counters
Date: Mon, 17 Jan 2011 10:45:46 GMT	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <20110117161742.5feb3761@kryten>

Commit-ID:  4bca770ede796a1ef7af9c983166d5608d9ccfaf
Author:     Anton Blanchard <>
AuthorDate: Mon, 17 Jan 2011 16:17:42 +1100
Committer:  Ingo Molnar <>
CommitDate: Mon, 17 Jan 2011 11:43:02 +0100

powerpc: perf: Fix frequency calculation for overflowing counters

When profiling a benchmark that is almost 100% userspace, I noticed some wildly
inaccurate profiles that showed almost all time spent in the kernel.

Closer examination shows we were programming a tiny number of cycles into the
PMU after each overflow (about ~200 away from the next overflow). This gets us
stuck in a loop which we eventually break out of by throttling the PMU (there
are regular throttle/unthrottle events in the log).

It looks like we aren't setting event->hw.last_period to something same and the
frequency to period calculations in perf are going haywire.

With the following patch we find the correct period after a few interrupts and
stay there. I also see no more throttle events.

Signed-off-by: Anton Blanchard <>
Acked-by: Benjamin Herrenschmidt <>
Cc: Paul Mackerras <>
Cc: Peter Zijlstra <>
Cc: Arnaldo Carvalho de Melo <>
LKML-Reference: <20110117161742.5feb3761@kryten>
Signed-off-by: Ingo Molnar <>
 arch/powerpc/kernel/perf_event.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/arch/powerpc/kernel/perf_event.c b/arch/powerpc/kernel/perf_event.c
index 5674807..ab6f6be 100644
--- a/arch/powerpc/kernel/perf_event.c
+++ b/arch/powerpc/kernel/perf_event.c
@@ -1212,6 +1212,7 @@ static void record_and_restart(struct perf_event *event, unsigned long val,
 			if (left <= 0)
 				left = period;
 			record = 1;
+			event->hw.last_period = event->hw.sample_period;
 		if (left < 0x80000000LL)
 			val = 0x80000000LL - left;

  reply	other threads:[~2011-01-17 10:46 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-01-17  5:17 [PATCH] " Anton Blanchard
2011-01-17 10:45 ` tip-bot for Anton Blanchard [this message]
2011-01-17 17:32 ` Scott Wood
2011-01-17 17:38   ` Peter Zijlstra
2011-01-18 10:44     ` [PATCH] powerpc: perf: Fix frequency calculation for overflowing counters (FSL version) Anton Blanchard
2011-01-19 19:19       ` [tip:perf/urgent] powerpc, " tip-bot for Anton Blanchard

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