linux-kernel - [tip:perf/urgent] powerpc: perf: Fix frequency calculation for overflowing counters

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Date:	Mon, 17 Jan 2011 10:45:46 GMT
From:	tip-bot for Anton Blanchard <anton@...ba.org>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, paulus@...ba.org, anton@...ba.org,
	hpa@...or.com, mingo@...hat.com, a.p.zijlstra@...llo.nl,
	acme@...stprotocols.net, benh@...nel.crashing.org,
	tglx@...utronix.de, mingo@...e.hu
Subject: [tip:perf/urgent] powerpc: perf: Fix frequency calculation for overflowing counters

Commit-ID:  4bca770ede796a1ef7af9c983166d5608d9ccfaf
Gitweb:     http://git.kernel.org/tip/4bca770ede796a1ef7af9c983166d5608d9ccfaf
Author:     Anton Blanchard <anton@...ba.org>
AuthorDate: Mon, 17 Jan 2011 16:17:42 +1100
Committer:  Ingo Molnar <mingo@...e.hu>
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 <anton@...ba.org>
Acked-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: linuxppc-dev@...ts.ozlabs.org
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Arnaldo Carvalho de Melo <acme@...stprotocols.net>
LKML-Reference: <20110117161742.5feb3761@...ten>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 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;
--
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