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

tip-bot for Anton Blanchard
2011-01-17T10:46:23Z

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