From: Anton Blanchard <anton@samba.org>
To: Paul Mackerras <paulus@samba.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Ingo Molnar <mingo@elte.hu>,
Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: [PATCH] powerpc: perf: Fix frequency calculation for overflowing counters
Date: Mon, 17 Jan 2011 16:17:42 +1100 [thread overview]
Message-ID: <20110117161742.5feb3761@kryten> (raw)
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>
---
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;
next reply other threads:[~2011-01-17 5:17 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-01-17 5:17 Anton Blanchard [this message]
2011-01-17 17:32 ` [PATCH] powerpc: perf: Fix frequency calculation for overflowing counters 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
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