linux-kernel - [PATCH] powerpc: perf: Fix frequency calculation for overflowing counters

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Message-ID: <20110117161742.5feb3761@kryten>
Date:	Mon, 17 Jan 2011 16:17:42 +1100
From:	Anton Blanchard <anton@...ba.org>
To:	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: [PATCH] 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>
---

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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