From 4bca770ede796a1ef7af9c983166d5608d9ccfaf Mon Sep 17 00:00:00 2001 From: Anton Blanchard <anton@samba.org> Date: Mon, 17 Jan 2011 16:17:42 +1100 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@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 file changed, 1 insertion(+) diff --git a/arch/powerpc/kernel/perf_event.c b/arch/powerpc/kernel/perf_event.c index 5674807057899c..ab6f6beadb5721 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; -- GitLab