Re: [PATCH] powerpc: perf: Fix frequency calculation for overflowing counters

Scott Wood
2011-01-17T17:33:08Z

On Mon, 17 Jan 2011 16:17:42 +1100
Anton Blanchard <anton@samba.org> wrote:

>
> 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;
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>

Does perf_event_fsl_emb.c need this as well (it has almost the same
record_and_restart code)?

-Scott