• Anton Blanchard's avatar
    powerpc: perf: Fix frequency calculation for overflowing counters · 4bca770e
    Anton Blanchard authored
    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: default avatarAnton Blanchard <anton@samba.org>
    Acked-by: default avatarBenjamin 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: default avatarIngo Molnar <mingo@elte.hu>
    4bca770e
perf_event.c 32.9 KB