Re: [PATCH] perf/Power: PERF_EVENT_IOC_ENABLE does not reenable event
From: Paul Mackerras <hidden>
Date: 2013-01-24 05:05:05
On Fri, Jan 11, 2013 at 11:11:17AM -0800, Sukadev Bhattiprolu wrote:
If we disable a perf event because we exceeded the specified ->event_limit,
power_pmu_stop() sets the PERF_HES_STOPPED flag on the event.
If the application then re-enables the event using PERF_EVENT_IOC_ENABLE
ioctl, we don't seem to ever clear this STOPPED flag. Consequently, the
user space is never notified of the event.
Following message has more background and test case.
http://lists.eecs.utk.edu/pipermail/ptools-perfapi/2012-October/002528.html
The problem reported there does not seem to occur on x86. My unverified theory:
Both x86 and Power clear the event->hw.state flag to 0 in their ->pmu_start()
operations. On X86 x86_pmu_start() is called from x86_pmu_enable(). But on
Power, power_pmu_start() is not called from power_pmu_enable().This code has changed a lot since I worked on it, but it seems like x86 has the STOPPED flag set whenever the event isn't currently active on a hardware counter, whereas we have it set only when the event has been throttled.
quoted hunk ↗ jump to hunk
Used the following test cases to verify that this patch works on latest PAPI. $ papi.git/src/ctests/nonthread PAPI_TOT_CYC@5000000 $ papi.git/src/ctests/overflow_single_event Signed-off-by: Sukadev Bhattiprolu <redacted> --- arch/powerpc/perf/core-book3s.c | 8 ++++++++ 1 files changed, 8 insertions(+), 0 deletions(-)diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c index aa2465e..a6faada 100644 --- a/arch/powerpc/perf/core-book3s.c +++ b/arch/powerpc/perf/core-book3s.c@@ -880,8 +880,16 @@ static int power_pmu_add(struct perf_event *event, int ef_flags) cpuhw->events[n0] = event->hw.config; cpuhw->flags[n0] = event->hw.event_base; + /* + * If this event was disabled in record_and_restart() because we + * exceeded the ->event_limit, this is probably a good time to + * re-enable the event ? If we don't reenable the event, we will + * never notify the user again about this event. + */
The comment seems a bit tentative. :) If the PERF_EF_START bit is set then we are being told to restart the event.
if (!(ef_flags & PERF_EF_START)) event->hw.state = PERF_HES_STOPPED | PERF_HES_UPTODATE; + else + event->hw.state &= ~PERF_HES_STOPPED;
This looks fine, though I think you could equally well just set event->hw.state to 0 in the else clause. That would clear the UPTODATE flag too, which is appropriate since we are about to put the event on a hardware counter. Paul.