[PATCH] perf/Power: PERF_EVENT_IOC_ENABLE does not reenable event

[Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>]

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().

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 <sukadev@linux.vnet.ibm.com>
---
 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.
+	 */
 	if (!(ef_flags & PERF_EF_START))
 		event->hw.state = PERF_HES_STOPPED | PERF_HES_UPTODATE;
+	else
+		event->hw.state &= ~PERF_HES_STOPPED;
 
 	/*
 	 * If group events scheduling transaction was started,
-- 
1.7.1