linux-kernel - [PATCH] perf/x86/intel: Fix debug_store reset field for freq events

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Message-Id: <20170714163551.19459-1-jolsa@kernel.org>
Date:   Fri, 14 Jul 2017 18:35:51 +0200
From:   Jiri Olsa <jolsa@...nel.org>
To:     Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andi Kleen <andi@...stfloor.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>
Subject: [PATCH] perf/x86/intel: Fix debug_store reset field for freq events

There's a bug in PEBs event enabling code, that prevents PEBS
freq events to work properly after non freq PEBS event was run.

freq events - perf_event_attr::freq set
              -F <freq> option of perf record

PEBS events - perf_event_attr::precise_ip > 0
              default for perf record

Like in following example with cpu 0 busy, we expect ~10000 samples
for following perf tool run:

  # perf record -F 10000 -C 0 sleep 1
  [ perf record: Woken up 2 times to write data ]
  [ perf record: Captured and wrote 0.640 MB perf.data (10031 samples) ]

Everything's fine, but once we run non freq PEBS event like:

  # perf record -c 10000 -C 0 sleep 1
  [ perf record: Woken up 4 times to write data ]
  [ perf record: Captured and wrote 1.053 MB perf.data (20061 samples) ]

the freq events start to fail like this:

  # perf record -F 10000 -C 0 sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.185 MB perf.data (40 samples) ]

The issue is in non freq PEBs event initialization of debug_store reset
field, which value is used to auto-reload the counter value after PEBS
event drain. This value is not being used for PEBS freq events, but once
we run non freq event it stays in debug_store data and screws the
sample_freq counting for PEBS freq events.

Setting the reset field to 0 for freq events.

Signed-off-by: Jiri Olsa <jolsa@...nel.org>
---
 arch/x86/events/intel/ds.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index c6d23ffe422d..2244bd8c09b1 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -889,6 +889,8 @@ void intel_pmu_pebs_enable(struct perf_event *event)
 	if (hwc->flags & PERF_X86_EVENT_AUTO_RELOAD) {
 		ds->pebs_event_reset[hwc->idx] =
 			(u64)(-hwc->sample_period) & x86_pmu.cntval_mask;
+	} else {
+		ds->pebs_event_reset[hwc->idx] = 0;
 	}
 }
 
-- 
2.9.4