linux-kernel - [PATCH 2/4] perf/x86/intel: fix event update for auto-reload

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Date:   Mon, 18 Dec 2017 03:34:49 -0800
From:   kan.liang@...ux.intel.com
To:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, jolsa@...hat.com, eranian@...gle.com,
        ak@...ux.intel.com, Kan Liang <kan.liang@...ux.intel.com>
Subject: [PATCH 2/4] perf/x86/intel: fix event update for auto-reload

From: Kan Liang <kan.liang@...ux.intel.com>

There is bug when mmap read event->count with large PEBS enabled.
Here is an example.
 #./read_count
 0x71f0
 0x122c0
 0x1000000001c54
 0x100000001257d
 0x200000000bdc5

There is auto-reload mechanism enabled for PEBS events in fixed period
mode. But the calculation of event->count does not take the auto-reload
values into account. Anyone who read the event->count will get wrong
result, e.g x86_pmu_read. Also, the calculation of hwc->period_left is
wrong either. It impacts the accuracy of period for the first record in
PEBS multiple records.

The issue is introduced with the auto-reload mechanism enabled by
commit 851559e35fd5 ("perf/x86/intel: Use the PEBS auto reload mechanism
when possible")

For the auto-reload before the last time, it went through the whole
period (reload value) every time. So period * times should be added into
the event->count.
For the last load, the elapsed delta (event-)time need to be corrected
by adding the period (reload value). Because the start point is -period.

Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
---
 arch/x86/events/core.c     | 14 ++++++++++++++
 arch/x86/events/intel/ds.c |  8 +++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 35552ea..f74e21d 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -100,6 +100,20 @@ u64 x86_perf_event_update(struct perf_event *event,
 	 * of the count.
 	 */
 	delta = (new_raw_count << shift) - (prev_raw_count << shift);
+
+	/*
+	 * Take auto-reload into account
+	 * For the auto-reload before the last time, it went through the
+	 * whole period (reload_val) every time.
+	 * Just simply add period * times to the event.
+	 *
+	 * For the last load, the elapsed delta (event-)time need to be
+	 * corrected by adding the period. Because the start point is -period.
+	 */
+	if (reload_times > 0) {
+		delta += (reload_val << shift);
+		local64_add(reload_val * (reload_times - 1), &event->count);
+	}
 	delta >>= shift;
 
 	local64_add(delta, &event->count);
diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index 0b693b7..f0f6026 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -1256,11 +1256,17 @@ static void __intel_pmu_pebs_event(struct perf_event *event,
 				   void *base, void *top,
 				   int bit, int count)
 {
+	struct hw_perf_event *hwc = &event->hw;
 	struct perf_sample_data data;
 	struct pt_regs regs;
 	void *at = get_next_pebs_record_by_bit(base, top, bit);
 
-	if (!intel_pmu_save_and_restart(event, 0, 0) &&
+	/*
+	 * Now, auto-reload is only enabled in fixed period mode.
+	 * The reload value is always hwc->sample_period.
+	 * May need to change it, if auto-reload is enabled in freq mode later.
+	 */
+	if (!intel_pmu_save_and_restart(event, hwc->sample_period, count - 1) &&
 	    !(event->hw.flags & PERF_X86_EVENT_AUTO_RELOAD))
 		return;
 
-- 
2.7.4