linux-kernel - [PATCH] perf/x86/intel: Fix inaccurate period in context switch for auto-reload

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Date:   Thu,  6 Dec 2018 15:04:18 -0800
From:   kan.liang@...ux.intel.com
To:     peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Cc:     ak@...ux.intel.com, Kan Liang <kan.liang@...ux.intel.com>
Subject: [PATCH] perf/x86/intel: Fix inaccurate period in context switch for auto-reload

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

When a task, which is sampled by a PEBS event with a fixed period, is
sched_in, the fixed period will always be used as new period for
counter. It's inaccurate, because the left period from last sched_out
isn't taken into account.

The auto-reload feature is implicitly enabled for a PEBS event with a
fixed period. The feature has specific
intel_pmu_save_and_restart_reload(), which never records the left
period. The period_left, which is used in perf_event_set_period() to
calculate the new period, is always the user defined fixed period.

With auto-reload, the counter always has a negative counter value. So
the left period is -value. Update the period_left in
intel_pmu_save_and_restart_reload().

Fixes: commit d31fc13fdcb2 ("perf/x86/intel: Fix event update for auto-reload")
Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
---
 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 b7b01d7..a04b52b 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -1402,6 +1402,8 @@ intel_pmu_save_and_restart_reload(struct perf_event *event, int count)
 	old = ((s64)(prev_raw_count << shift) >> shift);
 	local64_add(new - old + count * period, &event->count);

+	local64_set(&hwc->period_left, -new);
+
 	perf_event_update_userpage(event);

 	return 0;
-- 
2.7.4