linux-kernel - [PATCH] perf/x86/intel/ds: Fix counter backwards of non-precise events counters-snapshotting

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Message-Id: <20250204210514.4089680-1-kan.liang@linux.intel.com>
Date: Tue,  4 Feb 2025 13:05:14 -0800
From: kan.liang@...ux.intel.com
To: peterz@...radead.org,
	mingo@...hat.com,
	acme@...nel.org,
	namhyung@...nel.org,
	irogers@...gle.com,
	adrian.hunter@...el.com,
	linux-kernel@...r.kernel.org
Cc: ak@...ux.intel.com,
	eranian@...gle.com,
	dapeng1.mi@...ux.intel.com,
	Kan Liang <kan.liang@...ux.intel.com>
Subject: [PATCH] perf/x86/intel/ds: Fix counter backwards of non-precise events counters-snapshotting

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

The counter backwards may be observed in the PMI handler when
counters-snapshotting some non-precise events in the freq mode.

For the non-precise events, it's possible the counters-snapshotting
records a positive value for an overflowed PEBS event. Then the HW
auto-reload mechanism reset the counter to 0 immediately. Because the
pebs_event_reset is cleared in the freq mode, which doesn't set the
PERF_X86_EVENT_AUTO_RELOAD.
In the PMI handler, 0 will be read rather than the positive value
recorded in the counters-snapshotting record.

The counters-snapshotting case has to be specially handled. Since the
event value has been updated when processing the counters-snapshotting
record, only needs to set the new period for the counter via
x86_pmu_set_period().

Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
---
 arch/x86/events/intel/ds.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index 46aaaeae0c8d..e8f808905871 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -2377,8 +2377,25 @@ __intel_pmu_pebs_last_event(struct perf_event *event,
 			 */
 			intel_pmu_save_and_restart_reload(event, count);
 		}
-	} else
-		intel_pmu_save_and_restart(event);
+	} else {
+		/*
+		 * For a non-precise event, it's possible the
+		 * counters-snapshotting records a positive value for the
+		 * overflowed event. Then the HW auto-reload mechanism
+		 * reset the counter to 0 immediately, because the
+		 * pebs_event_reset is cleared if the PERF_X86_EVENT_AUTO_RELOAD
+		 * is not set. The counter backwards may be observed in a
+		 * PMI handler.
+		 *
+		 * Since the event value has been updated when processing the
+		 * counters-snapshotting record, only needs to set the new
+		 * period for the counter.
+		 */
+		if (is_pebs_counter_event_group(event))
+			static_call(x86_pmu_set_period)(event);
+		else
+			intel_pmu_save_and_restart(event);
+	}
 }
 
 static __always_inline void
-- 
2.38.1