linux-kernel - [PATCH V3] perf: Fix the throttle error of some clock events

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Message-Id: <20250604171554.3909897-1-kan.liang@linux.intel.com>
Date: Wed,  4 Jun 2025 10:15:54 -0700
From: kan.liang@...ux.intel.com
To: peterz@...radead.org,
	mingo@...hat.com,
	namhyung@...nel.org,
	irogers@...gle.com,
	mark.rutland@....com,
	linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org
Cc: eranian@...gle.com,
	ctshao@...gle.com,
	tmricht@...ux.ibm.com,
	Kan Liang <kan.liang@...ux.intel.com>,
	Leo Yan <leo.yan@....com>,
	Aishwarya TCV <aishwarya.tcv@....com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
Subject: [PATCH V3] perf: Fix the throttle error of some clock events

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

Both ARM and IBM CI reports RCU stall, which can be reproduced by the
below perf command.
  perf record -a -e cpu-clock -- sleep 2

The issue is introduced by the generic throttle patch set, which
unconditionally invoke the event_stop() when throttle is triggered.

The cpu-clock and task-clock are two special SW events, which rely on
the hrtimer. The throttle is invoked in the hrtimer handler. The
event_stop()->hrtimer_cancel() waits for the handler to finish, which is
a deadlock. Instead of invoking the stop(), the HRTIMER_NORESTART should
be used to stop the timer.

There may be two ways to fix it.
- Introduce a PMU flag to track the case. Avoid the event_stop in
  perf_event_throttle() if the flag is detected.
  It has been implemented in the
  https://lore.kernel.org/lkml/20250528175832.2999139-1-kan.liang@linux.intel.com/
  The new flag was thought to be an overkill for the issue.
- Add a check in the event_stop. Return immediately if the throttle is
  invoked in the hrtimer handler. Rely on the existing HRTIMER_NORESTART
  method to stop the timer.

The latter is implemented here.

Move event->hw.interrupts = MAX_INTERRUPTS before the stop(). It makes
the order the same as perf_event_unthrottle(). Except the patch, no one
checks the hw.interrupts in the stop(). There is no impact from the
order change.

Reported-by: Leo Yan <leo.yan@....com>
Reported-by: Aishwarya TCV <aishwarya.tcv@....com>
Closes: https://lore.kernel.org/lkml/20250527161656.GJ2566836@e132581.arm.com/
Reported-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
Closes: https://lore.kernel.org/lkml/djxlh5fx326gcenwrr52ry3pk4wxmugu4jccdjysza7tlc5fef@ktp4rffawgcw/
Reported-by: Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
Closes: https://lore.kernel.org/lkml/8e8f51d8-af64-4d9e-934b-c0ee9f131293@linux.ibm.com/
Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
---

Changes since V2:
- Apply a different way to fix the issue.
  Remove all Tested-by since a different way is applied
- Update the change log
- Add more Reported-by

 kernel/events/core.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f34c99f8ce8f..46441c23475d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2656,8 +2656,8 @@ static void perf_event_unthrottle(struct perf_event *event, bool start)
 
 static void perf_event_throttle(struct perf_event *event)
 {
-	event->pmu->stop(event, 0);
 	event->hw.interrupts = MAX_INTERRUPTS;
+	event->pmu->stop(event, 0);
 	if (event == event->group_leader)
 		perf_log_throttle(event, 0);
 }
@@ -11749,7 +11749,12 @@ static void perf_swevent_cancel_hrtimer(struct perf_event *event)
 {
 	struct hw_perf_event *hwc = &event->hw;
 
-	if (is_sampling_event(event)) {
+	/*
+	 * The throttle can be triggered in the hrtimer handler.
+	 * The HRTIMER_NORESTART should be used to stop the timer,
+	 * rather than hrtimer_cancel(). See perf_swevent_hrtimer()
+	 */
+	if (is_sampling_event(event) && (hwc->interrupts != MAX_INTERRUPTS)) {
 		ktime_t remaining = hrtimer_get_remaining(&hwc->hrtimer);
 		local64_set(&hwc->period_left, ktime_to_ns(remaining));
 
-- 
2.38.1