linux-kernel - [PATCH] locking/percpu-rwsem: Trigger contention tracepoints only if contended

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Date:   Wed,  8 Nov 2023 13:53:22 -0800
From:   Namhyung Kim <namhyung@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>
Cc:     Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] locking/percpu-rwsem: Trigger contention tracepoints only if contended

It mistakenly fires lock contention tracepoints always in the writer path.
It should be conditional on the try lock result.

Signed-off-by: Namhyung Kim <namhyung@...nel.org>
---
 kernel/locking/percpu-rwsem.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 185bd1c906b0..6083883c4fe0 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -223,9 +223,10 @@ static bool readers_active_check(struct percpu_rw_semaphore *sem)
 
 void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
 {
+	bool contended = false;
+
 	might_sleep();
 	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
-	trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
 
 	/* Notify readers to take the slow path. */
 	rcu_sync_enter(&sem->rss);
@@ -234,8 +235,11 @@ void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
 	 * Try set sem->block; this provides writer-writer exclusion.
 	 * Having sem->block set makes new readers block.
 	 */
-	if (!__percpu_down_write_trylock(sem))
+	if (!__percpu_down_write_trylock(sem)) {
+		trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);
 		percpu_rwsem_wait(sem, /* .reader = */ false);
+		contended = true;
+	}
 
 	/* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
 
@@ -247,7 +251,8 @@ void __sched percpu_down_write(struct percpu_rw_semaphore *sem)
 
 	/* Wait for all active readers to complete. */
 	rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
-	trace_contention_end(sem, 0);
+	if (contended)
+		trace_contention_end(sem, 0);
 }
 EXPORT_SYMBOL_GPL(percpu_down_write);
 
-- 
2.42.0.869.gea05f2083d-goog