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From:   Uros Bizjak <ubizjak@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     Uros Bizjak <ubizjak@gmail.com>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: [PATCH] sched/clock: Use try_cmpxchg64 in sched_clock_{local,remote}
Date:   Wed, 18 May 2022 20:49:53 +0200
Message-Id: <20220518184953.3446778-1-ubizjak@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Use try_cmpxchg64 instead of cmpxchg64 (*ptr, old, new) != old in
sched_clock_{local,remote}. x86 cmpxchg returns success in ZF flag,
so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
---
Patch requires commits 0aa7be05d83cc584da0782405e8007e351dfb6cc
and c2df0a6af177b6c06a859806a876f92b072dc624 from tip.git
---
 kernel/sched/clock.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index d9272d9061a3..e374c0c923da 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -287,7 +287,7 @@ notrace static u64 sched_clock_local(struct sched_clock_data *scd)
 	clock = wrap_max(clock, min_clock);
 	clock = wrap_min(clock, max_clock);
 
-	if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
+	if (!try_cmpxchg64(&scd->clock, &old_clock, clock))
 		goto again;
 
 	return clock;
@@ -349,7 +349,7 @@ notrace static u64 sched_clock_remote(struct sched_clock_data *scd)
 		val = remote_clock;
 	}
 
-	if (cmpxchg64(ptr, old_val, val) != old_val)
+	if (!try_cmpxchg64(ptr, &old_val, val))
 		goto again;
 
 	return val;
-- 
2.35.1