linux-kernel - [RFC PATCH 04/19] sched/fair: Use xchg() to set sd->nohz

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Message-ID: <20250904041516.3046-5-kprateek.nayak@amd.com>
Date: Thu, 4 Sep 2025 04:15:00 +0000
From: K Prateek Nayak <kprateek.nayak@....com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>, Thomas Gleixner
	<tglx@...utronix.de>, <linux-kernel@...r.kernel.org>
CC: Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
	<rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, K Prateek Nayak
	<kprateek.nayak@....com>, "Gautham R. Shenoy" <gautham.shenoy@....com>,
	Swapnil Sapkal <swapnil.sapkal@....com>
Subject: [RFC PATCH 04/19] sched/fair: Use xchg() to set sd->nohz_idle state

"sd->nohz_idle" currently relies on the fact that it is always modified
from the local CPU to keep consistency however, the subsequent commits
will use the rq->online() callback to correct the "sd->nohz_idle" status
from a remote CPU during sched domain rebuild.

Use xchg() instead to prevent double accounting of "nr_busy_cpus" if the
status update on the local CPU races with a correction from a remote
CPU.

No functional changes intended.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@....com>
Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
 kernel/sched/fair.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index af9f2c1e93f8..0e68371365a5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12455,7 +12455,9 @@ static void set_cpu_sd_state_busy(int cpu)
 	if (!sd || !sd->nohz_idle)
 		return;
 
-	sd->nohz_idle = 0;
+	if (!xchg(&sd->nohz_idle, 0))
+		return;
+
 	atomic_inc(&sd->shared->nr_busy_cpus);
 }
 
@@ -12483,7 +12485,9 @@ static void set_cpu_sd_state_idle(int cpu)
 	if (!sd || sd->nohz_idle)
 		return;
 
-	sd->nohz_idle = 1;
+	if (xchg(&sd->nohz_idle, 1))
+		return;
+
 	atomic_dec(&sd->shared->nr_busy_cpus);
 }
 
-- 
2.34.1