linux-kernel - [RESEND RFC PATCH v2 18/29] sched/fair: Reorder the barrier in nohz_balance_enter

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Message-ID: <20251208092744.32737-18-kprateek.nayak@amd.com>
Date: Mon, 8 Dec 2025 09:27:04 +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>
CC: <linux-kernel@...r.kernel.org>, 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>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>, Chen Yu <yu.c.chen@...el.com>
Subject: [RESEND RFC PATCH v2 18/29] sched/fair: Reorder the barrier in nohz_balance_enter_idle()

The smp_mb__after_atomic() in nohz_balance_enter_idle() is used to order
the setting of the nohz idle CPU on the "nohz.idle_cpus_mask" with the
write to "nohz.needs_update" as per the comment.

Since set_cpu_sd_state_idle() is now responsible for idle tracking using
"sd_nohz->shared", reorder the call to set_cpu_sd_state_idle() to come
before the barrier ensuring the CPU is tracked on the idle mask before
the update to "nohz.needs_update" is observed.

Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8f21b655fc40..5e5db24e1384 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12648,6 +12648,8 @@ void nohz_balance_enter_idle(int cpu)
 	cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
 	atomic_inc(&nohz.nr_cpus);
 
+	set_cpu_sd_state_idle(cpu);
+
 	/*
 	 * Ensures that if nohz_idle_balance() fails to observe our
 	 * @idle_cpus_mask store, it must observe the @has_blocked
@@ -12655,8 +12657,6 @@ void nohz_balance_enter_idle(int cpu)
 	 */
 	smp_mb__after_atomic();
 
-	set_cpu_sd_state_idle(cpu);
-
 	WRITE_ONCE(nohz.needs_update, 1);
 out:
 	/*
-- 
2.43.0