linux-kernel - [RFC PATCH 03/19] sched/fair: Use rq->nohz_tick_stopped in update_nohz

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Message-ID: <20250904041516.3046-4-kprateek.nayak@amd.com>
Date: Thu, 4 Sep 2025 04:14:59 +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 03/19] sched/fair: Use rq->nohz_tick_stopped in update_nohz_stats()

"rq->nohz_tick_stopped" always follows the state of CPU on
"nohz.idle_cpus_mask". Use the local rq indicator instead of checking
for the CPU on the "idle_cpus_mask".

Use READ_ONCE() and WRITE_ONCE() for "rq->nohz_tick_stopped" to ensure
update_nohz_stats() always sees the latest value.

This cleanup is necessary to avoid the number of references to the
global "nohz.idle_cpus_mask" to ease the transition to a distributed
nohz idle tracking strategy.

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 | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 61b59fd75ced..af9f2c1e93f8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12463,10 +12463,10 @@ void nohz_balance_exit_idle(struct rq *rq)
 {
 	WARN_ON_ONCE(rq != this_rq());
 
-	if (likely(!rq->nohz_tick_stopped))
+	if (likely(!READ_ONCE(rq->nohz_tick_stopped)))
 		return;
 
-	rq->nohz_tick_stopped = 0;
+	WRITE_ONCE(rq->nohz_tick_stopped, 0);
 	cpumask_clear_cpu(rq->cpu, nohz.idle_cpus_mask);
 	atomic_dec(&nohz.nr_cpus);
 
@@ -12514,14 +12514,14 @@ void nohz_balance_enter_idle(int cpu)
 	 * *_avg. The CPU is already part of nohz.idle_cpus_mask so the clear
 	 * of nohz.has_blocked can only happen after checking the new load
 	 */
-	if (rq->nohz_tick_stopped)
+	if (READ_ONCE(rq->nohz_tick_stopped))
 		goto out;
 
 	/* If we're a completely isolated CPU, we don't play: */
 	if (on_null_domain(rq))
 		return;
 
-	rq->nohz_tick_stopped = 1;
+	WRITE_ONCE(rq->nohz_tick_stopped, 1);
 
 	cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
 	atomic_inc(&nohz.nr_cpus);
@@ -12551,7 +12551,7 @@ static bool update_nohz_stats(struct rq *rq)
 	if (!rq->has_blocked_load)
 		return false;
 
-	if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask))
+	if (!READ_ONCE(rq->nohz_tick_stopped))
 		return false;
 
 	if (!time_after(jiffies, READ_ONCE(rq->last_blocked_load_update_tick)))
-- 
2.34.1