linux-kernel - [PATCH v3 1/3] sched/fair: Move checking for nohz cpus after time check

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Message-ID: <20260107065125.669668-2-sshegde@linux.ibm.com>
Date: Wed,  7 Jan 2026 12:21:23 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org
Cc: sshegde@...ux.ibm.com, kprateek.nayak@....com, juri.lelli@...hat.com,
        vschneid@...hat.com, tglx@...utronix.de, dietmar.eggemann@....com,
        anna-maria@...utronix.de, frederic@...nel.org, wangyang.guo@...el.com
Subject: [PATCH v3 1/3] sched/fair: Move checking for nohz cpus after time check

NOHZ idle load balancer is kicked off only after time check. So move
the atomic read after the time check to access it only when needed.

When there are no idle CPUs(100% busy), even if the flag gets set to
NOHZ_STATS_KICK | NOHZ_NEXT_KICK, find_new_ilb will fail and
there will be no NOHZ idle balance. The current behaviour is retained.

Note: This patch doesn't solve any cacheline overheads. No improvement
in performance apart from saving a few cycles of atomic_read.

Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 kernel/sched/fair.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9743fc0b225c..17e4e8ac5fca 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12451,20 +12451,24 @@ static void nohz_balancer_kick(struct rq *rq)
 	 */
 	nohz_balance_exit_idle(rq);
 
-	/*
-	 * None are in tickless mode and hence no need for NOHZ idle load
-	 * balancing:
-	 */
-	if (likely(!atomic_read(&nohz.nr_cpus)))
-		return;
-
 	if (READ_ONCE(nohz.has_blocked_load) &&
 	    time_after(now, READ_ONCE(nohz.next_blocked)))
 		flags = NOHZ_STATS_KICK;
 
+	/*
+	 * If none are in tickless mode, though flag maybe set,
+	 * idle load balancing is not done as find_new_ilb fails
+	 */
 	if (time_before(now, nohz.next_balance))
 		goto out;
 
+	/*
+	 * None are in tickless mode and hence no need for NOHZ idle load
+	 * balancing:
+	 */
+	if (likely(!atomic_read(&nohz.nr_cpus)))
+		return;
+
 	if (rq->nr_running >= 2) {
 		flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
 		goto out;
-- 
2.47.3