Commit 6b67c8a7 authored Jan 15, 2026 by Shrikanth Hegde Committed by Peter Zijlstra Jan 15, 2026

sched/fair: Move checking for nohz cpus after time check



Current code does.
- Read nohz.nr_cpus
- Check if the time has passed to do NOHZ idle balance

Instead do this.
- Check if the time has passed to do NOHZ idle balance
- Read nohz.nr_cpus

This will skip the read most of the time in normal system usage.
i.e when there are nohz.nr_cpus (system is not 100% busy).

Note that 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. In such cases there will be a very
narrow window where, kick_ilb will be called un-necessarily.
However current functionality is still retained.

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

Reviewed-and-tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/20260115073524.376643-2-sshegde@linux.ibm.com

parent 553255cc

kernel/sched/fair.c

+16 −7

Original line number	Diff line number	Diff line
		@@ -12441,20 +12441,29 @@ 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;

		/*
		* Most of the time system is not 100% busy. i.e nohz.nr_cpus > 0
		* Skip the read if time is not due.
		*
		* If none are in tickless mode, there maybe a narrow window
		* (28 jiffies, HZ=1000) where flags maybe set and kick_ilb called.
		* But idle load balancing is not done as find_new_ilb fails.
		* That's very rare. So read nohz.nr_cpus only if time is due.
		*/
		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;