From 6b67c8a72e56041f91f70ae5995bdb769761869a Mon Sep 17 00:00:00 2001 From: Shrikanth Hegde Date: Thu, 15 Jan 2026 13:05:22 +0530 Subject: 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 Signed-off-by: Shrikanth Hegde Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Vincent Guittot Link: https://patch.msgid.link/20260115073524.376643-2-sshegde@linux.ibm.com --- kernel/sched/fair.c | 23 ++++++++++++++++------- 1 file changed, 16 insertions(+), 7 deletions(-) (limited to 'kernel') diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index af120e88720f..9afe0c69a3c1 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -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; -- cgit v1.2.3