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

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index af120e88720f7e31e9d04afd13e64e5e1b6ba9ac..9afe0c69a3c110bce60142b3f1605720c41b431e 100644 (file)
--- 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;