[PATCH v5 1/3] sched/fair: Move checking for nohz cpus after time check

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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>
---
 kernel/sched/fair.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c921cdc6c3ed..a4910658c5d6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12445,20 +12445,29 @@ 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;
-- 
2.51.0