[PATCH v2 1/2] sched/fair: Add NOHZ balancer flag for nohz.next_balance updates

[Valentin Schneider <valentin.schneider@arm.com>]

A following patch will trigger NOHZ idle balances as a means to update
nohz.next_balance. Vincent noted that blocked load updates can have
non-negligible overhead, which should be avoided if the intent is to only
update nohz.next_balance.

Add a new NOHZ balance kick flag, NOHZ_NEXT_KICK. Gate NOHZ blocked load
update by the presence of NOHZ_STATS_KICK - currently all NOHZ balance
kicks will have the NOHZ_STATS_KICK flag set, so no change in behaviour is
expected.

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/fair.c  | 9 ++++++---
 kernel/sched/sched.h | 8 +++++++-
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 11d22943753f..5c88698c3664 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10506,7 +10506,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
 	 * setting the flag, we are sure to not clear the state and not
 	 * check the load of an idle cpu.
 	 */
-	WRITE_ONCE(nohz.has_blocked, 0);
+	if (flags & NOHZ_STATS_KICK)
+		WRITE_ONCE(nohz.has_blocked, 0);
 
 	/*
 	 * Ensures that if we miss the CPU, we must see the has_blocked
@@ -10528,13 +10529,15 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
 		 * balancing owner will pick it up.
 		 */
 		if (need_resched()) {
-			has_blocked_load = true;
+			if (flags & NOHZ_STATS_KICK)
+				has_blocked_load = true;
 			goto abort;
 		}
 
 		rq = cpu_rq(balance_cpu);
 
-		has_blocked_load |= update_nohz_stats(rq);
+		if (flags & NOHZ_STATS_KICK)
+			has_blocked_load |= update_nohz_stats(rq);
 
 		/*
 		 * If time for next balance is due,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 9a1c6aeb9165..b0f38b5d2550 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2695,12 +2695,18 @@ extern void cfs_bandwidth_usage_dec(void);
 #define NOHZ_BALANCE_KICK_BIT	0
 #define NOHZ_STATS_KICK_BIT	1
 #define NOHZ_NEWILB_KICK_BIT	2
+#define NOHZ_NEXT_KICK_BIT	3
 
+/* Run rebalance_domains() */
 #define NOHZ_BALANCE_KICK	BIT(NOHZ_BALANCE_KICK_BIT)
+/* Update blocked load */
 #define NOHZ_STATS_KICK		BIT(NOHZ_STATS_KICK_BIT)
+/* Update blocked load when entering idle */
 #define NOHZ_NEWILB_KICK	BIT(NOHZ_NEWILB_KICK_BIT)
+/* Update nohz.next_balance */
+#define NOHZ_NEXT_KICK		BIT(NOHZ_NEXT_KICK_BIT)
 
-#define NOHZ_KICK_MASK	(NOHZ_BALANCE_KICK | NOHZ_STATS_KICK)
+#define NOHZ_KICK_MASK	(NOHZ_BALANCE_KICK | NOHZ_STATS_KICK | NOHZ_NEXT_KICK)
 
 #define nohz_flags(cpu)	(&cpu_rq(cpu)->nohz_flags)
 
-- 
2.25.1