[PATCH v2] sched: fix nohz.next_balance update

[Vincent Guittot]

Since commit d4573c3e1c99 ("sched: Improve load balancing in the presence
of idle CPUs"), the ILB CPU starts with the idle load balancing of other
idle CPUs and finishes with itself in order to speed up the spread of tasks
in all idle CPUs.

The this_rq->next_balance is still used in nohz_idle_balance as an
intermediate step to gather the shortest next balance before updating
nohz.next_balance. But the former has not been updated yet and is likely to
be set with the current jiffies. As a result, the nohz.next_balance will be
set with current jiffies instead of the real next balance date. This
generates spurious kicks of nohz ilde balance.

nohz_idle_balance must set the nohz.next_balance without taking into
account this_rq->next_balance which is not updated yet. Then, this_rq will
update nohz.next_update with its next_balance once updated and if necessary.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---

change since v1:
- add #ifdef CONFIG_NO_HZ_COMMON for accessing nohz structure
- fix some typos

 kernel/sched/fair.c | 35 +++++++++++++++++++++++++++++++----
 1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 587a2f6..581378a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7779,8 +7779,23 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 	 * When the cpu is attached to null domain for ex, it will not be
 	 * updated.
 	 */
-	if (likely(update_next_balance))
+	if (likely(update_next_balance)) {
 		rq->next_balance = next_balance;
+
+#ifdef CONFIG_NO_HZ_COMMON
+		/*
+		 * If this cpu has been elected to perform the nohz idle
+		 * balance. Other idle cpus have already rebalanced with
+		 * nohz_idle_balance and the nohz.next_balance has been
+		 * updated accordingly. This cpu is now running the idle load
+		 * balance for itself and we need to update the
+		 * nohz.next_balance accordingly.
+		 */
+		if ((idle == CPU_IDLE) &&
+			time_after(nohz.next_balance, rq->next_balance))
+				nohz.next_balance = rq->next_balance;
+#endif
+	}
 }
 
 #ifdef CONFIG_NO_HZ_COMMON
@@ -7793,6 +7808,9 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 	int this_cpu = this_rq->cpu;
 	struct rq *rq;
 	int balance_cpu;
+	/* Earliest time when we have to do rebalance again */
+	unsigned long next_balance = jiffies + 60*HZ;
+	int update_next_balance = 0;
 
 	if (idle != CPU_IDLE ||
 	    !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
@@ -7824,10 +7842,19 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 			rebalance_domains(rq, CPU_IDLE);
 		}
 
-		if (time_after(this_rq->next_balance, rq->next_balance))
-			this_rq->next_balance = rq->next_balance;
+		if (time_after(next_balance, rq->next_balance)) {
+			next_balance = rq->next_balance;
+			update_next_balance = 1;
+		}
 	}
-	nohz.next_balance = this_rq->next_balance;
+
+	/*
+	 * next_balance will be updated only when there is a need.
+	 * When the cpu is attached to null domain for ex, it will not be
+	 * updated.
+	 */
+	if (likely(update_next_balance))
+		nohz.next_balance = next_balance;
 end:
 	clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
 }
-- 
1.9.1

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