From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Mon, 8 Jun 2020 20:47:49 +0100
Subject: [PATCH] sched/fair: fix NOHZ next idle balance

Patch-mainline: v5.9-rc1
Git-commit: 3ea2f097b17e13a8280f1f9386c331b326a3dbef
References: bnc#1155798 (CPU scheduler functional and performance backports)

With commit:
  'b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")'
rebalance_domains of the local cfs_rq happens before others idle cpus have
updated nohz.next_balance and its value is overwritten.

Move the update of nohz.next_balance for other idles cpus before balancing
and updating the next_balance of local cfs_rq.

Also, the nohz.next_balance is now updated only if all idle cpus got a
chance to rebalance their domains and the idle balance has not been aborted
because of new activities on the CPU. In case of need_resched, the idle
load balance will be kick the next jiffie in order to address remaining
ilb.

Reported-by: Peng Liu <iwtbavbm@gmail.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/sched/fair.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 16864566bf6d..6f879ebedf61 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9946,7 +9946,12 @@ static void kick_ilb(unsigned int flags)
 {
 	int ilb_cpu;
 
-	nohz.next_balance++;
+	/*
+	 * Increase nohz.next_balance only when if full ilb is triggered but
+	 * not if we only update stats.
+	 */
+	if (flags & NOHZ_BALANCE_KICK)
+		nohz.next_balance = jiffies+1;
 
 	ilb_cpu = find_new_ilb();
 
@@ -10264,6 +10269,14 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
 		}
 	}
 
+	/*
+	 * 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;
+
 	/* Newly idle CPU doesn't need an update */
 	if (idle != CPU_NEWLY_IDLE) {
 		update_blocked_averages(this_cpu);
@@ -10284,14 +10297,6 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
 	if (has_blocked_load)
 		WRITE_ONCE(nohz.has_blocked, 1);
 
-	/*
-	 * 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;
-
 	return ret;
 }