[tip:sched/core] sched/fair: Fix load_balance redo for !imbalance

[tip-bot for Vincent Guittot]

Commit-ID:  bb3485c8ace6475c269b1aa2da674490f455f412
Gitweb:     https://git.kernel.org/tip/bb3485c8ace6475c269b1aa2da674490f455f412
Author:     Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate: Fri, 7 Sep 2018 09:51:04 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 10 Sep 2018 10:13:49 +0200

sched/fair: Fix load_balance redo for !imbalance

It can happen that load_balance() finds a busiest group and then a
busiest rq but the calculated imbalance is in fact 0.

In such situation, detach_tasks() returns immediately and lets the
flag LBF_ALL_PINNED set. The busiest CPU is then wrongly assumed to
have pinned tasks and removed from the load balance mask. then, we
redo a load balance without the busiest CPU. This creates wrong load
balance situation and generates wrong task migration.

If the calculated imbalance is 0, it's useless to try to find a
busiest rq as no task will be migrated and we can return immediately.

This situation can happen with heterogeneous system or smp system when
RT tasks are decreasing the capacity of some CPUs.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: dietmar.eggemann@xxxxxxx
Cc: jhugo@xxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1536306664-29827-1-git-send-email-vincent.guittot@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f12d004be6a1..fc9a484ef82b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8275,7 +8275,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
 force_balance:
 	/* Looks like there is an imbalance. Compute it */
 	calculate_imbalance(env, &sds);
-	return sds.busiest;
+	return env->imbalance ? sds.busiest : NULL;
 
 out_balanced:
 	env->imbalance = 0;