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;
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