[PATCH v2] sched/fair: Fix wakeup_preempt_fair for not waking up task

[Vincent Guittot]

The assumption that p is always enqueued and not delayed, is only true for
wakeup. If p was moved while delayed, pick_next_entity will dequeue it
and the cfs might become empty. Test if there are still queued tasks
before trying again to determine if p could be the next one to be picked.

There are at least 2 cases:

When cfs becomes idle, it tries to pull tasks but if those pulled tasks are
delayed, they will be dequeued when attached to cfs.
  attach_tasks() -> attach_task() -> wakeup_preempt(rq, p, 0);

A misfit task running on cfs A triggers a load balance to be pulled on a
better cpu, the load balance on cfs B starts an active load balance to
pulled the running misfit task. If there is a delayed dequeue task on cfs A
, it can be pulled instead of the previously running misfit task.
  attach_one_task() -> attach_task() -> wakeup_preempt(rq, p, 0);

Fixes: ac8e69e69363 ("sched/fair: Fix wakeup_preempt_fair() vs delayed dequeue")
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---

Change since v1:
- Use cfs_rq->nr_queued instead of WF_TTWU flag to cover all cases instead
  of just wakeup.

I'm preparing patches to cover those useless newly idle and active load
balance task migration.

 kernel/sched/fair.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 728965851842..24087b0f25f6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9145,9 +9145,10 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
 
 	/*
 	 * Because p is enqueued, nse being null can only mean that we
-	 * dequeued a delayed task.
+	 * dequeued a delayed task. If there are still entities queued in
+	 * cfs, check if the next one will be p.
 	 */
-	if (!nse)
+	if (!nse && cfs_rq->nr_queued)
 		goto pick;
 
 	if (sched_feat(RUN_TO_PARITY))
-- 
2.43.0