sched/fair: Fix wakeup_preempt_fair() for not waking up task

Make sure to only call pick_next_entity() on an non-empty cfs_rq.

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: ac8e69e ("sched/fair: Fix wakeup_preempt_fair() vs delayed dequeue")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260503104503.1732682-1-vincent.guittot@linaro.org

Author: vingu-linaro

kernel/sched/fair.c

@@ -9174,9 +9174,10 @@ 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))