[tip: sched/core] sched/eevdf: Delayed dequeue task can't preempt

[tip-bot2 for Vincent Guittot]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     04998aa54848f15332202d0bea008d2ca1ed1713
Gitweb:        https://git.kernel.org/tip/04998aa54848f15332202d0bea008d2ca1ed1713
Author:        Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate:    Wed, 01 Jul 2026 18:49:20 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 02 Jul 2026 09:43:17 +02:00

sched/eevdf: Delayed dequeue task can't preempt

Load balancing can migrate delayed dequeue tasks to even the load between
CPUs.

sched_balance_rq()
  -> detach_task()
    -> deactivate_task(DEQUEUE_NOCLOCK)
    -> set_task_cpu(dst_cpu)

  -> attach_task()
    -> activate_task(ENQUEUE_NOCLOCK)
    -> wakeup_preempt()

A delayed task with shorter slice can be dequeued during pick_next_entity()
but then jump to preempt because eligible.

Always jump to update for delayed dequeue task even with shorter slice.

Fixes: ba0d3bf5f97b ("sched/eevdf: Cancel slice protection if short slice task is eligible")
Reported-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Link: https://patch.msgid.link/20260701164920.1571352-1-vincent.guittot@xxxxxxxxxx
---
 kernel/sched/fair.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fc6cd55..df8c9c2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9949,6 +9949,13 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
 		goto update;
 
 	/*
+	 * Do not preempt for tasks that are sched_delayed as it would violate
+	 * EEVDF to forcibly queue an ineligible task.
+	 */
+	if (pse->sched_delayed)
+		goto update;
+
+	/*
 	 * If @p has a shorter slice than current and @p is eligible, override
 	 * current's slice protection in order to allow preemption.
 	 */
@@ -9959,11 +9966,9 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
 
 	/*
 	 * Ignore wakee preemption on WF_FORK as it is less likely that
-	 * there is shared data as exec often follow fork. Do not
-	 * preempt for tasks that are sched_delayed as it would violate
-	 * EEVDF to forcibly queue an ineligible task.
+	 * there is shared data as exec often follow fork.
 	 */
-	if ((wake_flags & WF_FORK) || pse->sched_delayed)
+	if (wake_flags & WF_FORK)
 		goto update;
 
 	/* Prefer picking wakee soon if appropriate. */