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

[Vincent Guittot]

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>
---
 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 fc6cd55f9d22..df8c9c2c7918 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9948,6 +9948,13 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
 	if (unlikely(!normal_policy(p->policy)))
 		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. */
-- 
2.43.0