sched/fair: Properly deactivate sched_delayed task upon class change

__sched_setscheduler() goes through an enqueue/dequeue cycle like so:

  flags := DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
  prev_class->dequeue_task(rq, p, flags);
  new_class->enqueue_task(rq, p, flags);

when prev_class := fair_sched_class, this is followed by:

  dequeue_task(rq, p, DEQUEUE_NOCLOCK | DEQUEUE_SLEEP);

the idea being that since the task has switched classes, we need to drop
the sched_delayed logic and have that task be deactivated per its previous
dequeue_task(..., DEQUEUE_SLEEP).

Unfortunately, this leaves the task on_rq. This is missing the tail end of
dequeue_entities() that issues __block_task(), which __sched_setscheduler()
won't have done due to not using DEQUEUE_DELAYED - not that it should, as
it is pretty much a fair_sched_class specific thing.

Make switched_from_fair() properly deactivate sched_delayed tasks upon
class changes via __block_task(), as if a
  dequeue_task(..., DEQUEUE_DELAYED)
had been issued.

Fixes: 2e0199df ("sched/fair: Prepare exit/cleanup paths for delayed_dequeue")
Reported-by: "Paul E. McKenney" <paulmck@kernel.org>
Reported-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20240829135353.1524260-1-vschneid@redhat.com

kernel/sched/fair.c

@@ -5456,6 +5456,13 @@ static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
 
 static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq);
 
+static inline void finish_delayed_dequeue_entity(struct sched_entity *se)
+{
+	se->sched_delayed = 0;
+	if (sched_feat(DELAY_ZERO) && se->vlag > 0)
+		se->vlag = 0;
+}
+
 static bool
 dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 {
@@ -5531,11 +5538,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE)
 		update_min_vruntime(cfs_rq);
 
-	if (flags & DEQUEUE_DELAYED) {
-		se->sched_delayed = 0;
-		if (sched_feat(DELAY_ZERO) && se->vlag > 0)
-			se->vlag = 0;
-	}
+	if (flags & DEQUEUE_DELAYED)
+		finish_delayed_dequeue_entity(se);
 
 	if (cfs_rq->nr_running == 0)
 		update_idle_cfs_rq_clock_pelt(cfs_rq);
@@ -13107,11 +13111,16 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
 	 * and we cannot use DEQUEUE_DELAYED.
 	 */
 	if (p->se.sched_delayed) {
+		/* First, dequeue it from its new class' structures */
 		dequeue_task(rq, p, DEQUEUE_NOCLOCK | DEQUEUE_SLEEP);
-		p->se.sched_delayed = 0;
+		/*
+		 * Now, clean up the fair_sched_class side of things
+		 * related to sched_delayed being true and that wasn't done
+		 * due to the generic dequeue not using DEQUEUE_DELAYED.
+		 */
+		finish_delayed_dequeue_entity(&p->se);
 		p->se.rel_deadline = 0;
-		if (sched_feat(DELAY_ZERO) && p->se.vlag > 0)
-			p->se.vlag = 0;
+		__block_task(rq, p);
 	}
 }