Patch "sched/fair: Fix NEXT_BUDDY" has been added to the 6.12-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    sched/fair: Fix NEXT_BUDDY

to the 6.12-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     sched-fair-fix-next_buddy.patch
and it can be found in the queue-6.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 4ad329aa5f61d1ce6c81094d25ee4a35dc4e5ebb
Author: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Date:   Thu Nov 28 12:59:54 2024 +0530

    sched/fair: Fix NEXT_BUDDY
    
    [ Upstream commit 493afbd187c4c9cc1642792c0d9ba400c3d6d90d ]
    
    Adam reports that enabling NEXT_BUDDY insta triggers a WARN in
    pick_next_entity().
    
    Moving clear_buddies() up before the delayed dequeue bits ensures
    no ->next buddy becomes delayed. Further ensure no new ->next buddy
    ever starts as delayed.
    
    Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
    Reported-by: Adam Li <adamli@xxxxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
    Tested-by: Adam Li <adamli@xxxxxxxxxxxxxxxxxxxxxx>
    Link: https://lkml.kernel.org/r/670a0d54-e398-4b1f-8a6e-90784e2fdf89@xxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 782ce70ebd1b..c467e389cd6f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5484,6 +5484,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	bool sleep = flags & DEQUEUE_SLEEP;
 
 	update_curr(cfs_rq);
+	clear_buddies(cfs_rq, se);
 
 	if (flags & DEQUEUE_DELAYED) {
 		SCHED_WARN_ON(!se->sched_delayed);
@@ -5500,8 +5501,6 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 
 		if (sched_feat(DELAY_DEQUEUE) && delay &&
 		    !entity_eligible(cfs_rq, se)) {
-			if (cfs_rq->next == se)
-				cfs_rq->next = NULL;
 			update_load_avg(cfs_rq, se, 0);
 			se->sched_delayed = 1;
 			return false;
@@ -5526,8 +5525,6 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 
 	update_stats_dequeue_fair(cfs_rq, se, flags);
 
-	clear_buddies(cfs_rq, se);
-
 	update_entity_lag(cfs_rq, se);
 	if (sched_feat(PLACE_REL_DEADLINE) && !sleep) {
 		se->deadline -= se->vruntime;
@@ -8786,7 +8783,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	if (unlikely(throttled_hierarchy(cfs_rq_of(pse))))
 		return;
 
-	if (sched_feat(NEXT_BUDDY) && !(wake_flags & WF_FORK)) {
+	if (sched_feat(NEXT_BUDDY) && !(wake_flags & WF_FORK) && !pse->sched_delayed) {
 		set_next_buddy(pse);
 	}