[tip: sched/core] sched/fair: Fix NEXT_BUDDY

[tip-bot2 for K Prateek Nayak]

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

Commit-ID:     493afbd187c4c9cc1642792c0d9ba400c3d6d90d
Gitweb:        https://git.kernel.org/tip/493afbd187c4c9cc1642792c0d9ba400c3d6d90d
Author:        K Prateek Nayak <kprateek.nayak@xxxxxxx>
AuthorDate:    Thu, 28 Nov 2024 12:59:54 +05:30
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 09 Dec 2024 11:48:09 +01:00

sched/fair: Fix NEXT_BUDDY

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
---
 kernel/sched/fair.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 05b8f1e..9d7a2dd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5478,6 +5478,7 @@ 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);
@@ -5494,8 +5495,6 @@ 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;
@@ -5520,8 +5519,6 @@ 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;
@@ -8774,7 +8771,7 @@ 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);
 	}