[PATCH] sched/fair: Call update_util_est() after dequeue_entities()

[Qais Yousef]

update_util_est() reads task_util() at dequeue which is updated in
dequeue_entities(). To read the accurate util_avg at dequeue, make sure
to do the read after load_avg is updated in dequeue_entities().

util_est for a periodic task before

                                periodic-3114 util_est.enqueued running
   ┌───────────────────────────────────────────────────────────────────────────────────────────────┐
183┤                ▖▗  ▐▖         ▖ ▗▙   ▗   ▗▙▖▖       ▖▖   ▖       ▖▖        ▗  ▟  ▗▄▖          │
139┤               ▐▛█▜▙▞▀▄▄▞▚▄▟█▞▙█▄▟▀▚▄▄▞▚▄▄▟▀▀▛▄▝▄▄▄▙█▛▛█▛▜▛▄▄▀▄█▙▛▛▛▙▄▀▄▄▖▜▄▟█▟▀▜▟▄▜▀▄▄▟▙▖     │
 95┤              ▐▀    ▘   ▝   ▝        ▝▘        ▘   ▘▘       ▝▘       ▝▘  ▝    ▝        ▀       │
   │              ▛                                                                                │
 51┤             ▐▘                                                                                │
  7┤      ▖▗▗  ▗▄▐                                                                                 │
   └┬─────────┬──────────┬─────────┬──────────┬─────────┬──────────┬─────────┬──────────┬─────────┬┘
  0.00      0.65       1.30      1.96       2.61      3.26       3.91      4.57       5.22     5.87

and after

                                 periodic-2977 util_est.enqueued running
     ┌─────────────────────────────────────────────────────────────────────────────────────────────┐
157.0┤               ▙▄ ▗▄  ▗▄▄▄ ▗▄  ▗▄▄▄▗▄▄  ▗▄▄▖ ▄   ▄▄▄   ▄  ▄▖▖  ▄▄▄▄▄▖▖▝▙▄▄▄▄▄▄▖ ▗▄           │
119.5┤             ▗▄▌▘▀▀ ▀▀▀ ▝▀▀▘▝▀▀▀ ▝▀▘ ▝▀▀▘ ▀▝▀▘▀▀▀▘▝▀▀▀▀▀▀▀▘▝▝▀▀ ▀   ▝▝▀  ▀   ▀▀▀▀            │
 82.0┤             ▟                                                                               │
     │             ▌                                                                               │
 44.5┤             ▌                                                                               │
  7.0┤      ▗   ▗▖ ▌                                                                               │
     └┬─────────┬─────────┬──────────┬─────────┬─────────┬─────────┬──────────┬─────────┬─────────┬┘
    0.00      0.65      1.30       1.95      2.60      3.25      3.90       4.56      5.21     5.86

Note how the signal is noisier and can peak to 183 vs 157 now.

Fixes: b55945c500c5 ("sched: Fix pick_next_task_fair() vs try_to_wake_up() race")
Signed-off-by: Qais Yousef <qyousef@xxxxxxxxxxx>
---

This is split from [1] series where I stumbled upon this problem. AFAICS it
needs backporting all the way to 6.12 LTS.

[1] https://lore.kernel.org/lkml/20260504020003.71306-1-qyousef@xxxxxxxxxxx/

 kernel/sched/fair.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 728965851842..96ba97e5f4ae 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7401,6 +7401,8 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
  */
 static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 {
+	int ret;
+
 	if (task_is_throttled(p)) {
 		dequeue_throttled_task(p, flags);
 		return true;
@@ -7409,8 +7411,9 @@ static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	if (!p->se.sched_delayed)
 		util_est_dequeue(&rq->cfs, p);
 
+	ret = dequeue_entities(rq, &p->se, flags);
 	util_est_update(&rq->cfs, p, flags & DEQUEUE_SLEEP);
-	if (dequeue_entities(rq, &p->se, flags) < 0)
+	if (ret < 0)
 		return false;
 
 	/*
-- 
2.34.1