Re: [PATCH v2 10/10] sched/eevdf: Move to a single runqueue

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 20, 2026 at 06:32:11PM +0200, Vincent Guittot wrote:

> I finally fount the root cause of regression: the update of entity lag happened
> after the task has been dequeued which screwed update_entity_lag():
> 
> update_entity_lag must be called after updating curr and cfs_rd and before 
> clearing on_rq
> 
> With the fix below I'm back to original hackbench figures and maybe even a bit better.
> I haven't checked shceduling latency yet
> 
> ---
>  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 77d0e1937f2c..32fe57004f27 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5753,6 +5753,9 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>  
>  	update_stats_dequeue_fair(cfs_rq, se, flags);
>  
> +	if (entity_is_task(se))
> +		update_entity_lag(&rq_of(cfs_rq)->cfs, se);
> +
>  	se->on_rq = 0;
>  	account_entity_dequeue(cfs_rq, se);
>  
> @@ -7423,6 +7426,7 @@ static bool __dequeue_task(struct rq *rq, struct task_struct *p, int flags)
>  		if (sched_feat(DELAY_DEQUEUE) && delay &&
>  		    !entity_eligible(cfs_rq, se)) {
>  			update_load_avg(cfs_rq_of(se), se, 0);
> +			update_entity_lag(cfs_rq, se);
>  			set_delayed(se);
>  			return false;
>  		}
> @@ -7430,7 +7434,6 @@ static bool __dequeue_task(struct rq *rq, struct task_struct *p, int flags)
>  
>  	dequeue_hierarchy(p, flags);
>  
> -	update_entity_lag(cfs_rq, se);
>  	if (sched_feat(PLACE_REL_DEADLINE) && !task_sleep) {
>  		se->deadline -= se->vruntime;
>  		se->rel_deadline = 1;

Argh!!! Thank you! I've gone blind staring at all this :/

Would it not be simpler to just move the update_entity_lag() call up a
bit, like so?

---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7999,6 +7999,9 @@ static bool __dequeue_task(struct rq *rq
 
 	clear_buddies(cfs_rq, se);
 
+	update_curr(cfs_rq);
+	update_entity_lag(cfs_rq, se);
+
 	if (flags & DEQUEUE_DELAYED) {
 		WARN_ON_ONCE(!se->sched_delayed);
 	} else {
@@ -8022,7 +8025,6 @@ static bool __dequeue_task(struct rq *rq
 
 	dequeue_hierarchy(p, flags);
 
-	update_entity_lag(cfs_rq, se);
 	if (sched_feat(PLACE_REL_DEADLINE) && !task_sleep) {
 		se->deadline -= se->vruntime;
 		se->rel_deadline = 1;