Re: [PATCH v2] sched: fair: Prevent negative lag increase during delayed dequeue

[Peter Zijlstra]

On Tue, Mar 31, 2026 at 06:23:52PM +0200, Vincent Guittot wrote:
> +/*
> + * Delayed dequeue aims to reduce the negative lag of a dequeued task.
> + * While updating the lag of an entity, check that negative lag didn't increase
> + * during the delayed dequeue period which would be unfair.
> + * Similarly, check that the entity didn't gain positive lag when DELAY_ZERO is
> + * set.
> + *
> + * Return true if the lag has been adjusted.
> + */
> +static bool update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> +	s64 vlag;
> +
>  	WARN_ON_ONCE(!se->on_rq);
>  
> -	se->vlag = entity_lag(cfs_rq, se, avg_vruntime(cfs_rq));
> +	vlag = entity_lag(cfs_rq, se, avg_vruntime(cfs_rq));
> +
> +	if (se->sched_delayed)
> +		/* previous vlag < 0 otherwise se would not be delayed */
> +		se->vlag = clamp(vlag, se->vlag, sched_feat(DELAY_ZERO) ? 0 : S64_MAX);
> +	else
> +		se->vlag = vlag;
> +
> +	return (vlag != se->vlag);
>  }

Would you mind terribly if I write this like so?

---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -841,29 +841,32 @@ static s64 entity_lag(struct cfs_rq *cfs
 }
 
 /*
- * Delayed dequeue aims to reduce the negative lag of a dequeued task.
- * While updating the lag of an entity, check that negative lag didn't increase
+ * Delayed dequeue aims to reduce the negative lag of a dequeued task. While
+ * updating the lag of an entity, check that negative lag didn't increase
  * during the delayed dequeue period which would be unfair.
- * Similarly, check that the entity didn't gain positive lag when DELAY_ZERO is
- * set.
+ * Similarly, check that the entity didn't gain positive lag when DELAY_ZERO
+ * is set.
  *
  * Return true if the lag has been adjusted.
  */
-static bool update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
+static __always_inline
+bool update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	s64 vlag;
+	s64 vlag = entity_lag(cfs_rq, se, avg_vruntime(cfs_rq));
+	bool ret;
 
 	WARN_ON_ONCE(!se->on_rq);
 
-	vlag = entity_lag(cfs_rq, se, avg_vruntime(cfs_rq));
-
-	if (se->sched_delayed)
+	if (se->sched_delayed) {
 		/* previous vlag < 0 otherwise se would not be delayed */
-		se->vlag = clamp(vlag, se->vlag, sched_feat(DELAY_ZERO) ? 0 : S64_MAX);
-	else
-		se->vlag = vlag;
+		vlag = max(vlag, se->vlag);
+		if (sched_feat(DELAY_ZERO))
+			vlag = min(vlag, 0);
+	}
+	ret = (vlag == se->vlag);
+	se->vlag = vlag;
 
-	return (vlag != se->vlag);
+	return ret;
 }
 
 /*