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

Peter Zijlstra
2026-04-02T13:14:06Z

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;
}

/*