Commit c5cd6fd7 authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

sched/fair: Fix the negative lag increase fix 



Vincent reported that my rework of his original patch lost a little
something.

Specifically it got the return value wrong; it should not compare
against the old se->vlag, but rather against the current value. Since
the thing that matters is if the effective vruntime of an entity is
affected and the thing needs repositioning or not.

Fixes: 059258b0 ("sched/fair: Prevent negative lag increase during delayed dequeue")
Reported-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/20260423094107.GT3102624%40noisy.programming.kicks-ass.net
parent 254f4963

kernel/sched/fair.c

+10 −5
Original line number Diff line number Diff line
@@ -847,13 +847,19 @@ static s64 entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se, u64 avrunt 
 * Similarly, check that the entity didn't gain positive lag when DELAY_ZERO

 * is set.

 *

 * Return true if the lag has been adjusted.

 * Return true if the vlag has been modified. Specifically:

 *

 *   se->vlag != avg_vruntime() - se->vruntime

 *

 * This can be due to clamping in entity_lag() or clamping due to

 * sched_delayed. Either way, when vlag is modified and the entity is

 * retained, the tree needs to be adjusted.

 */

static __always_inline

bool update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)

{

	s64 vlag = entity_lag(cfs_rq, se, avg_vruntime(cfs_rq));

	bool ret;

	u64 avruntime = avg_vruntime(cfs_rq);

	s64 vlag = entity_lag(cfs_rq, se, avruntime);



	WARN_ON_ONCE(!se->on_rq);
@@ -863,10 +869,9 @@ bool update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se) 
		if (sched_feat(DELAY_ZERO))

			vlag = min(vlag, 0);

	}

	ret = (vlag == se->vlag);

	se->vlag = vlag;



	return ret;

	return avruntime - vlag != se->vruntime;

}



/*