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: 059258b0d424 ("sched/fair: Prevent negative lag increase during delayed dequeue")
Reported-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/20260423094107.GT3102624%40noisy.programming.kicks-ass.net

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 69361c63353ad5b3d23529a54a384a5a32ffe1db..615861d963571ec48ee1e1591906c02bfbb3d923 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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;
 }
 
 /*