Merge tag 'sched_urgent_for_v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull scheduler fixes from Borislav Petkov:

- Do not adjust the weight of empty group entities and avoid
scheduling artifacts

- Avoid scheduling lag by computing lag properly and thus address
an EEVDF entity placement issue

* tag 'sched_urgent_for_v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
sched/fair: Fix update_cfs_group() vs DELAY_DEQUEUE
sched/fair: Fix EEVDF entity placement bug causing scheduling lag

kernel/sched/fair.c

···
  *
  * XXX could add max_slice to the augmented data to track this.
  */
-static s64 entity_lag(u64 avruntime, struct sched_entity *se)
+static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
 	s64 vlag, limit;
 
-	vlag = avruntime - se->vruntime;
-	limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
-
-	return clamp(vlag, -limit, limit);
-}
-
-static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
-{
 	SCHED_WARN_ON(!se->on_rq);
 
-	se->vlag = entity_lag(avg_vruntime(cfs_rq), se);
+	vlag = avg_vruntime(cfs_rq) - se->vruntime;
+	limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
+
+	se->vlag = clamp(vlag, -limit, limit);
 }
 
 /*
···
 dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
 #endif
 
-static void reweight_eevdf(struct sched_entity *se, u64 avruntime,
-			   unsigned long weight)
-{
-	unsigned long old_weight = se->load.weight;
-	s64 vlag, vslice;
-
-	/*
-	 * VRUNTIME
-	 * --------
-	 *
-	 * COROLLARY #1: The virtual runtime of the entity needs to be
-	 * adjusted if re-weight at !0-lag point.
-	 *
-	 * Proof: For contradiction assume this is not true, so we can
-	 * re-weight without changing vruntime at !0-lag point.
-	 *
-	 *             Weight	VRuntime   Avg-VRuntime
-	 *     before    w          v            V
-	 *      after    w'         v'           V'
-	 *
-	 * Since lag needs to be preserved through re-weight:
-	 *
-	 *	lag = (V - v)*w = (V'- v')*w', where v = v'
-	 *	==>	V' = (V - v)*w/w' + v		(1)
-	 *
-	 * Let W be the total weight of the entities before reweight,
-	 * since V' is the new weighted average of entities:
-	 *
-	 *	V' = (WV + w'v - wv) / (W + w' - w)	(2)
-	 *
-	 * by using (1) & (2) we obtain:
-	 *
-	 *	(WV + w'v - wv) / (W + w' - w) = (V - v)*w/w' + v
-	 *	==> (WV-Wv+Wv+w'v-wv)/(W+w'-w) = (V - v)*w/w' + v
-	 *	==> (WV - Wv)/(W + w' - w) + v = (V - v)*w/w' + v
-	 *	==>	(V - v)*W/(W + w' - w) = (V - v)*w/w' (3)
-	 *
-	 * Since we are doing at !0-lag point which means V != v, we
-	 * can simplify (3):
-	 *
-	 *	==>	W / (W + w' - w) = w / w'
-	 *	==>	Ww' = Ww + ww' - ww
-	 *	==>	W * (w' - w) = w * (w' - w)
-	 *	==>	W = w	(re-weight indicates w' != w)
-	 *
-	 * So the cfs_rq contains only one entity, hence vruntime of
-	 * the entity @v should always equal to the cfs_rq's weighted
-	 * average vruntime @V, which means we will always re-weight
-	 * at 0-lag point, thus breach assumption. Proof completed.
-	 *
-	 *
-	 * COROLLARY #2: Re-weight does NOT affect weighted average
-	 * vruntime of all the entities.
-	 *
-	 * Proof: According to corollary #1, Eq. (1) should be:
-	 *
-	 *	(V - v)*w = (V' - v')*w'
-	 *	==>    v' = V' - (V - v)*w/w'		(4)
-	 *
-	 * According to the weighted average formula, we have:
-	 *
-	 *	V' = (WV - wv + w'v') / (W - w + w')
-	 *	   = (WV - wv + w'(V' - (V - v)w/w')) / (W - w + w')
-	 *	   = (WV - wv + w'V' - Vw + wv) / (W - w + w')
-	 *	   = (WV + w'V' - Vw) / (W - w + w')
-	 *
-	 *	==>  V'*(W - w + w') = WV + w'V' - Vw
-	 *	==>	V' * (W - w) = (W - w) * V	(5)
-	 *
-	 * If the entity is the only one in the cfs_rq, then reweight
-	 * always occurs at 0-lag point, so V won't change. Or else
-	 * there are other entities, hence W != w, then Eq. (5) turns
-	 * into V' = V. So V won't change in either case, proof done.
-	 *
-	 *
-	 * So according to corollary #1 & #2, the effect of re-weight
-	 * on vruntime should be:
-	 *
-	 *	v' = V' - (V - v) * w / w'		(4)
-	 *	   = V  - (V - v) * w / w'
-	 *	   = V  - vl * w / w'
-	 *	   = V  - vl'
-	 */
-	if (avruntime != se->vruntime) {
-		vlag = entity_lag(avruntime, se);
-		vlag = div_s64(vlag * old_weight, weight);
-		se->vruntime = avruntime - vlag;
-	}
-
-	/*
-	 * DEADLINE
-	 * --------
-	 *
-	 * When the weight changes, the virtual time slope changes and
-	 * we should adjust the relative virtual deadline accordingly.
-	 *
-	 *	d' = v' + (d - v)*w/w'
-	 *	   = V' - (V - v)*w/w' + (d - v)*w/w'
-	 *	   = V  - (V - v)*w/w' + (d - v)*w/w'
-	 *	   = V  + (d - V)*w/w'
-	 */
-	vslice = (s64)(se->deadline - avruntime);
-	vslice = div_s64(vslice * old_weight, weight);
-	se->deadline = avruntime + vslice;
-}
+static void place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags);
 
 static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 			    unsigned long weight)
 {
 	bool curr = cfs_rq->curr == se;
-	u64 avruntime;
 
 	if (se->on_rq) {
 		/* commit outstanding execution time */
 		update_curr(cfs_rq);
-		avruntime = avg_vruntime(cfs_rq);
+		update_entity_lag(cfs_rq, se);
+		se->deadline -= se->vruntime;
+		se->rel_deadline = 1;
 		if (!curr)
 			__dequeue_entity(cfs_rq, se);
 		update_load_sub(&cfs_rq->load, se->load.weight);
 	}
 	dequeue_load_avg(cfs_rq, se);
 
-	if (se->on_rq) {
-		reweight_eevdf(se, avruntime, weight);
-	} else {
-		/*
-		 * Because we keep se->vlag = V - v_i, while: lag_i = w_i*(V - v_i),
-		 * we need to scale se->vlag when w_i changes.
-		 */
-		se->vlag = div_s64(se->vlag * se->load.weight, weight);
-	}
+	/*
+	 * Because we keep se->vlag = V - v_i, while: lag_i = w_i*(V - v_i),
+	 * we need to scale se->vlag when w_i changes.
+	 */
+	se->vlag = div_s64(se->vlag * se->load.weight, weight);
+	if (se->rel_deadline)
+		se->deadline = div_s64(se->deadline * se->load.weight, weight);
 
 	update_load_set(&se->load, weight);
 
···
 	enqueue_load_avg(cfs_rq, se);
 	if (se->on_rq) {
 		update_load_add(&cfs_rq->load, se->load.weight);
+		place_entity(cfs_rq, se, 0);
 		if (!curr)
 			__enqueue_entity(cfs_rq, se);
 
···
 	struct cfs_rq *gcfs_rq = group_cfs_rq(se);
 	long shares;
 
-	if (!gcfs_rq)
+	/*
+	 * When a group becomes empty, preserve its weight. This matters for
+	 * DELAY_DEQUEUE.
+	 */
+	if (!gcfs_rq || !gcfs_rq->load.weight)
 		return;
 
 	if (throttled_hierarchy(gcfs_rq))
···
 
 	se->vruntime = vruntime - lag;
 
-	if (sched_feat(PLACE_REL_DEADLINE) && se->rel_deadline) {
+	if (se->rel_deadline) {
 		se->deadline += se->vruntime;
 		se->rel_deadline = 0;
 		return;