[PATCH v2 6/7] sched/fair: Revert 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag")

[Peter Zijlstra]

Zicheng Qu reported that, because avg_vruntime() always includes
cfs_rq->curr, when ->on_rq, place_entity() doesn't work right.

Specifically, the lag scaling in place_entity() relies on
avg_vruntime() being the state *before* placement of the new entity.
However in this case avg_vruntime() will actually already include the
entity, which breaks things.

Also, Zicheng Qu argues that avg_vruntime should be invariant under
reweight. IOW commit 6d71a9c61604 ("sched/fair: Fix EEVDF entity
placement bug causing scheduling lag") was wrong!

The issue reported in 6d71a9c61604 could possibly be explained by
rounding artifacts -- notably the extreme weight '2' is outside of the
range of avg_vruntime/sum_w_vruntime, since that uses
scale_load_down(). By scaling vruntime by the real weight, but
accounting it in vruntime with a factor 1024 more, the average moves
significantly. However, that is now cured.

Tested by reverting 66951e4860d3 ("sched/fair: Fix update_cfs_group()
vs DELAY_DEQUEUE") and tracing vruntime and vlag figures again.

Reported-by: Zicheng Qu <quzicheng@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: Shubhang Kaushik <shubhang@xxxxxxxxxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c |  148 +++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 124 insertions(+), 24 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -819,17 +819,22 @@ static inline u64 cfs_rq_max_slice(struc
  *
  *   -r_max < lag < max(r_max, q)
  */
-static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
+static s64 entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se, u64 avruntime)
 {
 	u64 max_slice = cfs_rq_max_slice(cfs_rq) + TICK_NSEC;
 	s64 vlag, limit;
 
-	WARN_ON_ONCE(!se->on_rq);
-
-	vlag = avg_vruntime(cfs_rq) - se->vruntime;
+	vlag = avruntime - se->vruntime;
 	limit = calc_delta_fair(max_slice, se);
 
-	se->vlag = clamp(vlag, -limit, limit);
+	return clamp(vlag, -limit, limit);
+}
+
+static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
+{
+	WARN_ON_ONCE(!se->on_rq);
+
+	se->vlag = entity_lag(cfs_rq, se, avg_vruntime(cfs_rq));
 }
 
 /*
@@ -3895,23 +3900,125 @@ dequeue_load_avg(struct cfs_rq *cfs_rq,
 		    se_weight(se) * -se->avg.load_sum);
 }
 
-static void place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags);
+static void
+rescale_entity(struct sched_entity *se, unsigned long weight, bool rel_vprot)
+{
+	unsigned long old_weight = se->load.weight;
+
+	/*
+	 * 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'
+	 */
+	se->vlag = div64_long(se->vlag * old_weight, weight);
+
+	/*
+	 * 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'
+	 */
+	if (se->rel_deadline)
+		se->deadline = div64_long(se->deadline * old_weight, weight);
+
+	if (rel_vprot)
+		se->vprot = div64_long(se->vprot * old_weight, weight);
+}
 
 static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 			    unsigned long weight)
 {
 	bool curr = cfs_rq->curr == se;
 	bool rel_vprot = false;
-	u64 vprot;
+	u64 avruntime = 0;
 
 	if (se->on_rq) {
 		/* commit outstanding execution time */
 		update_curr(cfs_rq);
-		update_entity_lag(cfs_rq, se);
-		se->deadline -= se->vruntime;
+		avruntime = avg_vruntime(cfs_rq);
+		se->vlag = entity_lag(cfs_rq, se, avruntime);
+		se->deadline -= avruntime;
 		se->rel_deadline = 1;
 		if (curr && protect_slice(se)) {
-			vprot = se->vprot - se->vruntime;
+			se->vprot -= avruntime;
 			rel_vprot = true;
 		}
 
@@ -3922,30 +4029,23 @@ static void reweight_entity(struct cfs_r
 	}
 	dequeue_load_avg(cfs_rq, se);
 
-	/*
-	 * 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 = div64_long(se->vlag * se->load.weight, weight);
-	if (se->rel_deadline)
-		se->deadline = div64_long(se->deadline * se->load.weight, weight);
-
-	if (rel_vprot)
-		vprot = div64_long(vprot * se->load.weight, weight);
+	rescale_entity(se, weight, rel_vprot);
 
 	update_load_set(&se->load, weight);
 
 	do {
 		u32 divider = get_pelt_divider(&se->avg);
-
 		se->avg.load_avg = div_u64(se_weight(se) * se->avg.load_sum, divider);
 	} while (0);
 
 	enqueue_load_avg(cfs_rq, se);
 	if (se->on_rq) {
-		place_entity(cfs_rq, se, 0);
 		if (rel_vprot)
-			se->vprot = se->vruntime + vprot;
+			se->vprot += avruntime;
+		se->deadline += avruntime;
+		se->rel_deadline = 0;
+		se->vruntime = avruntime - se->vlag;
+
 		update_load_add(&cfs_rq->load, se->load.weight);
 		if (!curr)
 			__enqueue_entity(cfs_rq, se);
@@ -5303,7 +5403,7 @@ place_entity(struct cfs_rq *cfs_rq, stru
 
 	se->vruntime = vruntime - lag;
 
-	if (se->rel_deadline) {
+	if (sched_feat(PLACE_REL_DEADLINE) && se->rel_deadline) {
 		se->deadline += se->vruntime;
 		se->rel_deadline = 0;
 		return;