+++ /dev/null
-From 5f1e9bf4a001dc935b5c913507a27be28e80dccc Mon Sep 17 00:00:00 2001
-From: Sasha Levin <sashal@kernel.org>
-Date: Thu, 9 Jan 2025 11:59:59 +0100
-Subject: sched/fair: Fix EEVDF entity placement bug causing scheduling lag
-
-From: Peter Zijlstra <peterz@infradead.org>
-
-[ Upstream commit 6d71a9c6160479899ee744d2c6d6602a191deb1f ]
-
-I noticed this in my traces today:
-
- turbostat-1222 [006] d..2. 311.935649: reweight_entity: (ffff888108f13e00-ffff88885ef38440-6)
- { weight: 1048576 avg_vruntime: 3184159639071 vruntime: 3184159640194 (-1123) deadline: 3184162621107 } ->
- { weight: 2 avg_vruntime: 3184177463330 vruntime: 3184748414495 (-570951165) deadline: 4747605329439 }
- turbostat-1222 [006] d..2. 311.935651: reweight_entity: (ffff888108f13e00-ffff88885ef38440-6)
- { weight: 2 avg_vruntime: 3184177463330 vruntime: 3184748414495 (-570951165) deadline: 4747605329439 } ->
- { weight: 1048576 avg_vruntime: 3184176414812 vruntime: 3184177464419 (-1049607) deadline: 3184180445332 }
-
-Which is a weight transition: 1048576 -> 2 -> 1048576.
-
-One would expect the lag to shoot out *AND* come back, notably:
-
- -1123*1048576/2 = -588775424
- -588775424*2/1048576 = -1123
-
-Except the trace shows it is all off. Worse, subsequent cycles shoot it
-out further and further.
-
-This made me have a very hard look at reweight_entity(), and
-specifically the ->on_rq case, which is more prominent with
-DELAY_DEQUEUE.
-
-And indeed, it is all sorts of broken. While the computation of the new
-lag is correct, the computation for the new vruntime, using the new lag
-is broken for it does not consider the logic set out in place_entity().
-
-With the below patch, I now see things like:
-
- migration/12-55 [012] d..3. 309.006650: reweight_entity: (ffff8881e0e6f600-ffff88885f235f40-12)
- { weight: 977582 avg_vruntime: 4860513347366 vruntime: 4860513347908 (-542) deadline: 4860516552475 } ->
- { weight: 2 avg_vruntime: 4860528915984 vruntime: 4860793840706 (-264924722) deadline: 6427157349203 }
- migration/14-62 [014] d..3. 309.006698: reweight_entity: (ffff8881e0e6cc00-ffff88885f3b5f40-15)
- { weight: 2 avg_vruntime: 4874472992283 vruntime: 4939833828823 (-65360836540) deadline: 6316614641111 } ->
- { weight: 967149 avg_vruntime: 4874217684324 vruntime: 4874217688559 (-4235) deadline: 4874220535650 }
-
-Which isn't perfect yet, but much closer.
-
-Reported-by: Doug Smythies <dsmythies@telus.net>
-Reported-by: Ingo Molnar <mingo@kernel.org>
-Tested-by: Ingo Molnar <mingo@kernel.org>
-Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
-Signed-off-by: Ingo Molnar <mingo@kernel.org>
-Fixes: eab03c23c2a1 ("sched/eevdf: Fix vruntime adjustment on reweight")
-Link: https://lore.kernel.org/r/20250109105959.GA2981@noisy.programming.kicks-ass.net
-Signed-off-by: Sasha Levin <sashal@kernel.org>
----
- kernel/sched/fair.c | 145 ++++++--------------------------------------
- 1 file changed, 18 insertions(+), 127 deletions(-)
-
-diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
-index 1ca96c99872f0..bf8153af96879 100644
---- a/kernel/sched/fair.c
-+++ b/kernel/sched/fair.c
-@@ -689,21 +689,16 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
- *
- * 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);
- }
-
- /*
-@@ -3774,137 +3769,32 @@ static inline void
- 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);
-
-@@ -3919,6 +3809,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
- 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);
-
-@@ -5359,7 +5250,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
-
- 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;
---
-2.39.5
-
projects / thirdparty / kernel / stable-queue.git / commitdiff
