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sched/eevdf: Fix vruntime adjustment on reweight

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vruntime of the (on_rq && !0-lag) entity needs to be adjusted when
it gets re-weighted, and the calculations can be simplified based
on the fact that re-weight won't change the w-average of all the
entities. Please check the proofs in comments.

But adjusting vruntime can also cause position change in RB-tree
hence require re-queue to fix up which might be costly. This might
be avoided by deferring adjustment to the time the entity actually
leaves tree (dequeue/pick), but that will negatively affect task
selection and probably not good enough either.

Fixes: 147f3efaa2 ("sched/fair: Implement an EEVDF-like scheduling policy")
Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20231107090510.71322-2-wuyun.abel@bytedance.com
This commit is contained in:
Abel Wu 2023-11-07 17:05:07 +08:00 committed by Peter Zijlstra
parent b85ea95d08
commit eab03c23c2
1 changed files with 128 additions and 23 deletions
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151
kernel/sched/fair.c
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@ -3666,41 +3666,140 @@ static inline void
dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
#endif
static void reweight_eevdf(struct cfs_rq *cfs_rq, struct sched_entity *se,
unsigned long weight)
{
unsigned long old_weight = se->load.weight;
u64 avruntime = avg_vruntime(cfs_rq);
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 = (s64)(avruntime - se->vruntime);
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 reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
unsigned long weight)
{
unsigned long old_weight = se->load.weight;
bool curr = cfs_rq->curr == se;
if (se->on_rq) {
/* commit outstanding execution time */
if (cfs_rq->curr == se)
if (curr)
update_curr(cfs_rq);
else
avg_vruntime_sub(cfs_rq, se);
__dequeue_entity(cfs_rq, se);
update_load_sub(&cfs_rq->load, se->load.weight);
}
dequeue_load_avg(cfs_rq, se);
update_load_set(&se->load, weight);
if (!se->on_rq) {
/*
* 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 * old_weight, weight);
se->vlag = div_s64(se->vlag * se->load.weight, weight);
} else {
s64 deadline = se->deadline - se->vruntime;
/*
* When the weight changes, the virtual time slope changes and
* we should adjust the relative virtual deadline accordingly.
*/
deadline = div_s64(deadline * old_weight, weight);
se->deadline = se->vruntime + deadline;
if (se != cfs_rq->curr)
min_deadline_cb_propagate(&se->run_node, NULL);
reweight_eevdf(cfs_rq, se, weight);
}
update_load_set(&se->load, weight);
#ifdef CONFIG_SMP
do {
u32 divider = get_pelt_divider(&se->avg);
@ -3712,8 +3811,17 @@ 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);
if (cfs_rq->curr != se)
avg_vruntime_add(cfs_rq, se);
if (!curr) {
/*
* The entity's vruntime has been adjusted, so let's check
* whether the rq-wide min_vruntime needs updated too. Since
* the calculations above require stable min_vruntime rather
* than up-to-date one, we do the update at the end of the
* reweight process.
*/
__enqueue_entity(cfs_rq, se);
update_min_vruntime(cfs_rq);
}
}
}
@ -3857,14 +3965,11 @@ static void update_cfs_group(struct sched_entity *se)
#ifndef CONFIG_SMP
shares = READ_ONCE(gcfs_rq->tg->shares);
if (likely(se->load.weight == shares))
return;
#else
shares = calc_group_shares(gcfs_rq);
shares = calc_group_shares(gcfs_rq);
#endif
reweight_entity(cfs_rq_of(se), se, shares);
if (unlikely(se->load.weight != shares))
reweight_entity(cfs_rq_of(se), se, shares);
}
#else /* CONFIG_FAIR_GROUP_SCHED */