Messages in this thread | | | Date | Fri, 17 Nov 2023 21:07:55 +0800 | Subject | Re: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6 sched/fair: Add lag based placement) | From | Abel Wu <> |
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On 11/17/23 8:37 PM, Peter Zijlstra Wrote: > On Fri, Nov 17, 2023 at 01:24:21PM +0100, Tobias Huschle wrote: >> On Fri, Nov 17, 2023 at 10:23:18AM +0100, Peter Zijlstra wrote: > >>> kworkers are typically not in cgroups and are part of the root cgroup, >>> but what's a vhost and where does it live? >> >> The qemu instances of the two KVM guests are placed into cgroups. >> The vhosts run within the context of these qemu instances (4 threads per guest). >> So they are also put into those cgroups. >> >> I'll answer the other questions you brought up as well, but I guess that one >> is most critical: >> >>> >>> After confirming both tasks are indeed in the same cgroup ofcourse, >>> because if they're not, vruntime will be meaningless to compare and we >>> should look elsewhere. >> >> In that case we probably have to go with elsewhere ... which is good to know. > > Ah, so if this is a cgroup issue, it might be worth trying this patch > that we have in tip/sched/urgent.
And please also apply this fix: https://lore.kernel.org/all/20231117080106.12890-1-s921975628@gmail.com/
> > I'll try and read the rest of the email a little later, gotta run > errands first. > > --- > > commit eab03c23c2a162085b13200d7942fc5a00b5ccc8 > Author: Abel Wu <wuyun.abel@bytedance.com> > Date: Tue Nov 7 17:05:07 2023 +0800 > > sched/eevdf: Fix vruntime adjustment on reweight > > 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: 147f3efaa241 ("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 > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 2048138ce54b..025d90925bf6 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -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 */
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