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sched/fair: Revert 6d71a9c616 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag")

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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 6d71a9c616 ("sched/fair: Fix EEVDF entity
placement bug causing scheduling lag") was wrong!

The issue reported in 6d71a9c616 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 66951e4860 ("sched/fair: Fix update_cfs_group()
vs DELAY_DEQUEUE") and tracing vruntime and vlag figures again.

Reported-by: Zicheng Qu <quzicheng@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Shubhang Kaushik <shubhang@os.amperecomputing.com>
Link: https://patch.msgid.link/20260219080625.066102672%40infradead.org
This commit is contained in:
Peter Zijlstra 2026-01-26 20:56:23 +01:00
parent 4823725d9d
commit 101f3498b4
1 changed files with 124 additions and 24 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

148
kernel/sched/fair.c
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@ -822,17 +822,22 @@ static inline u64 cfs_rq_max_slice(struct cfs_rq *cfs_rq);
*
* -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));
}
/*
@ -3898,23 +3903,125 @@ dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
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;
}
@ -3925,30 +4032,23 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
}
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);
@ -5306,7 +5406,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
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;