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sched/fair: Fix flat hierarchy

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When a fair task is enqueued, we must update curr and more precisely
its vruntime before placing the enqueued task so avg vruntime will take
into account the last exec phase.

Example:
TA is an always running task in cgroup G0.
TB is a short running task (cyclictest) in cgroup G1.
The lag of TB always increases up the clamp limit because TB is placed
before TA(curr) is updated (since the last tick). When curr(TA) is
finally updated, its last exec phase provide positive lag to TB

Because TA and TB don't belong to the same group, enqueue_hierarchy() will not
update TA's entity when updating curr but only G0's entity at root level.

The same applies when dequeuing.

This is because update_curr() uses ->h_curr, rather than ->curr, and therefore,
while it is invoked on the root cfs_rq, which contains all the eevdf bits, it
does not do the right thing.

Fixes: 85570f10a4 ("sched/eevdf: Move to a single runqueue")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260812125039.1717249-1-vincent.guittot@linaro.org
This commit is contained in:
Vincent Guittot 2026-08-12 14:50:39 +02:00 committed by Peter Zijlstra
parent 3f8fa8fe90
commit 68e3748781
1 changed files with 12 additions and 3 deletions
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15
kernel/sched/fair.c
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@ -7903,8 +7903,6 @@ requeue_delayed_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
WARN_ON_ONCE(!se->sched_delayed);
WARN_ON_ONCE(!se->on_rq);
update_curr(cfs_rq);
if (update_entity_lag(cfs_rq, se)) {
cfs_rq->h_nr_queued--;
if (se != cfs_rq->curr)
@ -7958,6 +7956,15 @@ static unsigned long enqueue_hierarchy(struct task_struct *p, int flags)
return weight;
}
/* Update curr's vruntime before placing entity or updating lag */
static inline void update_curr_eevdf(struct cfs_rq *cfs_rq)
{
if (!cfs_rq->curr)
return;
update_curr(cfs_rq_of(cfs_rq->curr));
}
/*
* The enqueue_task method is called before nr_running is
* increased. Here we update the fair scheduling stats and
@ -7985,6 +7992,8 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
if (!p->se.sched_delayed || (flags & ENQUEUE_DELAYED))
util_est_enqueue(cfs_rq, p);
update_curr_eevdf(cfs_rq);
if (flags & ENQUEUE_DELAYED) {
requeue_delayed_entity(cfs_rq, se);
return;
@ -8105,7 +8114,7 @@ static bool __dequeue_task(struct rq *rq, struct task_struct *p, int flags)
clear_buddies(cfs_rq, se);
update_curr(cfs_rq_of(se));
update_curr_eevdf(cfs_rq);
update_entity_lag(cfs_rq, se);
if (flags & DEQUEUE_DELAYED) {