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sched/core: Fix group_entity's share update

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The update of the share of a cfs_rq is done when its load_avg is updated
but before the group_entity's load_avg has been updated for the past time
slot. This generates wrong load_avg accounting which can be significant
when small tasks are involved in the scheduling.

Let take the example of a task a that is dequeued of its task group A:
   root
  (cfs_rq)
    \
    (se)
     A
    (cfs_rq)
      \
      (se)
       a

Task "a" was the only task in task group A which becomes idle when a is
dequeued.

We have the sequence:

- dequeue_entity a->se
    - update_load_avg(a->se)
    - dequeue_entity_load_avg(A->cfs_rq, a->se)
    - update_cfs_shares(A->cfs_rq)
	A->cfs_rq->load.weight == 0
        A->se->load.weight is updated with the new share (0 in this case)
- dequeue_entity A->se
    - update_load_avg(A->se) but its weight is now null so the last time
      slot (up to a tick) will be accounted with a weight of 0 instead of
      its real weight during the time slot. The last time slot will be
      accounted as an idle one whereas it was a running one.

If the running time of task a is short enough that no tick happens when it
runs, all running time of group entity A->se will be accounted as idle
time.

Instead, we should update the share of a cfs_rq (in fact the weight of its
group entity) only after having updated the load_avg of the group_entity.

update_cfs_shares() now takes the sched_entity as a parameter instead of the
cfs_rq, and the weight of the group_entity is updated only once its load_avg
has been synced with current time.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: pjt@google.com
Link: http://lkml.kernel.org/r/1482335426-7664-1-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Vincent Guittot 2016-12-21 16:50:26 +01:00 committed by Ingo Molnar
parent da9647e076
commit 89ee048f3c
1 changed files with 37 additions and 13 deletions
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50
kernel/sched/fair.c
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@ -2689,16 +2689,20 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);
static void update_cfs_shares(struct cfs_rq *cfs_rq)
static void update_cfs_shares(struct sched_entity *se)
{
struct cfs_rq *cfs_rq = group_cfs_rq(se);
struct task_group *tg;
struct sched_entity *se;
long shares;
tg = cfs_rq->tg;
se = tg->se[cpu_of(rq_of(cfs_rq))];
if (!se || throttled_hierarchy(cfs_rq))
if (!cfs_rq)
return;
if (throttled_hierarchy(cfs_rq))
return;
tg = cfs_rq->tg;
#ifndef CONFIG_SMP
if (likely(se->load.weight == tg->shares))
return;
@ -2707,8 +2711,9 @@ static void update_cfs_shares(struct cfs_rq *cfs_rq)
reweight_entity(cfs_rq_of(se), se, shares);
}
#else /* CONFIG_FAIR_GROUP_SCHED */
static inline void update_cfs_shares(struct cfs_rq *cfs_rq)
static inline void update_cfs_shares(struct sched_entity *se)
{
}
#endif /* CONFIG_FAIR_GROUP_SCHED */
@ -3582,10 +3587,18 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
if (renorm && !curr)
se->vruntime += cfs_rq->min_vruntime;
/*
* When enqueuing a sched_entity, we must:
* - Update loads to have both entity and cfs_rq synced with now.
* - Add its load to cfs_rq->runnable_avg
* - For group_entity, update its weight to reflect the new share of
* its group cfs_rq
* - Add its new weight to cfs_rq->load.weight
*/
update_load_avg(se, UPDATE_TG);
enqueue_entity_load_avg(cfs_rq, se);
update_cfs_shares(se);
account_entity_enqueue(cfs_rq, se);
update_cfs_shares(cfs_rq);
if (flags & ENQUEUE_WAKEUP)
place_entity(cfs_rq, se, 0);
@ -3657,6 +3670,15 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
* Update run-time statistics of the 'current'.
*/
update_curr(cfs_rq);
/*
* When dequeuing a sched_entity, we must:
* - Update loads to have both entity and cfs_rq synced with now.
* - Substract its load from the cfs_rq->runnable_avg.
* - Substract its previous weight from cfs_rq->load.weight.
* - For group entity, update its weight to reflect the new share
* of its group cfs_rq.
*/
update_load_avg(se, UPDATE_TG);
dequeue_entity_load_avg(cfs_rq, se);
@ -3681,7 +3703,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
/* return excess runtime on last dequeue */
return_cfs_rq_runtime(cfs_rq);
update_cfs_shares(cfs_rq);
update_cfs_shares(se);
/*
* Now advance min_vruntime if @se was the entity holding it back,
@ -3864,7 +3886,7 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
* Ensure that runnable average is periodically updated.
*/
update_load_avg(curr, UPDATE_TG);
update_cfs_shares(cfs_rq);
update_cfs_shares(curr);
#ifdef CONFIG_SCHED_HRTICK
/*
@ -4761,7 +4783,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
break;
update_load_avg(se, UPDATE_TG);
update_cfs_shares(cfs_rq);
update_cfs_shares(se);
}
if (!se)
@ -4820,7 +4842,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
break;
update_load_avg(se, UPDATE_TG);
update_cfs_shares(cfs_rq);
update_cfs_shares(se);
}
if (!se)
@ -9362,8 +9384,10 @@ int sched_group_set_shares(struct task_group *tg, unsigned long shares)
/* Possible calls to update_curr() need rq clock */
update_rq_clock(rq);
for_each_sched_entity(se)
update_cfs_shares(group_cfs_rq(se));
for_each_sched_entity(se) {
update_load_avg(se, UPDATE_TG);
update_cfs_shares(se);
}
raw_spin_unlock_irqrestore(&rq->lock, flags);
}