Re: [PATCH v2] sched: fix group_entity's share update

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 21/12/16 15:50, Vincent Guittot wrote:

IMHO, the overall idea makes sense to me. Just a couple of small
questions ...

> 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.

Why for small tasks? Is it because we use load =
scale_load_down(cfs_rq->load.weight) and not load = cfs_rq->avg.load_avg
in calc_cfs_shares()?

> 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

You mean A->cfs_rq->load.weight = 0 ?

>         A->se->load.weight is updated with the new share (0 in this case)

Shouldn't this be MIN_SHARES (2) instead of 0?

> - 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.

This is because we use 'se->on_rq * scale_load_down(se->load.weight)' in
__update_load_avg() as weight parameter for PELT load_avg update?

> update_cfs_shares() now take the sched_entity as 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.

[...]