Re: [PATCH v3] sched: fix first task of a task group is attached twice

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

On 30/05/16 16:52, Vincent Guittot wrote:
> The cfs_rq->avg.last_update_time is initialize to 0 with the main effect
> that the 1st sched_entity that will be attached, will keep its

attached in .task_move_group ?

I'm not sure if we can have a .switched_to() directly followed by a
.enqueue_task() into a cfs_rq with avg.last_update_time = 0.

> last_update_time set to 0 and will attached once again during the
> enqueue.
> Initialize cfs_rq->avg.last_update_time to 1 instead.

Maybe worth mentioning in the header:

This double se attaching for the first task which moves into the task
group owning this cfs_rq (.task_move_group() and .enqueue_task()) can
(obviously) only happen if CONFIG_FAIR_GROUP_SCHED is set.

The reason for this is that we set 'se->avg.last_update_time =
cfs_rq->avg.last_update_time' in attach_entity_load_avg() and use
'migrated = !sa->last_update_time' as a flag in
enqueue_entity_load_avg() to decide if we call attach_entity_load_avg()
(again) or only update se->avg .

Tested-by: Dietmar Eggemann <Dietmar.Eggemann@arm.com>

> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> 
> v3:
> - add initialization of load_last_update_time_copy for not 64bits system
> - move init into init_cfs_rq
> 
> v2:
> - rq_clock_task(rq_of(cfs_rq)) can't be used because lock is not held
> 
>  kernel/sched/fair.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 218f8e8..86be9c1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8459,6 +8459,16 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
>  	cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
>  #endif
>  #ifdef CONFIG_SMP
> +	/*
> +	 * Set last_update_time to something different from 0 to make
> +	 * sure the 1st sched_entity will not be attached twice: once
> +	 * when attaching the task to the group and one more time when
> +	 * enqueueing the task.
> +	 */
> +	cfs_rq->avg.last_update_time = 1;
> +#ifndef CONFIG_64BIT
> +	cfs_rq->load_last_update_time_copy = 1;
> +#endif
>  	atomic_long_set(&cfs_rq->removed_load_avg, 0);
>  	atomic_long_set(&cfs_rq->removed_util_avg, 0);
>  #endif