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

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Date:	Tue, 31 May 2016 03:48:02 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	peterz@...radead.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, dietmar.eggemann@....com
Subject: Re: [PATCH v3] sched: fix first task of a task group is attached
 twice

On Mon, May 30, 2016 at 05:52:20PM +0200, 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
> last_update_time set to 0 and will attached once again during the
> enqueue.
> Initialize cfs_rq->avg.last_update_time to 1 instead.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.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.
> +	 */

The first time: "once when attaching the task to the group".

That attaching is purely wrong, but will not have any effect (at least
load/util wise), because the task will later be inited in
init_entity_runnable_average().

The second time: "one more time when enqueueing the task".

In enqueue_entity_load_avg(), your patch will not have any effect either,
because of the above overwriting the new task's load in
init_entity_runnable_average(), no?