linux-kernel - Re: [PATCH v2] sched: fix first task of a task group is attached twice

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Message-ID: <57486C62.7090205@arm.com>
Date:	Fri, 27 May 2016 16:48:50 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
	mingo@...nel.org, linux-kernel@...r.kernel.org
Cc:	yuyang.du@...el.com
Subject: Re: [PATCH v2] sched: fix first task of a task group is attached
 twice

On 25/05/16 16:01, 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>
> ---
> 
> v2:
> - rq_clock_task(rq_of(cfs_rq)) can't be used because lock is not held
> 
>  kernel/sched/fair.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 218f8e8..3724656 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8586,6 +8586,14 @@ void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
>  		se->depth = parent->depth + 1;
>  	}
>  
> +	/*
> +	 * 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.
> +	 */
> +	tg->cfs_rq[cpu]->avg.last_update_time = 1;
> +
>  	se->my_q = cfs_rq;
>  	/* guarantee group entities always have weight */
>  	update_load_set(&se->load, NICE_0_LOAD);

So why not setting the last_update_time value for those cfs_rq's when
we have the lock? E.g. in task_move_group_fair() or attach_task_cfs_rq().

@@ -8490,12 +8493,20 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
 #ifdef CONFIG_FAIR_GROUP_SCHED
 static void task_move_group_fair(struct task_struct *p)
 {
+#ifdef CONFIG_SMP
+       struct cfs_rq *cfs_rq = NULL;
+#endif
+
        detach_task_cfs_rq(p);
        set_task_rq(p, task_cpu(p));
 
 #ifdef CONFIG_SMP
        /* Tell se's cfs_rq has been changed -- migrated */
        p->se.avg.last_update_time = 0;
+
+       cfs_rq = cfs_rq_of(&p->se);
+       if (!cfs_rq->avg.last_update_time)
+               cfs_rq->avg.last_update_time = rq_clock_task(rq_of(cfs_rq));
 #endif

or

@@ -8423,6 +8423,9 @@ static void attach_task_cfs_rq(struct task_struct *p)
        se->depth = se->parent ? se->parent->depth + 1 : 0;
 #endif
 
+       if (!cfs_rq->avg.last_update_time)
+               cfs_rq->avg.last_update_time = rq_clock_task(rq_of(cfs_rq));
+
        /* Synchronize task with its cfs_rq */
        attach_entity_load_avg(cfs_rq, se);