linux-kernel - Re: [PATCH v4 3/5] sched/fair: Skip detach sched avgs for new task when changing task groups

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Date:	Mon, 6 Jun 2016 15:03:38 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	peterz@...radead.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, bsegall@...gle.com, pjt@...gle.com,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com
Subject: Re: [PATCH v4 3/5] sched/fair: Skip detach sched avgs for new task
 when changing task groups

On Mon, 06 Jun, at 08:20:39AM, Yuyang Du wrote:
> Newly forked task has not been enqueued, so should not be removed from
> cfs_rq in task_move_group_fair(). To do so, we need to pass the fork
> information all the way from sched_move_task() to task_move_group_fair().
> 
> Signed-off-by: Yuyang Du <yuyang.du@...el.com>
> ---
>  kernel/sched/auto_group.c |    2 +-
>  kernel/sched/core.c       |    8 ++++----
>  kernel/sched/fair.c       |    8 ++++++--
>  kernel/sched/sched.h      |    4 ++--
>  4 files changed, 13 insertions(+), 9 deletions(-)

[...]

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 28b3415..370e284 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8429,9 +8429,13 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
>  }
>  
>  #ifdef CONFIG_FAIR_GROUP_SCHED
> -static void task_move_group_fair(struct task_struct *p)
> +static void task_move_group_fair(struct task_struct *p, bool fork)
>  {
> -	detach_task_cfs_rq(p);
> +	/*
> +	 * Newly forked task should not be removed from any cfs_rq
> +	 */
> +	if (!fork)
> +		detach_task_cfs_rq(p);
>  	set_task_rq(p, task_cpu(p));
>  	attach_task_cfs_rq(p);
>  	/*

Wouldn't it be more symmetric to add,

	if (p->se.avg.last_update_time)
		__update_load_avg(...)

to detach_entity_load_avg() so that there's no need to pass the @fork
parameter all the way down the stack, and more importantly, remember
to *not* call detach_task_cfs_rq() if the code changes in the future?

Additionally adding the above check looks like it would fix a race
that (I think) occurs if a newly forked task's class is changed before
it is enqueued in wake_up_new_task().