linux-kernel - Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to

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Message-ID: <20150908021030.GC6455@byungchulpark-X58A-UD3R>
Date:	Tue, 8 Sep 2015 11:10:30 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Wanpeng Li <wanpeng.li@...mail.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to_fair()

hello wanpeng,

On Mon, Sep 07, 2015 at 05:45:20PM +0800, Wanpeng Li wrote:
> The sleeper task will be normalized when moved from fair_sched_class, in 
> order that vruntime will be adjusted either the task is running or sleeping 
> when moved back. The nomalization in switch_to_fair for sleep task will 
> result in lose fair sleeper bonus in place_entity() once the vruntime - 
> cfs_rq->min_vruntime is big when moved from fair_sched_class.

it is nothing to do with normalization.

if vruntime - cfs_rq->min_vruntime is big even though place_entity() was
called when moved from fair class, then we actually expect that it still has
a big vruntime when moved back to fair class.

if we don't expect that it still has a big vruntime when moved back to fair
class, we need to consider other approaches e.g. to move a position calling 
place_entity() or to add place_entity() call properly ..

however we should not touch normalization logic. in other words, if we
normalized the vruntime when leaved, then we should necessarily restore the
vruntime to a non-normalized value when moved back.

> 
> This patch fix it by adjusting vruntime just during migrating as original 
> codes since the vruntime of the task has usually NOT been normalized in 
> this case.

could you explain this in detail? when is a vruntime not normalized?

thanks,
byungchul

> 
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  kernel/sched/fair.c |   11 +++++++----
>  1 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d26d3b7..eb9aa35 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8005,9 +8005,6 @@ static void attach_task_cfs_rq(struct task_struct *p)
>  
>  	/* Synchronize task with its cfs_rq */
>  	attach_entity_load_avg(cfs_rq, se);
> -
> -	if (!vruntime_normalized(p))
> -		se->vruntime += cfs_rq->min_vruntime;
>  }
>  
>  static void switched_from_fair(struct rq *rq, struct task_struct *p)
> @@ -8066,14 +8063,20 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>  static void task_move_group_fair(struct task_struct *p)
>  {
> +	struct sched_entity *se = &p->se;
> +	struct cfs_rq *cfs_rq = cfs_rq_of(se);
> +
>  	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;
> +	se->avg.last_update_time = 0;
>  #endif
>  	attach_task_cfs_rq(p);
> +
> +	if (!vruntime_normalized(p))
> +		se->vruntime += cfs_rq->min_vruntime;
>  }
>  
>  void free_fair_sched_group(struct task_group *tg)
> -- 
> 1.7.1
> 
> --
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