linux-kernel - Re: [RFC][PATCH 06/17] sched: Provide p->on

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Date:	Wed, 29 Dec 2010 22:14:46 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <oleg@...hat.com>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 06/17] sched: Provide p->on_rq

On Fri, Dec 24, 2010 at 01:23:44PM +0100, Peter Zijlstra wrote:
> Provide a generic p->on_rq because the p->se.on_rq semantics are
> unfavourable for lockless wakeups but needed for sched_fair.
> 
> In particular, p->on_rq is only cleared when we actually dequeue the
> task in schedule() and not on any random dequeue as done by things
> like __migrate_task() and __sched_setscheduler().
> 
> This also allows us to remove p->se usage from !sched_fair code.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  include/linux/sched.h   |    1 +
>  kernel/sched.c          |   36 ++++++++++++++++++------------------
>  kernel/sched_debug.c    |    2 +-
>  kernel/sched_rt.c       |   10 +++++-----
> Index: linux-2.6/kernel/sched_rt.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_rt.c
> +++ linux-2.6/kernel/sched_rt.c
> @@ -1132,7 +1132,7 @@ static void put_prev_task_rt(struct rq *
>  	 * The previous task needs to be made eligible for pushing
>  	 * if it is still active
>  	 */
> -	if (p->se.on_rq && p->rt.nr_cpus_allowed > 1)
> +	if (p->on_rq && p->rt.nr_cpus_allowed > 1)
>  		enqueue_pushable_task(rq, p);

Seems we need on_rt_rq(&p->rt) here, otherwise we enqueue the
task to pushable list when called from rt_mutex_setprio()/
__sched_setscheduler() etc. Thus add a little overhead.
Though we call dequeue_pushable_task() in set_curr_task_rt()
unconditionally.

Thanks,
Yong
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