linux-kernel - Re: [PATCH 22/22] sched: Remove TASK

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Message-ID: <4D797F92.6020504@am.sony.com>
Date:	Thu, 10 Mar 2011 17:49:06 -0800
From:	Frank Rowand <frank.rowand@...sony.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Chris Mason <chris.mason@...cle.com>,
	"Rowand, Frank" <Frank_Rowand@...yusa.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>,
	Yong Zhang <yong.zhang0@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 22/22] sched: Remove TASK_WAKING

On 03/02/11 09:38, Peter Zijlstra wrote:
> With the new locking TASK_WAKING has become obsolete, remove it.
> 
> Suggested-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> LKML-Reference: <new-submission>
> ---

< snip >

> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -2175,7 +2175,7 @@ void set_task_cpu(struct task_struct *p,
>  	 * We should never call set_task_cpu() on a blocked task,
>  	 * ttwu() will sort out the placement.
>  	 */
> -	WARN_ON_ONCE(p->state != TASK_RUNNING && p->state != TASK_WAKING &&
> +	WARN_ON_ONCE(p->state != TASK_RUNNING &&
>  			!(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE));
>  
>  #ifdef CONFIG_LOCKDEP
> @@ -2613,7 +2613,7 @@ try_to_wake_up(struct task_struct *p, un
>  	smp_rmb();
>  
>  	p->sched_contributes_to_load = !!task_contributes_to_load(p);
> -	p->state = TASK_WAKING;
> +	p->state = TASK_RUNNING;
>  
>  	if (p->sched_class->task_waking)
>  		p->sched_class->task_waking(p);

No harm if the coded as in the patch, but an alternate suggestion
if you like it:

The only reason left for "p->state = TASK_RUNNING;" here is when
cpu is remote.  If cpu is not remote then p->state will be set by:

   ttwu_queue()
      ttwu_do_activate()
         ttwu_do_wakeup()
            p->state = TASK_RUNNING;

It would be more clear that setting state to TASK_RUNNING is protecting
the process until it has been removed from the wake_list by
sched_ttwu_pending() by setting p->state = TASK_RUNNING in ttwu_queue_remote().

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