linux-kernel - Re: Bug in scheduler when using rt

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Date:	Fri, 21 Jan 2011 20:24:12 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mike Galbraith <efault@....de>, samu.p.onkalo@...ia.com,
	mingo@...e.hu,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	tglx <tglx@...utronix.de>, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Bug in scheduler when using rt_mutex

On Fri, Jan 21, 2011 at 12:08:56PM +0100, Peter Zijlstra wrote:
> > That's ok, we don't and aren't supposed to care what happens while he's
> > gone.  But we do have to make sure that vruntime is sane either when he
> > leaves, or when he comes back.  Seems to me the easiest is clip when he
> > leaves to cover him having slept a long time before leaving, then coming
> > back on us as a runner.  If he comes back as a sleeper, he'll be clipped
> > again anyway, so all is well.
> >
> > sched_fork() should probably zero child's vruntime too, so non-fair
> > children can't enter fair_class with some bogus lag they never had.
> 
> Something like so?
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -2624,6 +2624,8 @@ void sched_fork(struct task_struct *p, i
>  
>  	if (!rt_prio(p->prio))
>  		p->sched_class = &fair_sched_class;
> +	else
> +		p->se.vruntime = 0;

This can be moved to __sched_fork()

>  
>  	if (p->sched_class->task_fork)
>  		p->sched_class->task_fork(p);
> Index: linux-2.6/kernel/sched_fair.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_fair.c
> +++ linux-2.6/kernel/sched_fair.c
> @@ -4086,8 +4086,14 @@ static void switched_from_fair(struct rq
>  	 * have normalized the vruntime, if it was !on_rq, then only when
>  	 * the task is sleeping will it still have non-normalized vruntime.
>  	 */
> -	if (!se->on_rq && p->state != TASK_RUNNING)
> +	if (!se->on_rq && p->state != TASK_RUNNING) {
> +		/*
> +		 * Fix up our vruntime so that the current sleep doesn't
> +		 * cause 'unlimited' sleep bonus.
> +		 */
> +		place_entity(cfs_rq, se, 0);
>  		se->vruntime -= cfs_rq->min_vruntime;

Now I will say yes.
Though it's same to my suggestion which was rejected by myself :)

Thanks,
Yong
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