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

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Message-ID: <1295512625.8027.88.camel@marge.simson.net>
Date:	Thu, 20 Jan 2011 09:37:05 +0100
From:	Mike Galbraith <efault@....de>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, 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 Thu, 2011-01-20 at 15:06 +0800, Yong Zhang wrote:
> On Thu, Jan 20, 2011 at 2:12 PM, Mike Galbraith <efault@....de> wrote:
> > If the task returns as a sleeper, place entity() will be called when it
> > is awakened, so it's sleep credit will be clipped as usual.  So vruntime
> > can be much less than min_vruntime at class exit time, and it doesn't
> > matter, clipping on wakeup after re-entry takes care of it.. if that's
> > what you were thinking about.
> 
> For a sleep task which stay in sched_fair before it's waked:
> try_to_wake_up()
>   ttwu_activate()
>     activate_task()
>       enqueue_task_fair()
>         enqueue_entity()
>           place_entity()        <== clip vruntime
> 
> For a sleep task which promote to sched_rt when it's sleep:
> rt_mutex_setprio()
>   check_class_changed()
>     switch_from_fair()       <== vruntime -= min_vruntime
>       try_to_wake_up()
>         ...run then stay on rq
>         rt_mutex_setprio()
>           enqueue_task_fair()     <==vruntime += min_vruntime
> 
> The difference is that in the second case, place_entity() is not
> called, but wrt sched_fair, the task is a WAKEUP task.
> Then we place this task in sched_fair before where it should be.

D'oh.  You're right, he needs to be clipped before he leaves.

	-Mike

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