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

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Message-ID: <20110121131501.GA6051@zhy>
Date:	Fri, 21 Jan 2011 21:15:01 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Mike Galbraith <efault@....de>
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, Jan 20, 2011 at 11:07:27AM +0100, Mike Galbraith wrote:
> On Thu, 2011-01-20 at 17:07 +0800, Yong Zhang wrote:
> > On Thu, Jan 20, 2011 at 4:37 PM, Mike Galbraith <efault@....de> wrote:
> > > 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.
> > 
> > Exactly we should clip it when it comes back, because it still could
> > sleep for some time after it leaves ;)
> 
> 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.

OK, fair enough.

> 
> 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.

Yeah,
So I think Peter's new patch is taking care of all the issue now.

Thanks,
Yong
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