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

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Message-ID: <AANLkTi=WhLy=4MNdm-HaOUOmbKXsNSJojnzqRTYp6qop@mail.gmail.com>
Date:	Thu, 20 Jan 2011 17:07:17 +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 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 ;)

Thanks,
Yong

-- 
Only stand for myself
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