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

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Message-ID: <AANLkTi=7YyA-e8HAq+C_prK2EaZKcaTfzDJvmO3e3W=B@mail.gmail.com>
Date:	Thu, 20 Jan 2011 15:06:16 +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 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.

Thanks,
Yong

-- 
Only stand for myself
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