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

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Date:	Fri, 21 Jan 2011 12:08:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Yong Zhang <yong.zhang0@...il.com>, 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 11:07 +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.
>
> 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;
 
 	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;
+	}
 }
 
 /*

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