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

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Message-ID: <AANLkTikOoC_ESZSmbDrrnQa3MLr3xT-qFOWU433PrEVa@mail.gmail.com>
Date:	Thu, 20 Jan 2011 13:32:12 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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 12:27 PM, Yong Zhang <yong.zhang0@...il.com> wrote:
> on Thu, Jan 20, 2011 at 12:18 PM, Yong Zhang <yong.zhang0@...il.com> wrote:
>> On Wed, Jan 19, 2011 at 9:30 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>>> +static void switched_from_fair(struct rq *rq, struct task_struct *p)
>>> +{
>>> +       struct sched_entity *se = &p->se;
>>> +       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>>> +
>>> +       /*
>>> +        * Ensure the task's vruntime is normalized, so that when its
>>> +        * switched back to the fair class the enqueue_entity(.flags=0) will
>>> +        * do the right thing.
>>> +        *
>>> +        * If it was on_rq, then the dequeue_entity(.flags=0) will already
>>> +        * 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)
>>> +               se->vruntime -= cfs_rq->min_vruntime;
>>
>> Here it's possible that se->vruntime is little than cfs_rq->min_vruntime.
>
> Maybe we can:
>                       place_entity(cfs_rq, se, 0);
>                       se->vruntime -= cfs_rq->min_vruntime;

Hmmm, this doesn't make sense since we don't know how long the task
will sleep after we do this.
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