linux-kernel - Re: [BUGFIX][PATCH] Fix sched rt group scheduling when hierachy is enabled

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Date:	Fri, 4 Mar 2011 12:55:17 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <pzijlstr@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Bharata B Rao <bharata.rao@...ibm.com>
Subject: Re: [BUGFIX][PATCH] Fix sched rt group scheduling when hierachy is
 enabled

* Yong Zhang <yong.zhang0@...il.com> [2011-03-04 11:43:16]:

> On Thu, Mar 3, 2011 at 11:29 PM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > No, not really :) It is required, it is a backup check to see if we
> > have queued tasks, rt_time of 0 and the runqueue is not throttled, why
> > should it be dequeued?
> 
> But I can't see where that kind of rt_rq is dequeued, mind pointing it out?
>

So here is what I saw

1. sched_dequeue_stack called from the dequeue path dequeues the
queues and sets rt_nr_running to 0
2. Enqueuing fails because rt_throttled is set for the group_rq
(parent who is throttled)
3. This causes further enqueue to fail, since rt_nr_running did
not increment in step 2, eventually the timer decrements rt_time
to 0 and the task is never picked up.

-- 
	Three Cheers,
	Balbir
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