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

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Date:	Fri, 4 Mar 2011 14:05:52 +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

On Fri, Mar 4, 2011 at 2:02 PM, Yong Zhang <yong.zhang0@...il.com> wrote:
> On Fri, Mar 4, 2011 at 3:25 PM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>> * 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
>
> Whose rt_nr_running? group_rq's or parent of group_rq?
> For parent of group_rq, yes.
> For group_rq's, no, because we don't touch its rt_nr_running.
>

For parent of group_rq

>
>> , eventually the timer decrements rt_time
>> to 0 and the task is never picked up.
>
> If I understand correctly, you mean this:
>
> For a group(A) which has one task(b) attached but A is throttled,so
> A is unqueued now
> A.throttled == 1 && A.rt_nr_running == 1
>  deactivate_task(b); /* A.throttled == 1 && A.rt_nr_running == 0 */
>    do_sched_rt_period_timer(); /* A.run_time == 0 && A.throttled == 0*/

Note at some point rt_time becomes 0 and if enqueue is not set, the
next do_sched_rt_period_timer() is a NOP and does not enqueue back the
group

Balbir
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