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

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Date:	Fri, 4 Mar 2011 16:32:49 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	balbir@...ux.vnet.ibm.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 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.


> , 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*/
      sched_rt_rq_enqueue(A); /* fails due to A.rt_nr_running == 0 */

But this doesn't prevent task readded to group A and eventually
group A is added back.

Thanks,
Yong


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