linux-kernel - Re: [PATCH RFC] sched/deadline: support dl task migrate during cpu hotplug

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Message-ID: <54589A78.1060406@arm.com>
Date:	Tue, 04 Nov 2014 09:20:56 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] sched/deadline: support dl task migrate during cpu
 hotplug

Hi,

On 04/11/14 08:23, Wanpeng Li wrote:
> Hi Peter,
> On Tue, Nov 04, 2014 at 09:32:25AM +0100, Peter Zijlstra wrote:
>> On Tue, Nov 04, 2014 at 07:57:48AM +0800, Wanpeng Li wrote:
>>> On Mon, Nov 03, 2014 at 11:41:11AM +0100, Peter Zijlstra wrote:
>>
>>>> On Fri, Oct 31, 2014 at 03:28:17PM +0800, Wanpeng Li wrote:
>>>> So what is wrong with making dl_task_timer() deal with it? The timer
>>>> will still fire on the correct time, canceling it and or otherwise
>>>> messing with the CBS is wrong. Once it fires, all we need to do is
>>>> migrate it to another cpu (preferably one that is still online of course
>>>> :-).
>>
>>> Do you mean what I need to do is push the task to another cpu in dl_task_timer() 
>>> if rq is offline?
>>
>> That does indeed appear to be the sensible fix to me.
>>
>>> In addition, what will happen if dl task can't preempt on 
>>> another cpu?
>>
>> So if we find that the rq the task was on is no longer available, we
>> need to select a new rq, the 'right' rq would be the one running the
>> latest deadline.
>>
>> If it cannot preempt the latest (running) deadline, it was not eligible
>> for running in the first place so no worries, right?
> 
> I think this will lead to this deadline task cannot running on any rqs any more.
> If my understanding is not right, when it will be picked?
> 

I think you can just pick one random rq and move the task there. The
push/pull mechanism should then move it around properly.

Thanks,

- Juri

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