linux-kernel - Re: [PATCH 01/18] sched: select_task_rq

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Date:	Sun, 23 Dec 2012 20:17:06 +0800
From:	Alex Shi <lkml.alex@...il.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Alex Shi <alex.shi@...el.com>, rob@...dley.net,
	mingo@...hat.com, peterz@...radead.org, gregkh@...uxfoundation.org,
	andre.przywara@....com, rjw@...k.pl, paul.gortmaker@...driver.com,
	akpm@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, pjt@...gle.com,
	vincent.guittot@...aro.org
Subject: Re: [PATCH 01/18] sched: select_task_rq_fair clean up

On Fri, Dec 21, 2012 at 12:28 PM, Namhyung Kim <namhyung@...nel.org> wrote:
> Hi,
>
> On Tue, 11 Dec 2012 12:00:55 +0530, Preeti U. Murthy wrote:
>> On 12/11/2012 10:58 AM, Alex Shi wrote:
>>> On 12/11/2012 12:23 PM, Preeti U Murthy wrote:
>>>> Hi Alex,
>>>>
>>>> On 12/10/2012 01:52 PM, Alex Shi wrote:
>>>>> It is impossible to miss a task allowed cpu in a eligible group.
>>>>
>>>> The one thing I am concerned with here is if there is a possibility of
>>>> the task changing its tsk_cpus_allowed() while this code is running.
>>>>
>>>> i.e find_idlest_group() finds an idle group,then the tsk_cpus_allowed()
>>>> for the task changes,perhaps by the user himself,which might not include
>>>> the cpus in the idle group.After this find_idlest_cpu() is called.I mean
>>>> a race condition in short.Then we might not have an eligible cpu in that
>>>> group right?
>>>
>>> your worry make sense, but the code handle the situation, in
>>> select_task_rq(), it will check the cpu allowed again. if the answer is
>>> no, it will fallback to old cpu.
>>>>
>>>>> And since find_idlest_group only return a different group which
>>>>> excludes old cpu, it's also imporissible to find a new cpu same as old
>>>>> cpu.
>>
>> I doubt this will work correctly.Consider the following situation:sched
>> domain begins with sd that encloses both socket1 and socket2
>>
>> cpu0 cpu1  | cpu2 cpu3
>> -----------|-------------
>>  socket1   |  socket2
>>
>> old cpu = cpu1
>>
>> Iteration1:
>> 1.find_idlest_group() returns socket2 to be idlest.
>> 2.task changes tsk_allowed_cpus to 0,1
>> 3.find_idlest_cpu() returns cpu2
>
> AFAIK The tsk->cpus_allowed cannot be changed during the operation since
> it's protected by tsk->pi_lock.  I can see the following comment:

You are right. I misunderstand some comments in wake_up_new_task.


-- 
Thanks
    Alex
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