linux-kernel - Re: [PATCH v6 0/4] CPU hotplug, cpusets, suspend/resume: Fixes, cleanups and optimizations

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Message-ID: <4FE1F4FD.3050101@linux.vnet.ibm.com>
Date:	Wed, 20 Jun 2012 21:36:21 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, pjt@...gle.com,
	paul@...lmenage.org, akpm@...ux-foundation.org, rjw@...k.pl,
	nacc@...ibm.com, rientjes@...gle.com, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de, seto.hidetoshi@...fujitsu.com, tj@...nel.org,
	mschmidt@...hat.com, berrange@...hat.com,
	nikunj@...ux.vnet.ibm.com, vatsa@...ux.vnet.ibm.com,
	liuj97@...il.com, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH v6 0/4] CPU hotplug, cpusets, suspend/resume: Fixes, cleanups
 and optimizations

On 06/20/2012 08:17 PM, Ingo Molnar wrote:

> 
> * Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> 
>> On 06/20/2012 05:09 PM, Ingo Molnar wrote:
>>
>>> * Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
>>>
>>>> On 05/24/2012 07:46 PM, Srivatsa S. Bhat wrote:
>>>>
>>>>> Currently the kernel doesn't handle cpusets properly during 
>>>>> suspend/resume. After a resume, all non-root cpusets end up 
>>>>> having only 1 cpu (the boot cpu), causing massive 
>>>>> performance degradation of workloads. One major user of 
>>>>> cpusets is libvirt, which means that after a 
>>>>> suspend/hibernation cycle, all VMs suddenly end up running 
>>>>> terribly slow!
>>>>>
>>>>> Also, the kernel moves the tasks from one cpuset to another 
>>>>> during CPU hotplug in the suspend/resume path, leading to a 
>>>>> task-management nightmare after resume.
>>>>>
>>>>> Patch 1 fixes this by keeping cpusets unmodified in the 
>>>>> suspend/resume path. But to ensure we don't trip over, it 
>>>>> keeps the sched domains updated during every CPU hotplug in 
>>>>> the s/r path. This is a long standing issue and we need to 
>>>>> fix up stable kernels too.
>>>>>
>>>>> The rest of the patches in the series are mostly 
>>>>> cleanups/optimizations.
>>>>
>>>> Hi Peter,
>>>>
>>>> Would you be taking these patches through -tip for 3.6?
>>>
>>> They are now in tip:sched/core.
>>>
>>> Note that I removed the Cc:stable tag - it's not a regression 
>>> fix and such it is not eligible for immediate -stable backports.
>>>
>>> ( Once they are upstream and have been problem-free upstream for
>>>   several weeks then *maybe* we could forward the first commit
>>>   to -stable, as a super special exception. )
>>>
>>
>>
>> OK, I get the point of allowing it to cook in the mainline for 
>> a while before backporting to -stable and I totally agree with 
>> that, but why so much of uncertainty about whether the first 
>> commit should (eventually) even land in -stable or not? 
>> Distros have been struggling to deal with this bug in 
>> userspace and have failed, and AFAIK they are waiting for a 
>> proper kernel fix for this bug. Agreed, this is not a 
>> regression per se, but isn't this bug critical enough to 
>> qualify for -stable?
> 
> No, as a general rule only regression fixes are included in 
> -stable. The workflow is this: in the v3.4 -stable kernel we 
> included fixes that were introduced in the v3.4 merge window, 
> i.e. bugs that were introduced after v3.3 was released.
> 
> Not 'fixes' in general.
> 
> Fixes for "has been broken forever" problems (like this one) go 
> upstream and get released in the next stable kernel that gets 
> released - v3.6 in this case.
> 


Ah, ok.. Thanks for the explanation!
 
Regards,
Srivatsa S. Bhat

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