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

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Date:	Wed, 20 Jun 2012 19:47:44 +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 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?

Regards,
Srivatsa S. Bhat

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