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

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Message-ID: <20120620144735.GA2007@gmail.com>
Date:	Wed, 20 Jun 2012 16:47:35 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
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


* 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.

Thanks,

	Ingo
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