linux-kernel - Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets

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Date:	Wed, 08 Feb 2012 04:22:15 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	paul@...lmenage.org, mingo@...e.hu, rjw@...k.pl, tj@...nel.org,
	frank.rowand@...sony.com, pjt@...gle.com, tglx@...utronix.de,
	lizf@...fujitsu.com, prashanth@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, vatsa@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling
 related to cpusets

On Wed, 2012-02-08 at 00:25 +0530, Srivatsa S. Bhat wrote:
> There is a very long standing issue related to how cpusets handle CPU
> hotplug events. The problem is that when a CPU goes offline, it is removed
> from all cpusets. However, when that CPU comes back online, it is added
> *only* to the root cpuset. Which means, any task attached to a cpuset lower
> in the hierarchy will have one CPU less in its cpuset, though it had this
> CPU in its cpuset before the CPU went offline.

Yeah so? That's known behaviour..

> The issue gets enormously aggravated in the case of suspend/resume.

Why does suspend resume does this anyway? hotunplug is terribly
expensive, surely not doing it would make suspend ever so much faster?

>  During
> suspend, all non-boot CPUs are taken offline. Which means, all those CPUs
> get removed from all the cpusets. When the system resumes, all CPUs are
> brought back online; however, the newly onlined CPUs get added only to the
> root cpuset - and all other cpusets have cpuset.cpus = 0 (boot cpu alone)!
> This means, (as is obvious), all those tasks attached to non-root cpusets
> will be constrained to run only on one single cpu!
> 
> So, imagine the amount of performance degradation after suspend/resume!!
> 
> In particular, libvirt is one of the active users of cpusets. And apparently,
> people hit this problem long ago:
> https://bugzilla.redhat.com/show_bug.cgi?id=714271
> 
> But unfortunately this never got resolved since people probably thought that
> the bug was in libvirt... and all this time the kernel was the culprit!

/me boggles, why do you use cpusets on a system small enough to suspend,
and I'm so not going to ask about libvirt because I know I'll just get
sad.



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