Messages in this thread | | | Subject | Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets | From | Peter Zijlstra <> | Date | Wed, 08 Feb 2012 04:22:15 +0100 |
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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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