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SubjectRe: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets

* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Thu, 2012-02-09 at 16:11 +0100, Ingo Molnar wrote:
>
> > > My understanding of the code is that when a CPU is taken
> > > offline, it is removed from all the cpusets and then the
> > > scan_for_empty_cpusets() function is run to move tasks from
> > > empty cpusets to their parent cpusets.
> >
> > Why is that done that way? offlining a CPU should be an
> > invariant as far as cpusets are concerned.
>
> Can't, tasks need to run someplace. There's two choices, add a
> still online cpu to the now empty cpuset or move the tasks to
> a parent that still has online cpus.
>
> Both are destructive.

You aren't thinking hard enough ;-) There's several solutions
off the top of my mind:

1) refuse the "impossible" offlining of the CPU, with a clear
enough error to make it actionable

2) offer a 'forced' offlinign of a CPU that will SIGTERM all
tasks that are on the now offline CPU and can only be there.

3) offer a 'nice' offlining variant that moves all orphan tasks
to their or any other well-defined fallback CPU.

4) *allow* 'impossible' cpusets and just run them on CPU#0 or
any other natural approximation. Don't touch the cpuset!

All of these would be exception mechanisms with no need to do
anything at hot-replug time.

Thanks,

Ingo


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