Re: [PATCH V2] cpuset: Add knob to make allowed masks hotplug invariant on legacy hierarchy

[Peter Zijlstra]

On Sat, Apr 11, 2015 at 10:35:37AM +0200, Peter Zijlstra wrote:
> On Fri, Apr 10, 2015 at 07:41:52PM +0530, Preeti U Murthy wrote:
> > The cpus_allowed and mems_allowed masks of a cpuset get overwritten
> > after each hotplug operation on the legacy hierarchy of cgroups so as to
> > remain in sync with the online mask. But there are use cases which
> > expect user configured masks to remain unchanged.
> > 
> > For instance, when hotplugged out CPUs are brought back online, they
> > remain idle with none of the existing tasks allowed to run on them since
> > the cpus_allowed mask was overwritten to not include them when they were
> > offlined.
> > 
> > We cannot change the legacy hierarchy design now to keep the allowed
> > masks hotplug invariant since it is a user visible change. It was
> > suggested instead to add a knob in the root cpuset directory which
> > allows the user to specify if he wants the user configured masks to be
> > hotplug invariant [1]. This knob will enforce the choice throughout the
> > hierarchy. If the knob is set, the allowed maks will not be varied on
> > hotplug. It is also to to be noted that this knob will appear in the
> > root cgroup mounted on the legacy hierarchy alone since the default
> > hierarchy does not overwrite the allowed masks anyway.
> > 
> > Having said this, there are fair reasons to argue that the kernel is not
> > responsible for taking care of user configurations in the face of
> > hotplug. But one of the consequences of the current legacy hierarchy
> > design, is that CPUs are left out from being used at all on online
> > operations. The reason for this is not very obvious at first and several
> > users have raised the issue as a bug. Hence the patch was strongly
> > called for.
> > 
> > Moreover the default hierarchy keeps the allowed masks hotplug invariant
> > too. So the patch is not bringing about a fundamental change in the
> > design of cgroups.
> 
> What you've not explained is why you can use this knob but not use the
> other new mode?

Urgh, it looks like the new mode is only available through the default
hierarchy (whatever that is).

Would it not make sense to make that a mount option and limit the amount
of semantic variants of cpusets?
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