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? -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
