On 04/11/2015 02:05 PM, 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? It is claimed that the existing software which work with the legacy hierarchy break when cgroups are mounted on the default hierarchy due to poor backward compatibility. I will include this point in the changelog. Thanks Regards Preeti U Murthy > -- 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
