Hello. On Fri, Aug 27, 2021 at 06:50:10PM -0400, Waiman Long <llong@xxxxxxxxxx> wrote: > So the new rules will be: When I followed the thread, it seemed to me you're talking past each other a bit. I'd suggest the following terminology: - config space: what's written by the user and saved, - reality space: what's currently available (primarily subject to on-/offlinng but I think it'd be helpful to consider here also what's given by the parent), - effect space: what's actually possible and happening. Not all elements of config_space x reality_space (Cartesian product) can be represented in the effect_space (e.g. root partition with no (effective) cpus). IIUC, Waiman's "high bar" is supposed to be defined over transitions in the config_space. However, there can be independent changes in the reality_space so the rules should be actually formulated in the effect_space: The conditions for being a valid partition root rewritten into the effect space: > 1) The "cpuset.cpus" is not empty and the list of CPUs are exclusive. - effective CPUs are non-empty and exclusive wrt siblings - (E.g. setting empty cpuset.cpus might be possible but it invalidates the partition root, same as offlining or removal by an ancestor.) > 2) The parent cgroup is a partition root (can be an invalid one). - parent cgroup is a (valid) partition - (Being valid partition means owning "stolen" cpus from the parent, if the parent is not valid partition itself, you can't steal what is not owned.) - (And I think it's OK that: "the child partitions will stay invalid forever unless the parent become a valid partition again" [1].) > 3) The "cpuset.cpus" is a subset of the parent's cpuset.cpus.allowed. - I'm not sure what is the use of this condition (together with the rewrite of the 1st condition which covers effective cpus). I think it would make sense if being a valid parition root guaranteed that all configured cpuset.cpus will be available, however, that's not the case IIUC (e.g. due to offlining). > 4) No child cgroup with cpuset enabled. - A child cgroup with cpuset enabled is OK in the effect space (achievable by switching first and creating children later). - For technical reasons this may be a condition on the transitions in the config_space. Generally, most config changes should succeed and user should check (or watch) how they landed in combination with the reality_space. Regards, Michal [1] This follows the general model where ancestors can "preempt" resources from their subtree.
