On Wed, Feb 01, 2023 at 01:46:11PM -0500, Waiman Long wrote: > Note that using cpus_allowed directly in cgroup v2 may not be right because > cpus_allowed may have no relationship to effective_cpus at all in some > cases, e.g. > > root > | > V > A (cpus_allowed = 1-4, effective_cpus = 1-4) > | > V > B (cpus_allowed = 5-8, effective_cpus = 1-4) > > In the case of cpuset B, passing back cpus 5-8 as the allowed_cpus is wrong. I think my patch as written does the right thing here. Since the intersection of (1-4) and (5-8) is empty it will move up the hierarchy and we'll end up with (1-4) from the cgroup side of things. So the purpose of __cs_cpus_allowed() is to override the cpus_allowed of the root set and force it to cpu_possible_mask. Then cs_cpus_allowed() computes the intersection of cs->cpus_allowed and all it's parents. This will, in the case of B above, result in the empty mask. Then cpuset_cpus_allowed() has a loop that starts with task_cpu_possible_mask(), intersects that with cs_cpus_allowed() and if the intersection of that and cpu_online_mask is empty, moves up the hierarchy. Given cs_cpus_allowed(B) is the empty mask, we'll move to A. Note that since we force the mask of root to cpu_possible_mask, cs_cpus_allowed(root) will be a no-op and if we guarantee (in arch code) that cpu_online_mask always has a non-empty intersection with task_cpu_possible_mask(), this loop is guaranteed to terminate with a viable mask.
