On Wed, Feb 01, 2023 at 10:34:00PM -0500, Waiman Long wrote: > On 2/1/23 16:10, Peter Zijlstra wrote: > > 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. > > I will take a closer look at that tomorrow. I will be more comfortable > ack'ing that if this is specific to v1 cpuset instead of applying this in > both v1 and v2 since it is only v1 that is problematic. fwiw, the regression I'm seeing is with cgroup2. I haven't tried v1. WIll
