Messages in this thread | | | Date | Wed, 1 Feb 2023 22:10:46 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/2] cpuset: Fix cpuset_cpus_allowed() to not filter offline CPUs |
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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.
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