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The current cpuset code uses both cpu_active_mask and cpu_online_mask and it can be confusing which one should be used if we need to update the code. The top_cpuset is always synchronized to cpu_active_mask and we should avoid using cpu_online_mask as much as possible. An active CPU is always an online CPU, but not vice versa. cpu_active_mask and cpu_online_mask can differ during hotplug operations. A CPU is marked active at the last stage of CPU bringup (CPUHP_AP_ACTIVE). It is also the stage where cpuset hotplug code will be called to update the sched domains so that the scheduler can move a normal task to a newly active CPU or remove tasks away from a newly inactivated CPU. The online bit is set much earlier in the CPU bringup process and cleared much later in CPU teardown. If cpu_online_mask is used while a hotunplug operation is happening in parallel, we may leave an offline CPU in cpu_allowed or have a higher chance of leaving an offline CPU in some other masks. Avoid this problem by always using cpu_active_mask in the cpuset code and leave a comment as to why the use of cpu_online_mask is discouraged. Signed-off-by: Waiman Long <longman@redhat.com> Signed-off-by: Tejun Heo <tj@kernel.org>