Hello everyone :D I wonder that possible-CPU-mask(/sys/devices/system/cpu/possible) can be changed during runtime. I read that it is fixed at boot time, but I am not sure that it is really immutable even if some cgroup or virtualization magic is used. I am referring to /sys/devices/system/cpu/possible file to get to know the number of per-cpu areas. In userspace, I call `bpf_lookup_elem()` to get values at index from BPF array map of which type is BPF_MAP_TYPE_PERCPU_ARRAY. And the length of the gained values is the same with the number of per-cpu areas and in turn it is the same with the number of possible CPUs. I am anxious that this varies from time to time under some circumstances. So I checked some cgroup and virtualization use-cases which did not affect the possible-CPU-mask. $ docker run --cpuset-cpus=0-3 -it ubuntu:20.10 bash # cgroup cpuset $ virsh setvcpus --current ubuntu20.10 5 # hotplug cpu while guest os is running.. But while conducting this I realized that it's not possible to prove the immutability of possible-CPU-mask using inductive method. Can anyone explain that it will not happen that possible-CPU-mask changes after boot-time even with cgroup magic or some tricks from outside of hypervisors? Thanks,
