Hot-Add CPU not working on Ubuntu

 Problem

Although Hot added CPUs are identified and registered by the kernel, CPUs are not brought online.

 Solution

Create a udev rule to automatically enable the hot added CPU as follows.

1. Create a new udev rule 99-vmware-cpuhotplug-udev.rules under /etc/udev/rules.d directory, then add the following line into the file:  

   # VMware CPU HotPlug automatization # Automatically bring on-line added processors. # SUBSYSTEM=="cpu", ACTION=="add", TEST=="online", ATTR{online}=="0", ATTR{online}="1"

2. Restart the udev:

   systemctl restart udev

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