CPU Offline in Linux OS

 Some might notice that, CPU getting offline after complete Linux installation. 

If you have CPU with 4 Core, you can see that only 1 core is activated. (HyperThreading is disabled for this case).

What could be a root caused on this. Re-installed with Windows OS, all 4 cores are running perfectly. 

So, this could be a kernel issue that caused CPU offline and with this all CPU core able to run accordingly. 

This due to spectra and meltdown and caused the CPU offline. So, to enable back modify the grub is the way to disable this mitigation.

cat /etc/default/grub. Then check and append with mitigations=off

GRUB_CMDLINE_LINUX_DEFAULT="mitigations=off"

Ensure to put under "DEFAULT" to ensure it take place accordingly. Then, update grub config as below;

Open the terminal app

Edit the /etc/default/grub as per your needs. For example:

sudo vim /etc/default/grub

Type the following command as root user:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

Reboot your Linux box

sudo reboot

After system rebooted, please check CPU using your preferred method.