CPUs online or offline

Check CPU status

# get the list of offline CPUs
cat /sys/devices/system/cpu/offline

cat /sys/devices/system/cpu/online  
# get the list of online CPUs

# get the list of CPUs
ls -ld /sys/devices/system/cpu/cpu* 

# print individual CPU's online/offline status
cat /sys/devices/system/cpu/cpu*/online 

# set online CPU <number>
echo 1 > /sys/devices/system/cpu/cpu<number>/online 
or 
sudo chcpu -e <number>

# set offline CPU <number>
echo 0 > /sys/devices/system/cpu/cpu<number>/online
or 
sudo chcpu -d <number>

smpboot: native_cpu_up: bad cpu 255

This thread shows system IOMMU enable in BIOS would be required to enable the last CPU core on Gigabyte system. However, it would be also related to other systems' BIOS configuration.

smpboot: native_cpu_up is also related to x2APIC features cause operating systems cannot use x2APIC unless Processor x2APIC Support is set to "Force Enabled" prior to booting OS.

x2APIC is depending on the BIOS configuration and type/version of OS,

• Windows Server 2016
  • Windows Server 2016 supports AMD EPYC 7xx2-series CPUs, but this O.S. is limited to no more than 255 logical processors. Windows Server 2016 does not support X2APIC mode, and you should disable this mode in BIOS.
  • AMD CBS-> CPU Common Options-> Performance-> CCD/Core/Thread Enablement ->Accept-> SMT control->Disabled (For ASUS BIOS)