CPUs online or offline
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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[1] 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[2] features cause operating systems cannot use x2APIC unless Processor x2APIC Support is set to "Force Enabled" prior to booting.