Is it possible to "hotplug" a CPU on a running Red Hat Enterprise Linux 5 system?

Issue

• Is it possible to "hotplug" a CPU on a running Red Hat Enterprise Linux system?
• How to dynamically enable or disable a CPU on a running system.

Resolution

Red Hat Enterprise Linux 5 supports the cpu-hotplug mechanism, which allows for CPUs to be dynamically disabled and re-enabled on a system without requiring a system reboot.
In order to disable a CPU core in a running machine, use the file /sys/devices/system/cpu/cpuX/online.

The following command will disable a CPU:

    # echo 0 > /sys/devices/system/cpu/cpuX/online

Where X is the ID of the CPU as determined from /proc/cpuinfo.

To re-enable the CPU, run:

   # echo 1 > /sys/devices/system/cpu/cpuX/online

Note: CPU hot-plugging will only work if physical hotplug is supported by the hardware.
Note: There are some architectures in which a CPU cannot be disabled due to a dependency on a certain CPU. In such cases you will notice that the /sys/devices/system/cpu/cpuN/online file is missing or not writeable.

For further details on CPU hotplugging, see this file:
/usr/share/doc/kernel-doc-2.6.18/Documentation/cpu-hotplug.txt
The documentation files are in the kernel-doc package, which can be installed from RHN.