Setting CPUs online or offline

Red Hat Enterprise Linux 9.2 LPAR mode z/VM guest KVM guest

Use the chcpu command or the online sysfs attribute of a logical CPU to set a CPU online or offline.

Before you begin

  • Daemon processes like cpuplugd can change the state of any CPU at any time. Such changes can interfere with manual changes.

Procedure

  1. Optional: Rescan the CPUs to ensure that Linux® has a current list of configured CPUs.
    To initiate a rescan, issue the chcpu command with the -r option.
    # chcpu -r
    Alternatively, you can write 1 to /sys/devices/system/cpu/rescan.

    You might need a rescan for Linux on z/VM® after one or more CPUs have been added to the z/VM guest virtual machine by the z/VM hypervisor. Linux in LPAR mode automatically detects newly available CPUs.

    Linux on KVM and Linux in LPAR mode automatically detect newly available CPUs.

  2. Change the online state of a CPU by issuing a command of this form: 
    # chcpu -e|-d <N>
    where
    <N>
    is the number of the logical CPU.
    -e
    sets an offline CPU online. Only CPUs that are in the configuration state configured can be set online. For Linux on z/VM, all CPUs are in the configured state.
    -d
    sets an online CPU offline.
    Alternatively, you can write 1 to the online sysfs attribute of a CPU to set it online, or 0 to set it offline.
    Examples:
    • The following chcpu commands force a CPU rescan, and then set the logical CPU with number 2 online.
      # chcpu -r
# chcpu -e 2
      The following commands achieve the same results by writing 1 to the online sysfs attribute of the CPU.
      # echo 1 > /sys/devices/system/cpu/rescan
# echo 1 > /sys/devices/system/cpu/cpu2/online
    • The following chcpu command sets the logical CPU with number 2 offline.
      # chcpu -d 2
      The following command achieves the same results by writing 0 to the online sysfs attribute of the CPU.
      # echo 0 > /sys/devices/system/cpu/cpu2/online