Setting CPUs online or offline

6.10 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

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