3.4. Disabling C-states by using a kernel command line option

Procedure

1. Display the idle driver the system uses:

   # cat /sys/devices/system/cpu/cpuidle/current_driver
   intel_idle

   For details about the driver, see the /usr/share/doc/kernel-doc-<version>/Documentation/admin-guide/pm/cpuidle.rst file provided by the kernel-doc package.

2. If the host uses the intel_idle driver, set the intel_idle.max_cstate kernel parameter to define the highest C-state that CPU cores should be able to use:

   # grubby --update-kernel=ALL --args="intel_idle.max_cstate=0"

   Setting intel_idle.max_cstate=0 disables the intel_idle driver. Consequently, the kernel uses the acpi_idle driver that uses the C-state values set in the EFI firmware. For this reason, also set processor.max_cstate to override these C-state settings.

3. On every host, independent from the CPU vendor, set the highest C-state that CPU cores should be able to use:

   # grubby --update-kernel=ALL --args="processor.max_cstate=0"

   重要

   If you set processor.max_cstate=0 in addition to intel_idle.max_cstate=0, the acpi_idle driver overrides the value of processor.max_cstate and sets it to 1. As a result, with processor.max_cstate=0 intel_idle.max_cstate=0, the highest C-state the kernel will use is C1, not C0.

4. Restart the host for the changes to take effect:

   # reboot

Verification

1. Display the maximum C-state:

   # cat /sys/module/processor/parameters/max_cstate
   1

2. If the host uses the intel_idle driver, display the maximum C-state:

   # cat /sys/module/intel_idle/parameters/max_cstate
   0