CPU Hotplug
The user may take one or more APU cores on-line and off-line as needed through the CPU Hotplug control interface.
Kernel configurations required:
- Kernel Features
- [*] Support for hot-pluggable CPUs
See also:
- https://www.kernel.org/doc/Documentation/cpu-hotplug.txt
- http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/arm/idle-states.txt
For example, to take CPU3 off-line:
$ echo 0 > /sys/devices/system/cpu/cpu3/online
CPU Idle
If enabled, the kernel may cut power to individual APU cores when they are idling. The kernel configurations required are:
- CPU Power Management
- CPU Idle
- [*] CPU idle PM support
-
Arm CPU Idle Drivers
- [*] Generic Arm/Arm64 CPU idle Driver
- CPU Idle
See also:
- https://www.kernel.org/doc/Documentation/cpuidle/core.txt
- https://www.kernel.org/doc/Documentation/cpuidle/driver.txt
- https://www.kernel.org/doc/Documentation/cpuidle/governor.txt
- https://www.kernel.org/doc/Documentation/cpuidle/sysfs.txt
Below is the sysfs interface for cpuidle.
$ ls -lR /sys/devices/system/cpu/cpu0/cpuidle/
/sys/devices/system/cpu/cpu0/cpuidle/:
drwxr-xr-x 2 root root 0 Jun 10 21:55 state0
drwxr-xr-x 2 root root 0 Jun 10 21:55 state1
/sys/devices/system/cpu/cpu0/cpuidle/state0:
-r--r--r-- 1 root root 4096 Jun 10 21:55 desc
-rw-r--r-- 1 root root 4096 Jun 10 21:55 disable
-r--r--r-- 1 root root 4096 Jun 10 21:55 latency
-r--r--r-- 1 root root 4096 Jun 10 21:55 name
-r--r--r-- 1 root root 4096 Jun 10 21:55 power
-r--r--r-- 1 root root 4096 Jun 10 21:55 residency
-r--r--r-- 1 root root 4096 Jun 10 21:55 time
-r--r--r-- 1 root root 4096 Jun 10 21:55 usage
/sys/devices/system/cpu/cpu0/cpuidle/state1:
-r--r--r-- 1 root root 4096 Jun 10 21:55 desc
-rw-r--r-- 1 root root 4096 Jun 10 21:55 disable
-r--r--r-- 1 root root 4096 Jun 10 21:55 latency
-r--r--r-- 1 root root 4096 Jun 10 21:55 name
-r--r--r-- 1 root root 4096 Jun 10 21:55 power
-r--r--r-- 1 root root 4096 Jun 10 21:55 residency
-r--r--r-- 1 root root 4096 Jun 10 21:55 time
-r--r--r-- 1 root root 4096 Jun 10 21:55 usage
where:
- desc: Small description about the idle state (string)
- disable: Option to disable this idle state (bool)
- latency: Latency to exit out of this idle state (in microseconds)
- name: Name of the idle state (string)
- power: Power consumed while in this idle state (in milliwatts)
- time: Total time spent in this idle state (in microseconds)
- usage: Number of times this state was entered (count)
Below is the sysfs interface for cpuidle governors.
$ ls -lR /sys/devices/system/cpu/cpuidle/
/sys/devices/system/cpu/cpuidle/:
-r--r--r-- 1 root root 4096 Jun 10 21:55 current_driver
-r--r--r-- 1 root root 4096 Jun 10 21:55 current_governor_ro
CPU Frequency
If enabled, the CPU cores may switch between different operation clock frequencies. The kernel configurations required are:
- CPU Frequency scaling
- [*] CPU Frequency scaling
- Default CPUFreq governor
- Userspace
- CPU Power Management
- [*] CPU Frequency scaling
- Default CPUFreq governor
- Userspace
- <*> Generic DT based cpufreq driver
Look up the available CPU speeds:
$ cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cpu_freq
Select the 'userspace' governor for CPU frequency control:
$ echo userspace > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
Look up the current CPU speed (same for all cores):
$ cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cpu_freq
Change the CPU speed (same for all cores):
$ echo <freq> > /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed
For details on adding and changing CPU frequencies, see the Linux kernel documentation on Generic Operating Points.