Power Management for the CPU - 2020.2 English

CPU Hotplug

The user may take one or more APU cores on-line and off-line as needed via 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

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.