RZ-G/RZG2 kernel

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RZ/G2 Kernel and Linux FAQ

CPU Hotplug

You can enable and disable CPU cores by writing to a sysfs value.
This is helpful for when you want to experiment with the performance of your application if you were to use a processor with less CPU cores.

For example, this command will disable the 2nd core.

$ echo 0 > /sys/devices/system/cpu/cpu1/online

More detailed information can be found here: https://www.cyberciti.biz/faq/debian-rhel-centos-redhat-suse-hotplug-cpu

Power Saving

  • In Linux, this is a mechanism that is generally supported by all kernels.(it may depend on the version)
  • The Renesas kernel has support them.

About power consumption in RZ/G2 series, we have some supported features to save power cost in default environment:

  • CPUHotplug: Turn on/off CPU in runtime.
  • CPUIdle: Support 2 modes to turn off clock or power domain of CPU when CPU is idle (nothing to do).
    • Sleep mode: put in sleep state.
    • Core standby mode: put in shutdown state. It is described in devicetree of each SoC => It has deeper state than sleep mode so that save more power.
  • CPUFreq: there are 6 governors to support "Dynamic Frequency Scaling":
    • Performance: The frequency is always set maximum => It is using as default in our current environment.
    • Powersave: The frequency is always set minimum.
    • Ondemand: If CPU load is bigger than 95%, the frequency is set max. If CPU load is equal to or less than 95%, the frequency is set based on CPU load.
    • Conservative: If CPU load is bigger than 80%, the frequency is set one level higher than current frequency. If CPU load is equal to or less than 20%, the frequency is set one level lower than current frequency.
    • Userspace: It sets frequency which is defined by user in runtime.
    • Schedutil: Schedutil governor is driven by scheduler. It uses scheduler-provided CPU utilization information as input for making its decisions by formula: freq_next= 1.25 * freq_max* util_of_CPU.
  • Power Domain: it is supported as default by Linux Power Management Framework. If a module is not use, system will disable its clock and power domain automatically.

Therefore, select proper method will be based on user's purpose. Here are my examples:

  • Want to use with best performance: disable CPUIdle + use performance frequency governor.
  • Want to use less power: enable CPUIdle + use powersave frequency governor.
  • Want to balance performance and power: we can use schedutil.
  • Want to modify frequency as user's purpose: use userspance frequency governor.
  • If user is running realtime environment, I suggest using performance governor to ensure the minimum latency.

Here are some commands to check frequency value and frequency governor in linux:

  • Check available CPU frequency:
cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_frequencies
  • Check available CPU frequency governor:
cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_governors
  • Change to other governor:
echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor (performance/userspace/schedutil/...)
  • Check current frequency:
cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq