【os】Lock cpu freq on Linux and Android

Overview

Sometimes, we need to lock the cpu freq, so that:

• profile source code with timer functions
• etc.

Precondition

Need super user permision.

• Android
  • devices must be rooted first, or else you only have read-only permision to files in /sys/devices/system/cpu/cpu0/cpufreq/...
• Linux
  • Check if you have files in /sys/devices/system/cpu/cpu0/cpufreq/...
  • You have to use super user.

How to lock

• set scaling_governor to userspace
• set scaling_setspeed to freq you want.

See more details here:

#!/bin/sh

cpucount=`cat /proc/cpuinfo|grep processor|wc -l`

if [ "x$1" = "x-h" ]; then
    echo "There are ${cpucount} cpu on this device."
    echo "This script will lock one of the cpu to the lowest freq."
    echo "For example: '$ ./lock_to_lowest_freq.sh 0' will:"
    echo "    lock cpu0 to the lowest freq." 
    echo "    The lowest freq is the smallest one in /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies"
    exit
fi

if [ "x$1" = "x" ]; then
    printf "You must enter a cpu number, which is 0~%d.
" $((cpucount-1))
    echo "Use -h to see more details."
    exit
fi

if [ $1 -lt 0 -o $1 -ge $cpucount ]; then
    printf "You must enter a cpu number, which is 0~%d.
" $((cpucount-1))
    echo "Use -h to see more details."
    exit
fi

FLROOT=/sys/devices/system/cpu/cpu${1}/cpufreq/

echo "Available frequencies are: c"
cat ${FLROOT}scaling_available_frequencies

echo "Available governors are: c"
cat ${FLROOT}scaling_available_governors

echo "Current cpu${1} governor is: c"
cat ${FLROOT}scaling_governor

echo "Current cpu${1} freq is:  c"
cat ${FLROOT}cpuinfo_cur_freq

target_freq=`cat ${FLROOT}scaling_available_frequencies | sed 's/ /
/g' | sort -n | sed '/^$/d' | head -n 1` # get min freq
echo "This script will set cpu${1} governor to 'userspace', and set its freq to ${target_freq}."
echo -n "Continue? [y(default)/n]: "
read in
if [ -n "$in" -a ! "$in" = "y" ] ; then
    exit
fi

echo userspace > ${FLROOT}scaling_governor
echo ${target_freq} > ${FLROOT}scaling_setspeed

echo "Current cpu${1} freq is: c"
cat ${FLROOT}cpuinfo_cur_freq

Exceptions

For Android, sometimes only CPU0 /sys/devices/system/cpu/cpu0 has a directory named cpufreq/. Other CPU only has symlink file, which refer to cpu0/cpufreq/. Or other CPUs don't have cpufreq directory at all. For this situation, we only lock CPU0's freq, and we assume that all other CPUs' freq is locked to the same freq. We haven't find any documents or website to prove this, but we highly guess it's true from our profiling results.

Ref

• https://www.kernel.org/doc/Documentation/cpu-freq/user-guide.txt
  • This page has a detailed explaination of files in /sys/devices/system/cpu/cpu0/cpufreq/...
• https://software.intel.com/sites/default/files/comment/1716807/how-to-change-frequency-on-linux-pub.txt
  • This page has detailed explaination on how to lock cpu freq
  • a nice script to lock all cpu freq, which is at the bottom of the page.