【Linux】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:

 1 #!/bin/sh

 2 

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

 4 

 5 if [ "x$1" = "x-h" ]; then

 6     echo "There are ${cpucount} cpu on this device."

 7     echo "This script will lock one of the cpu to the lowest freq."

 8     echo "For example: '$ ./lock_to_lowest_freq.sh 0' will:"

 9     echo "    lock cpu0 to the lowest freq." 

10     echo "    The lowest freq is the smallest one in /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies"

11     exit

12 fi

13 

14 if [ "x$1" = "x" ]; then

15     printf "You must enter a cpu number, which is 0~%d.\n" $((cpucount-1))

16     echo "Use -h to see more details."

17     exit

18 fi

19 

20 if [ $1 -lt 0 -o $1 -ge $cpucount ]; then

21     printf "You must enter a cpu number, which is 0~%d.\n" $((cpucount-1))

22     echo "Use -h to see more details."

23     exit

24 fi

25 

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

27 

28 echo "Available frequencies are: \c"

29 cat ${FLROOT}scaling_available_frequencies

30 

31 echo "Available governors are: \c"

32 cat ${FLROOT}scaling_available_governors

33 

34 echo "Current cpu${1} governor is: \c"

35 cat ${FLROOT}scaling_governor

36 

37 echo "Current cpu${1} freq is:  \c"

38 cat ${FLROOT}cpuinfo_cur_freq

39 

40 target_freq=`cat ${FLROOT}scaling_available_frequencies | sed 's/ /\n/g' | sort -n | sed '/^$/d' | head -n 1` # get min freq

41 echo "This script will set cpu${1} governor to 'userspace', and set its freq to ${target_freq}."

42 echo -n "Continue? [y(default)/n]: "

43 read in

44 if [ -n "$in" -a ! "$in" = "y" ] ; then

45     exit

46 fi

47 

48 echo userspace > ${FLROOT}scaling_governor

49 echo ${target_freq} > ${FLROOT}scaling_setspeed

50 

51 echo "Current cpu${1} freq is: \c"

52 cat ${FLROOT}cpuinfo_cur_freq

lock_low_freq.sh

 

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.