How to turn off CPU cores on Samsung Galaxy A8? I need to turn off CPU cores in order to make energy measurements of sets of cpu cores in isolation (meaning that I want to run an application on a set of cpu cores and turn off the cpu cores that are not in the set). I looked at the documentation on the android linux kernel source and it says the following:


  Q: How do i logically offline a CPU? 
  A: Do the following. 
      #echo 0 &gt; /sys/devices/system/cpu/cpuX/online 
  Once the logical offline is successful, check 
      #cat /proc/interrupts 
  You should now not see the CPU that you removed. Also online file will report
  the state as 0 when a cpu if offline and 1 when its online. 
      #To display the current cpu state. 
      #cat /sys/devices/system/cpu/cpuX/online 


But when I write 0 to the online file, it still reports the CPU as being online even with superuser permissions and marking the file as writable (chmod +w). 

I imagine that the kernel is enabling the cpu again after I disabled it. Also I did not try to turn off CPU 0 because in the same documentation it says that it's not removable in some architectures, this behavior is happening on other CPUs.

Do I need to rebuild the kernel in order to enable the cpu hotplug or is this an architecture specific problem? Maybe my cpu doesn't support hotplugs.

What I've tried by now


Writing to /sys/devices/system/cpu/cpuN/online with a rooted device. (echo "0" &gt; ./sys/device/...../online)
Changing the kernel to a custom one and trying the step above again. (Kernel installed was Quantum Kernel)
Using kernel adiutor to turn off the CPUs on the CPU tab (this turns off the CPU for a while then it turns back on again)
Using su on adb shell to change file permissions on sysfs (chmod 664 /sys/devices/...../online) and then trying to write to it.


Device Information 
Model: Samsung Galaxy A8 (SM-A530F) 
Android Version: Android Pie (9) 
Kernel: Quantum Kernel V4 
Kernel version: 4.4.111-Quantum_pie.V4.0-A530F 
Processor: Samsung Exynos Octa 7885 
Architecture: aarch64 

EDIT: Forgot to mention that my device is rooted.
This helps on Galaxy-S7:
echo 0 &gt; /sys/devices/system/cpu/cpuhotplug/enabled
echo 0 &gt; /sys/devices/system/cpu/cpu0/online