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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 > /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" > ./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.

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