C-States

Problem

When running Linux, especially older 6.x kernels, you might find that the CPU doesn't use C-states below C1, constantly running on higher frequences and eating power.

Solution

For some reason I couldn't make it work with either acpi_cpufreq or amd_pstate, but CoreFreq turned out to be a perfect solution once again.

Assuming that you already have it installed and working (otherwise check this guide first):

# /etc/kernel/cmdline
... initcall_blacklist=acpi_cpufreq_init nmi_watchdog=0 idle=halt amd_pstate=disable tsc=unstable nowatchdog

# /etc/modprobe.d/blacklist.conf
k10temp
acpi_cpufreq

# /etc/modprobe.d/corefreqk.conf
options corefreqk Register_ClockSource=1 Register_CPU_Freq=1 Register_Governor=1 Register_CPU_Idle=1 Override_SubCstate="1,1,1,1,1,1,0,0"

Don't forget to run update-initramfs -u -k all and reboot after making these changes.

To check that it works, run corefreq-cli and go first to Menu > Settings, make sure that CPU-IDLE driver is ON:

Then go to Menu > Kernel data, make sure that CPU-Idle driver is set to corefreqk-idle and that the Idle Limit is set to C6:

Finally, in the Frequency or Idle C-States views you'll be able to see that the CPU now actually goes to lower frequencies/power: