AMD pstate=active gives lower idle on Arch vs OpenWrt ... why [solved]

graysky · 2024-09-01 12:30:43

If I boot into Arch on a mini PC (AMD Ryzen 7/5800U) with amd-pstate=active under linux-lts (currently 6.6.48), I find that while idle, most of the cores sit at 400 MHz. If I boot the identical hardware into OpenWrt on the same 6.6.48 kernel with the same kernel parameter for pstates=active, I am finding that while idle (all daemons disabled) the cores sit at 1384 MHz.

Both distros are showing the same values for:

# cat /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
powersave
# cat /sys/devices/system/cpu/amd_pstate/status
active
# cat /sys/devices/system/cpu/cpufreq/policy0/scaling_driver
amd-pstate-epp

I am wondering what kernel config differences in Arch's linux-lts might account for this.

Our linux-lts config
OpenWrt's config

Last edited by graysky (2024-09-03 00:16:44)

agapito · 2024-09-01 15:51:19

Are you sure OpenWrt is using the powersave governor? It's not even compiled in their kernel according to the config file:

# CONFIG_CPU_FREQ_DEFAULT_GOV_POWERSAVE is not set
# CONFIG_CPU_FREQ_GOV_POWERSAVE is not set

graysky · 2024-09-01 17:34:02

I modified the config inserting the Arch kernel-lts lines but I get the same result under OpenWrt:

# cat /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
powersave

seth · 2024-09-01 20:04:14

Does amd_pstate_epp build and is it loaded?
Have you tried to build the OpenWRT kernel w/ the arch config to see whether this is actually the cause (or to boot OpenWRT w/ the arch kernel, assuming that's possible)

graysky · 2024-09-01 20:43:21

Yes, it is loaded, the following on booted into OpenWrt. Building with the Arch config would be tough since OpenWrt splits their config options in two or three files for x86/86 (1. generic 2. x86 (I think) and 3. x86/64).

# cat /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
powersave
# cat /sys/devices/system/cpu/amd_pstate/status
active
# cat /sys/devices/system/cpu/cpufreq/policy0/scaling_driver
amd-pstate-epp

graysky · 2024-09-03 00:16:29

I figured out the cause, Arch uses a tickless timer and OpenWrt does not. Building a tickless timer on OpenWrt fixes it.

Arch Linux

#1 2024-09-01 12:30:43

AMD pstate=active gives lower idle on Arch vs OpenWrt ... why [solved]

#2 2024-09-01 15:51:19

Re: AMD pstate=active gives lower idle on Arch vs OpenWrt ... why [solved]

#3 2024-09-01 17:34:02

Re: AMD pstate=active gives lower idle on Arch vs OpenWrt ... why [solved]

#4 2024-09-01 20:04:14

Re: AMD pstate=active gives lower idle on Arch vs OpenWrt ... why [solved]

#5 2024-09-01 20:43:21

Re: AMD pstate=active gives lower idle on Arch vs OpenWrt ... why [solved]

#6 2024-09-03 00:16:29

Re: AMD pstate=active gives lower idle on Arch vs OpenWrt ... why [solved]

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