The ondemand governor does not work properly with the 4.14.x kernel vs the 4.9.x kernel

[graysky2]

I am running Arch ARM (armv7h) on both a Raspberry Pi 3 Model B Plus Rev 1.3 and a Raspberry Pi 2 Model B Rev 1.1. When I boot either of these using the identical micro SD, I find that under kernel version 4.14.29 (Arch ARM's official current version), the ondemand governor has the idle machine throttled up to the maximum frequency for that hardware as judged by running a simple script to query it and a few other parameters (shown at the bottom of the report). When I boot into kernel version 4.9.80 (that I built from be97feb), the idle frequency is the minimum as expected.

For example, when booted into kernel version 4.9.80 on the RPi3B+ (I get identical results on the RPi2):

% uptime
 06:31:24 up 14 min,  1 users,  load average: 0.00, 0.00, 0.01

% watch -t ~/bin/temps
CPU temp           :   37°C
CPU freq           :   600 MHz (ondemand)

Letting the watch command run for a few minutes, I never saw the frequency increase (as expected since the machine is idle).

Same hardware after booting into the 4.14.29 kernel:

% uptime
 06:57:15 up 6 min,  1 user,  load average: 0.00, 0.01, 0.00

% cat /sys/devices/system/cpu/cpufreq/policy0/scaling_min_freq 
600000

% watch -t ~/bin/temps
CPU temp           :   43°C
CPU freq           :   1400 MHz (ondemand)

Letting the watch command run for a few minutes, I never saw the frequency decrease. As you see the idle temp is also higher under the this kernel.

The script ~/bin/temps is simply:

#!/bin/sh
freq=$(cat /sys/devices/system/cpu/cpufreq/policy0/scaling_cur_freq)
gov=$(cat /sys/devices/system/cpu/cpufreq/policy0/scaling_governor)
temp=$(cat /sys/devices/virtual/thermal/thermal_zone0/temp)

echo "CPU temp           :   $(( temp / 1000 ))°C"
echo "CPU freq           :   $(bc <<< "scale=0; $freq/1000") MHz ($gov)"

[mkreisl]

Can't confirm
CPU runs constantly @ 600MHz when idle (XBian, kernel 4.14.27+)

[graysky2]

Can't confirm. CPU runs constantly @ 600MHz when idle (XBian, kernel 4.14.27+)

I will try writing out XBian_Latest_rpi3.img to a spare micro SD card to confirm as well. If it is an Arch-ARM-specific problem, I'm wondering how to diagnose. Differences in kernel configs (xbian vs Arch ARM)?

[pelwell]

On-demand CPU scaling is working fine under Raspbian on a 4.14.29 kernel.

On an idle machine, run vcgencmd measure_clock arm to confirm.

[graysky2]

@pelwell - Thanks for the command.
On Arch ARM armv7h 4.14.29-1:

% vcgencmd measure_clock arm
frequency(45)=1400146000

On Arch ARM armv7h with the 2.9.80 kernel I described above:

% vcgencmd measure_clock arm
frequency(45)=600000000

[pelwell]

That does look broken. One for the Arch devs, I think, since RPi and XBian kernels don't share the problem.

If the Arch maintainers want to compare notes with us we're happy to chat.

[MilhouseVH]

Not seeing any issues with ondemand and frequency scaling with LibreELEC builds using 4.14.29 - all seems normal.

[graysky2]

I can't explain it either. I am putting Raspbian on a spare card to see what I can learn. I will close this since it's not general. Thank you all for the replies!