linux-kernel - [RFC] why do sensors break CPU scaling

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Date:   Wed, 20 Nov 2019 21:42:12 +0100
From:   Tom Psyborg <pozega.tomislav@...il.com>
To:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [RFC] why do sensors break CPU scaling

Hi

Recently I've needed to set lowest CPU scaling profile, running ubuntu
16.04.06 I used standard approach - echoing powersave to
/sys/devices/system/cpu/cpu*/cpufreq/scaling_governor.
This did not work as the
/sys/devices/system/cpu/cpu*/cpufreq/cpuinfo_cur_freq kept returning
max scaling freq.

During testing of ubuntu 19.10 I've found that the above approach
actually does work, but as long as there are no xsensors (or just
sensors from cli) being run.
cpuinfo_cur_freq in this case was returning variable values +- 1% of
around 1.4GHz.
As soon as I issue sensors command cpuinfo_cur_freq jumps to 3.5GHz
for a fraction of second and returns back to 1.4GHz afterwards. If
running xsensors it keeps bouncing between 1.4 and 3.5GHz all the
time.

Rebooted back to 16.04.6 and was able to set lowest CPU scaling freq
as well, but issuing sensors command here once just breaks CPU scaling
that now remains at about 3.5GHz.
It can be set to lowest scaling freq again without rebooting but it
needs to change scaling_governor for all cores to something else and
then back to powersave.

Why is this happening, shouldn't sensors command just read temp/fan
values without writing to any of the CPU control registers?