[systemd-devel] Disabling cpufreq/boost at boot time sometimes fails

[Thomas HUMMEL]

On 13/07/2022 17:12, killermoehre at gmx.net wrote:
>> This must explain why my modprobe.d (of acpi_cpufreq) seems to always work but not why tmpfiles.d or a .service unit :
> 
> Actually, your modprobe.d is much too late


Well, maybe we don't talk about the same thing: I'm only interested in 
/sys/devices/system/cpu/cpufreq/boost file.

I can echo 0 or 1 into this file anytime long after boot and verify 
(running 'stress' for instance) in /proc/cpuinfo that core frequencies 
are boosted or limited (lscpu does not seem to update the info though).

Besides, rmmod'ing the acpi_cpufreq module makes the 'boost' file vanish 
(which seems normal according documentation)

> The driver absolutely loaded, as stated earlier. What I find interesting is the error message you get with
>> /sys/devices/system/cpu/cpufreq/boost: Permission denied

Agreed. Assuming module is loaded (hence file present) maybe there's a 
critical section where it does not have the correct permission ? However 
a standard systemd service (with default dependencies) comes quite late 
in the boot process...

> Did it failed because the file didn’t exists? Maybe the path you used is wrong?

I don't know. I initially thought about a race thinking that it was some 
systemd services (udevd.service ?) that created or chmoded the file. But 
(see above) this does not seem to be the case or is it ?


> In this case I think your best bet is to disable the most option in the BIOS/UEFI. At least https://urldefense.com/v3/__https://docs.kernel.org/admin-guide/pm/cpufreq.html*rationale-for-boost-control-knob__;Iw!!JFdNOqOXpB6UZW0!t1eSenFnShc2J6NFFCaKdU-KiALOpOCGtva-oRXD7dih4zXs-yXPrA0wcas3OKKLGw4wKo646cvZDartCoacDVmvAi4$  is speaking of that.
> If you don’t have such option in the BIOS/UEFI settings, you could try some udev rule reacting to the /sys entry. Something like
> 
> `/etc/udev/rules.d/20-disable-cpu-boost.rules`
> ```
> KERNEL=="cpu", ATTR{cpufreq/boost}=="1", ATTR{cpufreq/boost}:="0"
> ```

Thanks yes, I thought about it. Would like to understand what's failing 
in my original naive setting (tmpfiles or service).

Thanks again anyway.

--
Thomas HUMMEL