Cannot put CPU0 offline?

Xoores · n00b Joined: 15 Feb 2022 Posts: 5 Location: EU

Hi,

I'm trying to get CPU hotplug working on i9 CPU - and it works for all cpus (threads) but CPU0. I know that hotplug for CPU0 has to be enabled separately by adding `cpu0_hotplug` or setting a config option in a kernel config (kernel.org doc).

However on 6.10.0-gentoo this option does not seem to exist and that cmdline option does nothing.

What am I doing wrong?

Thanks

myga · Tux's lil' helper Joined: 12 Jun 2023 Posts: 121

Are you using sys-kernel/gentoo-sources? How are you searching for that option?
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Hu · Administrator Joined: 06 Mar 2007 Posts: 22618

This was (mostly) removed in x86/topology: Remove CPU0 hotplug option (v6.4-rc2-5-ge59e74dc48a3). I see a few documentation remnants, but the Kconfig option for setting a default, and the code for reacting to it, are gone. As such, it seems like a documentation bug that you can still find current documentation that describes it.

DaggyStyle · Watchman Joined: 22 Mar 2006 Posts: 5929

small question, if you offline cpu0, this means there is no bsp cpu and afaik, x64 doesn't support alternate bsps like aarch64 does.
how will your system work?
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Xoores · n00b Joined: 15 Feb 2022 Posts: 5 Location: EU

Hu · Administrator Joined: 06 Mar 2007 Posts: 22618

As I read the justification for removal, the feature you want never really worked fully, so the code that pretended to support this was removed after 11 years of waiting for it to be finished.

Xoores · n00b Joined: 15 Feb 2022 Posts: 5 Location: EU

Yeah, probably. I honestly hope that they will introduce some better way to handle P-/E-cores. Seems like this is a direction modern hardware is taking and it would be nice to have some direct in-kernel support for this kind of things.

I don't really think that blindly putting the cores offline is the best approach here because current implementation in kernel does not seem to be too happy as there is a detectable "hiccup" when changing core state. However this is the only way I found out to have atleast some way of taking advantage of having different core types at the moment (without having to code something).

I did not do any *exact* tests yet (since I could not put P-cores fully to sleep so far), but I'd say it makes a huge difference on my computer - disabling P-cores almost doubles my battery life. I'm still fighting with a new hardware so there are few things to be ironed out, but that's the price I have to pay

DaggyStyle · Watchman Joined: 22 Mar 2006 Posts: 5929

Xoores · n00b Joined: 15 Feb 2022 Posts: 5 Location: EU