Re: Arches that don't support PREEMPT

John Paul Adrian Glaubitz
2023-09-19T17:58:50Z

Hi Linus!

On Tue, 2023-09-19 at 10:25 -0700, Linus Torvalds wrote:
> On Tue, 19 Sept 2023 at 06:48, John Paul Adrian Glaubitz
> <glaubitz@physik.fu-berlin.de> wrote:
> >
> > As Geert poined out, I'm not seeing anything particular problematic with the
> > architectures lacking CONFIG_PREEMPT at the moment. This seems to be more
> > something about organizing KConfig files.
>
> It can definitely be problematic.
>
> Not the Kconfig file part, and not the preempt count part itself.
>
> But the fact that it has never been used and tested means that there
> might be tons of "this architecture code knows it's not preemptible,
> because this architecture doesn't support preemption".
>
> So you may have basic architecture code that simply doesn't have the
> "preempt_disable()/enable()" pairs that it needs.
>
> PeterZ mentioned the generic entry code, which does this for the entry
> path. But it actually goes much deeper: just do a
>
> git grep preempt_disable arch/x86/kernel
>
> and then do the same for some other architectures.
>
> Looking at alpha, for example, there *are* hits for it, so at least
> some of the code there clearly *tries* to do it. But does it cover all
> the required parts? If it's never been tested, I'd be surprised if
> it's all just ready to go.

Thanks for the detailed explanation.

> I do think we'd need to basically continue to support ARCH_NO_PREEMPT
> - and such architectures migth end up with the worst-cast latencies of
> only scheduling at return to user space.

Great to hear, thank you.

And, yes, eventually I would be happy to help get alpha and m68k converted.

Adrian

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