Re: Arches that don't support PREEMPT

[Thomas Gleixner]

On Tue, Sep 19 2023 at 10:25, Linus Torvalds wrote:
> On Tue, 19 Sept 2023 at 06:48, John Paul Adrian Glaubitz
> <glaubitz@xxxxxxxxxxxxxxxxxxx> 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.
>
> 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.

The only thing these architectures should gain is the preempt counter
itself, but yes the extra preemption points are not mandatory to
have, i.e. we simply do not enable them for the nostalgia club.

The removal of cond_resched() might cause latencies, but then I doubt
that these museus pieces are used for real work :)

Thanks,

        tglx