linux-kernel - Re: Arches that don't support PREEMPT

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Message-ID: <64746d98b60f1677cecd26501511f140688801e1.camel@physik.fu-berlin.de>
Date:   Tue, 19 Sep 2023 19:58:31 +0200
From:   John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org, mgorman@...e.de,
        rostedt@...dmis.org, jon.grimm@....com, bharata@....com,
        raghavendra.kt@....com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com, jgross@...e.com, andrew.cooper3@...rix.com,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Brian Cain <bcain@...cinc.com>,
        linux-hexagon@...r.kernel.org,
        Richard Henderson <richard.henderson@...aro.org>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>, linux-alpha@...r.kernel.org
Subject: Re: Arches that don't support PREEMPT

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@...sik.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

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
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