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

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Message-ID: <CAEdQ38H8QZvnwTMJ+-y56je7--Z2Ru-4kmPo7zcW3X8mxfXv1Q@mail.gmail.com>
Date:   Tue, 19 Sep 2023 10:57:24 -0400
From:   Matt Turner <mattst88@...il.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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, 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@...r.kernel.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>,
        linux-alpha@...r.kernel.org
Subject: Re: Arches that don't support PREEMPT

On Tue, Sep 19, 2023 at 10:51 AM H. Peter Anvin <hpa@...or.com> wrote:
>
> On September 19, 2023 7:17:04 AM PDT, Thomas Gleixner <tglx@...utronix.de> wrote:
> >On Tue, Sep 19 2023 at 15:48, John Paul Adrian Glaubitz wrote:
> >> On Tue, 2023-09-19 at 15:42 +0200, Peter Zijlstra wrote:
> >>> > The agreement to kill off ia64 wasn't an invitation to kill off other stuff
> >>> > that people are still working on! Can we please not do this?
> >>>
> >>> If you're working on one of them, then surely it's a simple matter of
> >>> working on adding CONFIG_PREEMPT support :-)
> >>
> >> 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.
> >>
> >> I find it a bit unfair that maintainers of architectures that have huge companies
> >> behind them use their manpower to urge less popular architectures for removal just
> >> because they don't have 150 people working on the port so they can keep up with
> >> design changes quickly.
> >
> >I don't urge for removal. I just noticed that these four architectures
> >lack PREEMPT support. The only thing which is missing is the actual
> >preemption point in the return to kernel code path.
> >
> >But otherwise it should just work, which I obviously can't confirm :)
> >
> >Even without that preemption point it should build and boot. There might
> >be some minor latency issues when that preemption point is not there,
> >but adding it is not rocket science either. It's probably about 10 lines
> >of ASM code, if at all.
> >
> >Though not adding that might cause a blocking issue for the rework of
> >the whole preemption logic in order to remove the sprinkled around
> >cond_resched() muck or force us to maintain some nasty workaround just
> >for the benefit of a few stranglers.
> >
> >So I can make the same argument the other way around, that it's
> >unjustified that some architectures which are just supported for
> >nostalgia throw roadblocks into kernel developemnt.
> >
> >If my ALPHA foo wouldn't be very close to zero, I'd write that ASM hack
> >myself, but that's going to cost more of my and your time than it's
> >worth the trouble,
> >
> >Hmm. I could delegate that to Linus, he might still remember :)
> >
> >Thanks,
> >
> >        tglx
>
> Does *anyone* actually run Alpha at this point?

I do, as part of maintaining the Gentoo distribution for Alpha.

I'm listed in MAINTAINERS, but really only so I can collect patches
send them to Linus after testing. I don't have copious amounts of free
time to be proactive in kernel development and it's also not really my
area of expertise so I'm nowhere near effective at it.

I would be happy to test any patches sent my way (but I acknowledge
that writing these patches wouldn't be high on anyone's priority list,
etc)

(A video my friend Ian and I made about a particularly large
AlphaServer I have in my basement, in case anyone is interested:
https://www.youtube.com/watch?v=z658a8Js5qg)