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From:   Matt Turner <mattst88@gmail.com>
Date:   Tue, 19 Sep 2023 10:57:24 -0400
Message-ID: <CAEdQ38H8QZvnwTMJ+-y56je7--Z2Ru-4kmPo7zcW3X8mxfXv1Q@mail.gmail.com>
Subject: Re: Arches that don't support PREEMPT
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de,
        dave.hansen@linux.intel.com, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org, mgorman@suse.de,
        rostedt@goodmis.org, jon.grimm@amd.com, bharata@amd.com,
        raghavendra.kt@amd.com, boris.ostrovsky@oracle.com,
        konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        linux-m68k@vger.kernel.org, Richard Weinberger <richard@nod.at>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        linux-um@lists.infradead.org, Brian Cain <bcain@quicinc.com>,
        linux-hexagon@vger.kernel.org,
        Richard Henderson <richard.henderson@linaro.org>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        linux-alpha@vger.kernel.org
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On Tue, Sep 19, 2023 at 10:51=E2=80=AFAM H. Peter Anvin <hpa@zytor.com> wro=
te:
>
> On September 19, 2023 7:17:04 AM PDT, Thomas Gleixner <tglx@linutronix.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 oth=
er 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 wi=
th the
> >> architectures lacking CONFIG_PREEMPT at the moment. This seems to be m=
ore
> >> something about organizing KConfig files.
> >>
> >> I find it a bit unfair that maintainers of architectures that have hug=
e 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 kee=
p 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=3Dz658a8Js5qg)