Message-ID: <7EB81196-3A32-4638-A076-0C0CFF722996@zytor.com>
Date:   Tue, 19 Sep 2023 07:50:36 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Peter Zijlstra <peterz@...radead.org>
CC:     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>,
        Matt Turner <mattst88@...il.com>, linux-alpha@...r.kernel.org
Subject: Re: Arches that don't support PREEMPT

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?

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