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

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Date:   Tue, 19 Sep 2023 16:16:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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, 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

On Tue, Sep 19, 2023 at 03:48:09PM +0200, 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.

The plan in the parent thread is to remove PREEMPT_NONE and
PREEMPT_VOLUNTARY and only keep PREEMPT_FULL.

> 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.

PREEMPT isn't something new. Also, I don't think the arch part for
actually supporting it is particularly hard, mostly it is sticking the
preempt_schedule_irq() call in return from interrupt code path.

If you convert the arch to generic-entry (a much larger undertaking)
then you get this for free.