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Date:   Tue, 19 Sep 2023 16:16:27 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com,
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Subject: Re: Arches that don't support PREEMPT
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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.