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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 20 Sep 2023 09:29:21 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
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        Geert Uytterhoeven <geert@linux-m68k.org>,
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Subject: Re: Arches that don't support PREEMPT
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* Thomas Gleixner <tglx@linutronix.de> wrote:

> On Tue, Sep 19 2023 at 10:25, Linus Torvalds wrote:
> > On Tue, 19 Sept 2023 at 06:48, John Paul Adrian Glaubitz
> > <glaubitz@physik.fu-berlin.de> wrote:
> >>
> >> 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.
> >
> > It can definitely be problematic.
> >
> > Not the Kconfig file part, and not the preempt count part itself.
> >
> > But the fact that it has never been used and tested means that there
> > might be tons of "this architecture code knows it's not preemptible,
> > because this architecture doesn't support preemption".
> >
> > So you may have basic architecture code that simply doesn't have the
> > "preempt_disable()/enable()" pairs that it needs.
> >
> > PeterZ mentioned the generic entry code, which does this for the entry
> > path. But it actually goes much deeper: just do a
> >
> >     git grep preempt_disable arch/x86/kernel
> >
> > and then do the same for some other architectures.
> >
> > Looking at alpha, for example, there *are* hits for it, so at least
> > some of the code there clearly *tries* to do it. But does it cover all
> > the required parts? If it's never been tested, I'd be surprised if
> > it's all just ready to go.
> >
> > I do think we'd need to basically continue to support ARCH_NO_PREEMPT
> > - and such architectures migth end up with the worst-cast latencies of
> > only scheduling at return to user space.
> 
> The only thing these architectures should gain is the preempt counter 
> itself, [...]

And if any of these machines are still used, there's the small benefit of 
preempt_count increasing debuggability of scheduling in supposedly 
preempt-off sections that were ignored silently previously, as most of 
these architectures do not even enable CONFIG_DEBUG_ATOMIC_SLEEP=y in their 
defconfigs:

  $ for ARCH in alpha hexagon m68k um; do git grep DEBUG_ATOMIC_SLEEP arch/$ARCH; done
  $

Plus the efficiency of CONFIG_DEBUG_ATOMIC_SLEEP=y is much reduced on 
non-PREEMPT kernels to begin with: it will basically only detect scheduling 
in hardirqs-off critical sections.

So IMHO there's a distinct debuggability & robustness plus in enabling the 
preemption count on all architectures, even if they don't or cannot use the 
rescheduling points.

> [...] but yes the extra preemption points are not mandatory to have, i.e. 
> we simply do not enable them for the nostalgia club.
> 
> The removal of cond_resched() might cause latencies, but then I doubt 
> that these museus pieces are used for real work :)

I'm not sure we should initially remove *explicit* legacy cond_resched() 
points, except from high-freq paths where they hurt - and of course remove 
them from might_sleep().

Thanks,

	Ingo