Re: Arches that don't support PREEMPT

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Sep 19, 2023, at 10:16, Peter Zijlstra wrote:
> 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.
...
>
> 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.

I checked the default configurations for both in-kernel targets and
general-purpose distros and was surprised to learn that very few
actually turn on full preemption by default:

- All distros I looked at (rhel, debian, opensuse) use PREEMPT_VOLUNTARY
  by default, though they usually also set PREEMPT_DYNAMIC to let users
  override it at boot time.

- The majority (220) of all defconfig files in the kernel don't select
  any preemption options, and just get PREEMPT_NONE automatically.
  This includes the generic configs for armv7, s390 and mips.
  
- A small number (24) set PREEMPT_VOLUNTARY, but this notably
  includes x86 and ppc64. x86 is the only one of those that sets
  PREEMPT_DYNAMIC

- CONFIG_PREEMPT=y (full preemption) is used on 89 defconfigs,
  including arm64 and a lot of the older arm32, arc and
  mips platforms.

If we want to have a chance of removing both PREEMPT_NONE and
PREEMPT_VOLUNTARY, I think we should start with changing the
defaults first, so defconfigs that don't specify anything else
get PREEMPT=y, and distros that use PREEMPT_VOLUNTARY use it
use it in the absence of a command line argument. If that
doesn't cause too many regressions, the next step might be
to hide the choice under CONFIG_EXPERT until all m68k and
alpha no longer require PREEMPT_NONE.

     Arnd