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
