Re: [LSF/MM/BPF TOPIC] Improve this_cpu_ops performance for ARM64 (and potentially other architectures)

[Heiko Carstens <hca@linux.ibm.com>]

On Wed, Feb 11, 2026 at 03:14:57PM -0800, Yang Shi wrote:
> Design
> ======
> To improve the performance for this_cpu_ops on ARM64 and potentially
> some other non-x86 architectures, I and Christopher Lameter proposed
> the below solution.

...

> I have done some PoC on ARM64. Hopefully I can post them to the
> mailing list to ease the discussion before the conference.

FWIW, from an s390 perspective we have been relying on that preempt_disable()
and preempt_enable() pairs for this_cpu*() ops are optimized away at compile
time for PREEMPT_NONE and PREEMPT_VOLUNTARY. Which is what all distributions
did, until now.

With 7dadeaa6e851 ("sched: Further restrict the preemption modes") this is not
the case anymore. So the overhead will be there for every "up-to-date"
architecture except x86. I've been looking at some ways how to get rid of this
extra overhead for s390. E.g. we have an "access register mode", which can be
sort of compared to the x86 segment register approach, but switching the CPU
to this mode and back is more expensive than the preempt_disable() and
preempt_enable() pairs.

To make the long story short: I'm very interested in your approch from an s390
perspective (aka "other non-x86 architectures").