[External] Re: [PATCH v9 1/3] riscv: implement this_cpu operations

[yunhui cui]

Hi David,

On Mon, Nov 9, 2026 at 5:33 AM David Laight
<david.laight.linux at gmail.com> wrote:
>
> On Sun,  8 Nov 2026 19:04:04 +0800
> Yunhui Cui <cuiyunhui at bytedance.com> wrote:
>
> > RISC-V has no asm/percpu.h, so this_cpu_*() read-modify-write
> > operations use generic implementations that disable local interrupts.
> >
> > Implement RISC-V-specific this_cpu operations with preemption disabled
> > to keep accesses on the current CPU.
>
> Have you looked at some of the schemes (I think arm64 has the latest,
> but also s390) that do most per-cpu ops without disabling preemption.
> In particular they save the pre-emption check when pre-emption
> is enabled.
>
> Basically the code tells the scheduler which register contains the
> per-cpu data so that it can be 'magically' flipped during a context
> switch.

Many thanks for pointing this out and suggesting this optimization. This
is a very useful suggestion. In the common case no preemption or migration
occurs, so the preemption bookkeeping and reschedule check are unnecessary
overhead.

Implementing this scheme for RISC-V will require additional changes beyond
the current series. If this series is otherwise acceptable, I would like
to land the current this_cpu_* support first and then prepare a follow-up
series using this approach.

Thanks,
Yunhui

>
> David