Re: [PATCH v4 1/1] powerpc: enable dynamic preemption

[Jirka Hladky]

On Thu, Jul 30, 2026 at 4:53 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> But is this really a fundamental RISC cost?  For example, does arm64
> see the same performance issues?

We tested arm64 (Ampere Altra Max) with the same controlled
experiment -- two 6.18 kernels, both voluntary, differing only in
PREEMPT_DYNAMIC:

Arch     PREEMPT_DYNAMIC   kill bogo-ops/sec   Delta
-------  ---------------   -----------------   -----
ppc64le  off               108,836
ppc64le  on                 68,197             -37.3%
aarch64  off                 5,538
aarch64  on                  5,082              -8.2%

arm64 sees -8.2% vs ppc64le's -37.3%. So arm64 is affected but
much less severely.

> In particular, I can see why the preempt_count() operations need to be
> interrupt-safe, but I don't see why you would need barriers.  And
> doesn't powerpc still use software interrupt disabling?  If so, why
> not use that to simply software-disable interrupts around the
> preempt_count() operations?
>
> What am I missing here?

That's a good question -- I don't know enough about the powerpc
preempt_count implementation to answer this. Shrikanth, could you
comment on whether removing the barriers or using software interrupt
disabling around preempt_count is feasible?

Thank you
Jirka