Re: [PATCH] arm64: remove HAVE_CMPXCHG_LOCAL

[Will Deacon]

On Tue, Feb 17, 2026 at 01:53:19PM +0000, Catalin Marinas wrote:
> On Mon, Feb 16, 2026 at 08:59:17PM +0530, Dev Jain wrote:
> > On 16/02/26 4:30 pm, Will Deacon wrote:
> > > On Sun, Feb 15, 2026 at 11:39:44AM +0800, Jisheng Zhang wrote:
> > >> It turns out the generic disable/enable irq this_cpu_cmpxchg
> > >> implementation is faster than LL/SC or lse implementation. Remove
> > >> HAVE_CMPXCHG_LOCAL for better performance on arm64.
> > >>
> > >> Tested on Quad 1.9GHZ CA55 platform:
> > >> average mod_node_page_state() cost decreases from 167ns to 103ns
> > >> the spawn (30 duration) benchmark in unixbench is improved
> > >> from 147494 lps to 150561 lps, improved by 2.1%
> > >>
> > >> Tested on Quad 2.1GHZ CA73 platform:
> > >> average mod_node_page_state() cost decreases from 113ns to 85ns
> > >> the spawn (30 duration) benchmark in unixbench is improved
> > >> from 209844 lps to 212581 lps, improved by 1.3%
> > >>
> > >> Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> > >> ---
> > >>  arch/arm64/Kconfig              |  1 -
> > >>  arch/arm64/include/asm/percpu.h | 24 ------------------------
> > >>  2 files changed, 25 deletions(-)
> > > That is _entirely_ dependent on the system, so this isn't the right
> > > approach. I also don't think it's something we particularly want to
> > > micro-optimise to accomodate systems that suck at atomics.
> > 
> > Hi Will,
> > 
> > As I mention in the other email, the suspect is not the atomics, but
> > preempt_disable(). On Apple M3, the regression reported in [1] resolves
> > by removing preempt_disable/enable in _pcp_protect_return. To prove
> > this another way, I disabled CONFIG_ARM64_HAS_LSE_ATOMICS and the
> > regression worsened, indicating that at least on Apple M3 the
> > atomics are faster.
> 
> Then why don't we replace the preempt disabling with local_irq_save()
> in the arm64 code and still use the LSE atomics?

Even better, work on making preempt_disable() faster as it's used in many
other places. Of course, if people want to hack the .config, they could
also change the preemption mode...

Will