Re: [PATCH] arm64: remove HAVE_CMPXCHG_LOCAL

[Dev Jain <dev.jain@arm.com>]

On 15/02/26 9:09 am, 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@kernel.org>
> ---

Thanks. This concurs with my investigation on [1]. The problem
isn't really LL/SC/LSE but preempt_disable()/enable() in
this_cpu_* [1, 2].

I think you should only remove the selection of the config,
but keep the code? We may want to switch this on again if
the real issue gets solved.

[1] https://lore.kernel.org/all/5a6782f3-d758-4d9c-975b-5ae4b5d80d4e@arm.com/
[2] https://lore.kernel.org/all/CAHbLzkpcN-T8MH6=W3jCxcFj1gVZp8fRqe231yzZT-rV_E_org@mail.gmail.com/

>  arch/arm64/Kconfig              |  1 -
>  arch/arm64/include/asm/percpu.h | 24 ------------------------
>  2 files changed, 25 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 38dba5f7e4d2..5e7e2e65d5a5 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -205,7 +205,6 @@ config ARM64
>  	select HAVE_EBPF_JIT
>  	select HAVE_C_RECORDMCOUNT
>  	select HAVE_CMPXCHG_DOUBLE
> -	select HAVE_CMPXCHG_LOCAL
>  	select HAVE_CONTEXT_TRACKING_USER
>  	select HAVE_DEBUG_KMEMLEAK
>  	select HAVE_DMA_CONTIGUOUS
> diff --git a/arch/arm64/include/asm/percpu.h b/arch/arm64/include/asm/percpu.h
> index b57b2bb00967..70ffe566cb4b 100644
> --- a/arch/arm64/include/asm/percpu.h
> +++ b/arch/arm64/include/asm/percpu.h
> @@ -232,30 +232,6 @@ PERCPU_RET_OP(add, add, ldadd)
>  #define this_cpu_xchg_8(pcp, val)	\
>  	_pcp_protect_return(xchg_relaxed, pcp, val)
>  
> -#define this_cpu_cmpxchg_1(pcp, o, n)	\
> -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> -#define this_cpu_cmpxchg_2(pcp, o, n)	\
> -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> -#define this_cpu_cmpxchg_4(pcp, o, n)	\
> -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> -#define this_cpu_cmpxchg_8(pcp, o, n)	\
> -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> -
> -#define this_cpu_cmpxchg64(pcp, o, n)	this_cpu_cmpxchg_8(pcp, o, n)
> -
> -#define this_cpu_cmpxchg128(pcp, o, n)					\
> -({									\
> -	typedef typeof(pcp) pcp_op_T__;					\
> -	u128 old__, new__, ret__;					\
> -	pcp_op_T__ *ptr__;						\
> -	old__ = o;							\
> -	new__ = n;							\
> -	preempt_disable_notrace();					\
> -	ptr__ = raw_cpu_ptr(&(pcp));					\
> -	ret__ = cmpxchg128_local((void *)ptr__, old__, new__);		\
> -	preempt_enable_notrace();					\
> -	ret__;								\
> -})
>  
>  #ifdef __KVM_NVHE_HYPERVISOR__
>  extern unsigned long __hyp_per_cpu_offset(unsigned int cpu);