Re: [PATCH 1/2] riscv: percpu:Add riscv percpu operations

[Christoph Lameter]

On Wed, 26 Oct 2022, Wen Yao wrote:

> This patch use riscv AMO(Atomic Memory Operation) instructions to
> optimise some this_cpu_and this_cpu_or this_cpu_add operations.
> It reuse cmpxchg_local() to impletment this_cpu_cmpxchg macros.
> It reuse xchg_relaxed() to impletment this_cpu_xchg macros.

Are you sure that these changes gives you any benefit vs disabling preempt
or irq offs? I dont know too much about atomics on riscv but it looks like
you are using full atomics. The performance penalty for the use of those
is usually drastic. Often irq/preempt off is better.

Could you run some of the synthetic tests to establish the benefit? F.e.
run the synthetic tests for the slub allcator with and without these
patches.


> +			__asm__ __volatile__(                                  \
> +				"amo" #asm_op ".w"                             \

amo = atomic operation?