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

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Date:   Sun, 30 Oct 2022 14:19:22 +0100 (CET)
From:   Christoph Lameter <cl@...two.de>
To:     Wen Yao <haiwenyao@...ontech.com>
cc:     paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, dennis@...nel.org, tj@...nel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 1/2] riscv: percpu:Add riscv percpu operations

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?