linux-kernel - Re: [PATCH V2] asm-generic: ticket-lock: Optimize arch_spin_value

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Date:   Mon, 31 Jul 2023 10:56:23 +0100
From:   Will Deacon <will@...nel.org>
To:     guoren@...nel.org
Cc:     David.Laight@...LAB.COM, peterz@...radead.org, mingo@...hat.com,
        longman@...hat.com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V2] asm-generic: ticket-lock: Optimize
 arch_spin_value_unlocked

On Sun, Jul 30, 2023 at 10:33:08PM -0400, guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
> 
> The arch_spin_value_unlocked would cause an unnecessary memory
> access to the contended value. Although it won't cause a significant
> performance gap in most architectures, the arch_spin_value_unlocked
> argument contains enough information. Thus, remove unnecessary
> atomic_read in arch_spin_value_unlocked().
> 
> The caller of arch_spin_value_unlocked() could benefit from this
> change. Currently, the only caller is lockref.
> 
> Signed-off-by: Guo Ren <guoren@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: David Laight <David.Laight@...LAB.COM>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> ---
> Changelog
> V2:
>  - Fixup commit log with Waiman advice.
>  - Add Waiman comment in the commit msg.
> ---
>  include/asm-generic/spinlock.h | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)

Acked-by: Will Deacon <will@...nel.org>

It would be nice to see some numbers showing this actually helps, though.

Will