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

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Date:   Mon, 31 Jul 2023 19:27:19 +0800
From:   bibo mao <maobibo@...ngson.cn>
To:     guoren@...nel.org, David.Laight@...LAB.COM, will@...nel.org,
        peterz@...radead.org, mingo@...hat.com, longman@...hat.com
Cc:     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



在 2023/7/31 10:33, guoren@...nel.org 写道:
> 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(-)
> 
> diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
> index fdfebcb050f4..90803a826ba0 100644
> --- a/include/asm-generic/spinlock.h
> +++ b/include/asm-generic/spinlock.h
> @@ -68,11 +68,18 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
>  	smp_store_release(ptr, (u16)val + 1);
>  }
>  
> +static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> +{
> +	u32 val = lock.counter;
> +
> +	return ((val >> 16) == (val & 0xffff));
> +}
I do not know much about lock, will it be cached in register without memory
access again like READ_ONCE or atomic_read?

Regards
Bibo Mao
> +
>  static __always_inline int arch_spin_is_locked(arch_spinlock_t *lock)
>  {
> -	u32 val = atomic_read(lock);
> +	arch_spinlock_t val = READ_ONCE(*lock);
>  
> -	return ((val >> 16) != (val & 0xffff));
> +	return !arch_spin_value_unlocked(val);
>  }
>  
>  static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
> @@ -82,11 +89,6 @@ static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
>  	return (s16)((val >> 16) - (val & 0xffff)) > 1;
>  }
>  
> -static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> -{
> -	return !arch_spin_is_locked(&lock);
> -}
> -
>  #include <asm/qrwlock.h>
>  
>  #endif /* __ASM_GENERIC_SPINLOCK_H */