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

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Message-ID: <0e39d62d-44bc-731e-471e-4df621b4cdd5@redhat.com>
Date:   Sat, 22 Jul 2023 22:07:19 -0400
From:   Waiman Long <longman@...hat.com>
To:     guoren@...nel.org, David.Laight@...LAB.COM, will@...nel.org,
        peterz@...radead.org, mingo@...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] asm-generic: ticket-lock: Optimize
 arch_spin_value_unlocked

On 7/19/23 03:00, guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
>
> Using arch_spinlock_is_locked would cause another 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().

AFAICS, only one memory access is needed for the current 
arch_spinlock_is_locked(). So your description isn't quite right. OTOH, 
caller of arch_spin_value_unlocked() could benefit from this change. 
Currently, the only caller is lockref.

Other than that, the patch looks good to me.

Cheers,
Longman

>
> Signed-off-by: Guo Ren <guoren@...nel.org>
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Cc: David Laight <David.Laight@...LAB.COM>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
> Changelog:
> This patch is separate from:
> https://lore.kernel.org/linux-riscv/20220808071318.3335746-1-guoren@kernel.org/
>
> Peter & David have commented on it:
> https://lore.kernel.org/linux-riscv/YsK4Z9w0tFtgkni8@hirez.programming.kicks-ass.net/
> ---
>   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));
> +}
> +
>   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 */