lkml.org 
[lkml]   [2023]   [Jul]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH] asm-generic: ticket-lock: Optimize arch_spin_value_unlocked
From
On 7/19/23 03:00, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.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@kernel.org>
> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> Cc: David Laight <David.Laight@ACULAB.COM>
> Cc: Peter Zijlstra <peterz@infradead.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 */

\
 
 \ /
  Last update: 2023-07-23 04:08    [W:0.059 / U:0.340 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site