Messages in this thread | | | Date | Sat, 22 Jul 2023 22:07:19 -0400 | Subject | Re: [PATCH] asm-generic: ticket-lock: Optimize arch_spin_value_unlocked | From | Waiman Long <> |
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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 */
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