Re: [PATCH] asm-generic: ticket-lock: Optimize arch_spin_value_unlocked

[Guo Ren]

On Sat, Jul 22, 2023 at 10:07:19PM -0400, Waiman Long wrote:
> On 7/19/23 03:00, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > 
> > 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,
Okay, I would improve it. Here means "arch_spin_value_unlocked using
arch_spinlock_is_locked" would cause "an" unnecessary ...

> caller of arch_spin_value_unlocked() could benefit from this change.
> Currently, the only caller is lockref.
Thx for comment, I would add it in the commit msg.

New version is here:
https://lore.kernel.org/linux-riscv/20230731023308.3748432-1-guoren@xxxxxxxxxx/

> 
> Other than that, the patch looks good to me.
> 
> Cheers,
> Longman
> 
> > 
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Cc: David Laight <David.Laight@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > ---
> > Changelog:
> > This patch is separate from:
> > https://lore.kernel.org/linux-riscv/20220808071318.3335746-1-guoren@xxxxxxxxxx/
> > 
> > Peter & David have commented on it:
> > https://lore.kernel.org/linux-riscv/YsK4Z9w0tFtgkni8@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > ---
> >   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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