Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for locklessupdate of refcount

[Waiman Long]

On 06/26/2013 05:22 PM, Andi Kleen wrote:
> On Wed, Jun 26, 2013 at 05:07:02PM -0400, Waiman Long wrote:
> > On 06/26/2013 04:17 PM, Andi Kleen wrote:
> > > > + * The combined data structure is 8-byte aligned. So proper placement of this
> > > > + * structure in the larger embedding data structure is needed to ensure that
> > > > + * there is no hole in it.
> > > On i386 u64 is only 4 bytes aligned. So you need to explicitely align
> > > it to 8 bytes. Otherwise you risk the two members crossing a cache line, which
> > > would be really expensive with atomics.
> > Do you mean the original i386 or the i586 that are now used by most
> > distribution now? If it is the former, I recall that i386 is now no
> > longer supported.
> I mean i386, as in the 32bit x86 architecture.
>
> > I also look around some existing codes that use cmpxchg64. It
> > doesn't seem like they use explicit alignment. I will need more
> > investigation to see if it is a real problem.
> Adding the alignment is basically free. If 32bit users don't enforce
> it they're likely potentially broken yes, but they may be lucky.

You are right. I will added the 8-byte alignment attribute to the union 
definition and document that in the code.

> > > > +	get_lock = ((threshold>= 0)&&   (old.count == threshold));
> > > > +	if (likely(!get_lock&&   spin_can_lock(&old.lock))) {
> > > What is that for? Why can't you do the CMPXCHG unconditially ?
> > An unconditional CMPXCHG can be as bad as acquiring the spinlock. So
> > we need to check the conditions are ready before doing an actual
> > CMPXCHG.
> But this isn't a cheap check. Especially spin_unlock_wait can be
> very expensive.
> And all these functions have weird semantics. Perhaps just a quick
> spin_is_locked.

In the uncontended case, doing spin_unlock_wait will be similar to 
spin_can_lock. This, when combined with a cmpxchg, is still faster than 
doing 2 atomic operations in spin_lock/spin_unlock.

In the contended case, doing spin_unlock_wait won't be more expensive 
than doing spin_lock. Without doing that, most of the threads will fall 
back to acquiring the lock negating any performance benefit. I had tried 
that without spin_unlock_wait and there wasn't that much performance 
gain in the AIM7 short workload. BTW, spin_can_lock is just the negation 
of spin_is_locked.

Regards,
Longman
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