Re: [PATCH] lockref: use cmpxchg64 explicitly for lockless updates

[Will Deacon <will.deacon@arm.com>]

On Fri, Sep 20, 2013 at 11:08:06AM +0100, Will Deacon wrote:
> On Thu, Sep 19, 2013 at 07:11:32PM +0100, Linus Torvalds wrote:
> > From a performance standpoint, lockref really is expected to mainly
> > help with big machines. Only insane people would do big machines with
> > 32-bit kernels these days.
> 
> Our definitions of "big" machines probably differ significantly, but it
> would be interesting to see if this *does* make a difference on some of the
> multi-cluster ARMv7 hardware.

[...]

> > In other words, I'd actually like to see some numbers if there are
> > loads where this actually helps and matters...
> 
> That's fair enough; I just saw the new lockref stuff, thought "that's a cool
> hack" then looked at playing with it on ARM. I'll go see what this AIM7
> thing is all about...

Right, turns out I can get some interesting numbers from your simple t.c
program on my dual-cluster, 5 CPU ARMv7 machine. The new cmpxchg-based lockref
code gives ~50% improvement, but the fun part is that implementing cmpxchg64
without memory barriers doubles this win to ~100% over current mainline.

If we can guarantee that the CODE just messes around with the lockref, those
barriers probably aren't needed...

As for AIM7/re-aim, I'm having a hard time getting repeatable numbers out of
it to establish a baseline, so it's not proving to be especially helpful.

Will