[PATCH 6/6] powerpc: Use lwsync for acquire barrier if CPU supports it

[Nick Piggin]

On Wed, Feb 10, 2010 at 10:10:25PM +1100, Anton Blanchard wrote:
> 
> Nick Piggin discovered that lwsync barriers around locks were faster than isync
> on 970. That was a long time ago and I completely dropped the ball in testing
> his patches across other ppc64 processors.
> 
> Turns out the idea helps on other chips. Using a microbenchmark that
> uses a lot of threads to contend on a global pthread mutex (and therefore a
> global futex), POWER6 improves 8% and POWER7 improves 2%. I checked POWER5
> and while I couldn't measure an improvement, there was no regression.

Ah, good to see this one come back. I also tested tbench over localhost
btw which actually did show some speedup on the G5. 

BTW. this was the last thing left:
http://www.mail-archive.com/linuxppc-dev@lists.ozlabs.org/msg29738.html

Don't know if you took a look at that again, but maybe it's worth
looking at. Hmm, we do actually seem to be growing number of smp_mb*
calls in core kernel too.