Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

From: Torsten Duwe
Date: Thu Feb 06 2014 - 12:37:42 EST

On Thu, Feb 06, 2014 at 05:38:37PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 06, 2014 at 11:37:37AM +0100, Torsten Duwe wrote:
> > x86 has them, MIPS has them, ARM has them, even ia64 has them:
> > ticket locks. They reduce memory bus and cache pressure especially
> > for contended spinlocks, increasing performance.
> >
> > This patch is a port of the x86 spin locks, mostly written in C,
> > to the powerpc, introducing inline asm where needed. The pSeries
> > directed yield for vCPUs is taken care of by an additional "holder"
> > field in the lock.
> >
> A few questions; what's with the ppc64 holder thing? Not having a 32bit
> spinlock_t is sad.

I must admit that I haven't tested the patch on non-pseries ppc64 nor on
ppc32. Only ppc64 has the ldarx and I tried to atomically replace the
holder along with the locks. That might prove unneccessary.

> Can you pair lwarx with sthcx ? I couldn't immediately find the answer
> in the PowerISA doc. If so I think you can do better by being able to
> atomically load both tickets but only storing the head without affecting
> the tail.

V2.06b, Book II, Chapter 3, "sthcx" says:
| If a reservation exists and the length associated [...] is not 2 bytes,
| it is undefined whether (RS)_48:63 are stored [...]

That doesn't make me feel comfortable :(


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