Re: [PATCH v5 1/6] qspinlock: powerpc support qspinlock

On Fri, Jun 03, 2016 at 02:33:47PM +1000, Benjamin Herrenschmidt wrote:

 - For the above, can you show (or describe) where the qspinlock
   improves things compared to our current locks.

So currently PPC has a fairly straight forward test-and-set spinlock
IIRC. You have this because LPAR/virt muck and lock holder preemption
issues etc..

qspinlock is 1) a fair lock (like ticket locks) and 2) provides
out-of-word spinning, reducing cacheline pressure.

Esp. on multi-socket x86 we saw the out-of-word spinning being a big win
over our ticket locks.

And fairness, brought to us by the ticket locks a long time ago,
eliminated starvation issues we had, where a spinner local to the holder
would 'always' win from a spinner further away. So under heavy enough
local contention, the spinners on 'remote' CPUs would 'never' get to own
the lock.

pv-qspinlock tries to preserve the fairness while allowing limited lock
stealing and explicitly managing which vcpus to wake.

While there's
   theory and to some extent practice on x86, it would be nice to
   validate the effects on POWER.

Right; so that will have to be from benchmarks which I cannot help you
with ;-)