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

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Date:	Mon, 6 Jun 2016 17:59:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	virtualization@...ts.linux-foundation.org, paulus@...ba.org,
	mpe@...erman.id.au, mingo@...hat.com, paulmck@...ux.vnet.ibm.com,
	waiman.long@....com
Subject: 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 ;-)