From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3rNpvH133nzDq5c for ; Tue, 7 Jun 2016 08:16:26 +1000 (AEST) Message-ID: <1465249297.4274.72.camel@kernel.crashing.org> Subject: Re: [PATCH v5 1/6] qspinlock: powerpc support qspinlock From: Benjamin Herrenschmidt To: Peter Zijlstra Cc: xinhui , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, virtualization@lists.linux-foundation.org, paulus@samba.org, mpe@ellerman.id.au, mingo@redhat.com, paulmck@linux.vnet.ibm.com, waiman.long@hpe.com Date: Tue, 07 Jun 2016 07:41:37 +1000 In-Reply-To: <20160606155907.GH30909@twins.programming.kicks-ass.net> References: <1464859370-5162-1-git-send-email-xinhui.pan@linux.vnet.ibm.com> <1464859370-5162-3-git-send-email-xinhui.pan@linux.vnet.ibm.com> <1464917520.26773.11.camel@kernel.crashing.org> <1464917548.26773.12.camel@au1.ibm.com> <57510353.1020209@linux.vnet.ibm.com> <1464928427.26773.26.camel@kernel.crashing.org> <20160606155907.GH30909@twins.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Mon, 2016-06-06 at 17:59 +0200, Peter Zijlstra wrote: > 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. Thanks Peter. I think I understand the theory, but I'd like see it translate into real numbers. > 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. I think our HW has tweaks to avoid that from happening with the simple locks in the underlying ll/sc implementation. In any case, what I'm asking is actual tests to verify it works as expected for us. > pv-qspinlock tries to preserve the fairness while allowing limited lock > stealing and explicitly managing which vcpus to wake. Right. > > > > 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 ;-) Precisely :-) This is what I was asking for ;-) Cheers, Ben.