From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753011AbcFFP7W (ORCPT ); Mon, 6 Jun 2016 11:59:22 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:43526 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752973AbcFFP7S (ORCPT ); Mon, 6 Jun 2016 11:59:18 -0400 Date: Mon, 6 Jun 2016 17:59:07 +0200 From: Peter Zijlstra To: Benjamin Herrenschmidt 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 Subject: Re: [PATCH v5 1/6] qspinlock: powerpc support qspinlock Message-ID: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1464928427.26773.26.camel@kernel.crashing.org> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 ;-)