From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: Preemptable Ticket Spinlock Date: Mon, 22 Apr 2013 21:49:07 +0200 Message-ID: <1366660147.6454.6.camel@laptop> References: <51745650.9050204@redhat.com> <1366631460.4443.3.camel@laptop> <51753289.70406@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: Jiannan Ouyang , LKML , Raghavendra K T , Avi Kivity , Gleb Natapov , Ingo Molnar , Marcelo Tosatti , Srikar , "H. Peter Anvin" , "Nikunj A. Dadhania" , KVM , Thomas Gleixner , Chegu Vinod , "Andrew M. Theurer" , Srivatsa Vaddagiri , Andrew Jones , Karen Noel To: Rik van Riel Return-path: Received: from 173-166-109-252-newengland.hfc.comcastbusiness.net ([173.166.109.252]:40179 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754927Ab3DVTtZ (ORCPT ); Mon, 22 Apr 2013 15:49:25 -0400 Received: from dhcp-089-099-019-018.chello.nl ([89.99.19.18] helo=dyad.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.80.1 #2 (Red Hat Linux)) id 1UUMjr-0003av-RL for kvm@vger.kernel.org; Mon, 22 Apr 2013 19:49:24 +0000 In-Reply-To: <51753289.70406@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, 2013-04-22 at 08:52 -0400, Rik van Riel wrote: > On 04/22/2013 07:51 AM, Peter Zijlstra wrote: > > On Sun, 2013-04-21 at 17:12 -0400, Rik van Riel wrote: > >> > >> If we always incremented the ticket number by 2 (instead of 1), then > >> we could use the lower bit of the ticket number as the spinlock. > > > > ISTR that paravirt ticket locks already do that and use the lsb to > > indicate the unlock needs to perform wakeups. > > > > Also, since all of this is virt nonsense, shouldn't it live in the > > paravirt ticket lock code and leave the native code as is? > > Sure, but that is still no reason not to have the virt > implementation be as fast as possible, and share the same > data type as the non-virt implementation. It has to share the same data-type.. > Also, is it guaranteed that the native spin_lock code has > not been called yet before we switch over to the paravirt > functions? > > If the native spin_lock code has been called already at > that time, the native code would still need to be modified > to increment the ticket number by 2, so we end up with a > compatible value in each spin lock's .tickets field, and > prevent a deadlock after we switch over to the paravirt > variant. I thought the stuff already made it upstream, but apparently not; the lastest posting I'm aware of is here: https://lkml.org/lkml/2012/5/2/105 That stuff changes the normal ticket increment as well..