From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753346Ab2EBFkE (ORCPT ); Wed, 2 May 2012 01:40:04 -0400 Received: from e28smtp06.in.ibm.com ([122.248.162.6]:49061 "EHLO e28smtp06.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751524Ab2EBFkC (ORCPT ); Wed, 2 May 2012 01:40:02 -0400 Message-ID: <4FA0C8A7.9000001@linux.vnet.ibm.com> Date: Wed, 02 May 2012 13:39:51 +0800 From: Xiao Guangrong User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.1) Gecko/20120216 Thunderbird/10.0.1 MIME-Version: 1.0 To: Takuya Yoshikawa CC: Marcelo Tosatti , Avi Kivity , LKML , KVM Subject: Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault References: <4F9776D2.7020506@linux.vnet.ibm.com> <4F9777A4.208@linux.vnet.ibm.com> <20120426234535.GA5057@amt.cnet> <4F9A3445.2060305@linux.vnet.ibm.com> <20120427145213.GB28796@amt.cnet> <20120429175004.b54d8c095a60d98c8cdbc942@gmail.com> In-Reply-To: <20120429175004.b54d8c095a60d98c8cdbc942@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit x-cbid: 12050205-9574-0000-0000-0000027186C5 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/29/2012 04:50 PM, Takuya Yoshikawa wrote: > On Fri, 27 Apr 2012 11:52:13 -0300 > Marcelo Tosatti wrote: > >> Yes but the objective you are aiming for is to read and write sptes >> without mmu_lock. That is, i am not talking about this patch. >> Please read carefully the two examples i gave (separated by "example)"). > > The real objective is not still clear. > > The ~10% improvement reported before was on macro benchmarks during live > migration. At least, that optimization was the initial objective. > > But at some point, the objective suddenly changed to "lock-less" without > understanding what introduced the original improvement. > > Was the problem really mmu_lock contention? > Takuya, i am so tired to argue the advantage of lockless write-protect and lockless O(1) dirty-log again and again. > If the path being introduced by this patch is really fast, isn't it > possible to achieve the same improvement still using mmu_lock? > > > Note: During live migration, the fact that the guest gets faulted is > itself a limitation. We could easily see noticeable slowdown of a > program even if it runs only between two GET_DIRTY_LOGs. > Obviously no. It depends on what the guest is doing, from my autotest test, it very easily to see that, the huge improvement is on bench-migration not pure-migration. > >> The rules for code under mmu_lock should be: >> >> 1) Spte updates under mmu lock must always be atomic and >> with locked instructions. >> 2) Spte values must be read once, and appropriate action >> must be taken when writing them back in case their value >> has changed (remote TLB flush might be required). > > Although I am not certain about what will be really needed in the > final form, if this kind of maybe-needed-overhead is going to be > added little by little, I worry about possible regression. Well, will you suggest Linus to reject all patches and stop all discussion for the "possible regression" reason?