From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754679Ab2ECAQG (ORCPT ); Wed, 2 May 2012 20:16:06 -0400 Received: from mail-pb0-f46.google.com ([209.85.160.46]:60834 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754309Ab2ECAQE (ORCPT ); Wed, 2 May 2012 20:16:04 -0400 Date: Thu, 3 May 2012 09:15:58 +0900 From: Takuya Yoshikawa To: Xiao Guangrong Cc: Marcelo Tosatti , Avi Kivity , LKML , KVM Subject: Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault Message-Id: <20120503091558.866e158916f0dd67daf5a9a2@gmail.com> In-Reply-To: <4FA0C8A7.9000001@linux.vnet.ibm.com> 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> <4FA0C8A7.9000001@linux.vnet.ibm.com> X-Mailer: Sylpheed 3.2.0beta3 (GTK+ 2.24.6; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 02 May 2012 13:39:51 +0800 Xiao Guangrong wrote: > > 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. You are missing my point. Please do not take my comments as an objection to your whole work: whey do you feel so? I thought that your new fast-page-fault path was fast and optimized the guest during dirty logging. So in this v4, you might get a similar result even before dropping mmu_lock, without 07/10?, if the problem Marcelo explained was not there. Of course there is a problem of mmu_lock contention. What I am suggesting is to split that problem and do measurement separately so that part of your work can be merged soon. Your guest size and workload was small to make get_dirty hold mmu_lock long time. If you want to appeal the real value of lock-less, you need to do another measurment. But this is your work and it's up to you. Although I was thinking to help your measurement, I cannot do that knowing the fact that you would not welcome my help. > > 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? My concern was for Marcelo's examples, not your current implementation. If you can show explicitely what will be needed in the final form, I do not have any concern. Sorry for disturbing. Thanks, Takuya