From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752767AbbGNSrX (ORCPT ); Tue, 14 Jul 2015 14:47:23 -0400 Received: from g4t3425.houston.hp.com ([15.201.208.53]:48495 "EHLO g4t3425.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752125AbbGNSrW (ORCPT ); Tue, 14 Jul 2015 14:47:22 -0400 Message-ID: <55A55934.7020606@hp.com> Date: Tue, 14 Jul 2015 14:47:16 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Peter Zijlstra CC: Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, Scott J Norton , Douglas Hatch Subject: Re: [PATCH 6/7] locking/qspinlock: A fairer queued unfair lock References: <1436647018-49734-1-git-send-email-Waiman.Long@hp.com> <1436647018-49734-7-git-send-email-Waiman.Long@hp.com> <20150712082105.GQ19282@twins.programming.kicks-ass.net> In-Reply-To: <20150712082105.GQ19282@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/12/2015 04:21 AM, Peter Zijlstra wrote: > On Sat, Jul 11, 2015 at 04:36:57PM -0400, Waiman Long wrote: >> For a virtual guest with the qspinlock patch, a simple unfair byte lock >> will be used if PV spinlock is not configured in or the hypervisor >> isn't either KVM or Xen. > Why do we care about this case enough to add over 300 lines of code? From my testing, I found the queued unfair lock to be superior to both the byte lock or the PV qspinlock when the VM is overcommitted. My current opinion is to use PV qspinlock for VMs that are not likely to run into the overcommited problem. For other VMs that are overcommitted, it will be better to use the queued unfair lock. However, this is a choice that the system administrators have to made. That is also the reason why I sent out another patch to add a KVM command line option to disable PV spinlock like what Xen already has. In this way, depending on how the kernel is booted, we can choose either PV qspinlock or queued unfair lock. Cheers, Longman