Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754585AbdFWGt2 (ORCPT ); Fri, 23 Jun 2017 02:49:28 -0400 Received: from mail-oi0-f67.google.com ([209.85.218.67]:34517 "EHLO mail-oi0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754334AbdFWGt0 (ORCPT ); Fri, 23 Jun 2017 02:49:26 -0400 Subject: Re: [PATCH 0/2] x86/idle: add halt poll support To: Wanpeng Li References: <1498130534-26568-1-git-send-email-root@ip-172-31-39-62.us-west-2.compute.internal> Cc: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Paolo Bonzini , the arch/x86 maintainers , Jonathan Corbet , tony.luck@intel.com, Borislav Petkov , Peter Zijlstra , mchehab@kernel.org, Andrew Morton , krzk@kernel.org, jpoimboe@redhat.com, Andy Lutomirski , Christian Borntraeger , Thomas Garnier , Robert Gerst , Mathias Krause , douly.fnst@cn.fujitsu.com, Nicolai Stange , Frederic Weisbecker , dvlasenk@redhat.com, Daniel Bristot de Oliveira , yamada.masahiro@socionext.com, mika.westerberg@linux.intel.com, Chen Yu , aaron.lu@intel.com, Steven Rostedt , Kyle Huey , Len Brown , Prarit Bhargava , hidehiro.kawai.ez@hitachi.com, fengtiantian@huawei.com, pmladek@suse.com, jeyu@redhat.com, Larry.Finger@lwfinger.net, zijun_hu@htc.com, luisbg@osg.samsung.com, johannes.berg@intel.com, niklas.soderlund+renesas@ragnatech.se, zlpnobody@gmail.com, Alexey Dobriyan , fgao@48lvckh6395k16k5.yundunddos.com, ebiederm@xmission.com, Subash Abhinov Kasiviswanathan , Arnd Bergmann , Matt Fleming , Mel Gorman , "linux-kernel@vger.kernel.org" , linux-doc@vger.kernel.org, linux-edac@vger.kernel.org, kvm From: Yang Zhang Message-ID: Date: Fri, 23 Jun 2017 14:49:04 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2887 Lines: 69 On 2017/6/23 12:35, Wanpeng Li wrote: > 2017-06-23 12:08 GMT+08:00 Yang Zhang : >> On 2017/6/22 19:50, Wanpeng Li wrote: >>> >>> 2017-06-22 19:22 GMT+08:00 root : >>>> >>>> From: Yang Zhang >>>> >>>> Some latency-intensive workload will see obviously performance >>>> drop when running inside VM. The main reason is that the overhead >>>> is amplified when running inside VM. The most cost i have seen is >>>> inside idle path. >>>> This patch introduces a new mechanism to poll for a while before >>>> entering idle state. If schedule is needed during poll, then we >>>> don't need to goes through the heavy overhead path. >>>> >>>> Here is the data i get when running benchmark contextswitch >>>> (https://github.com/tsuna/contextswitch) >>>> before patch: >>>> 2000000 process context switches in 4822613801ns (2411.3ns/ctxsw) >>>> after patch: >>>> 2000000 process context switches in 3584098241ns (1792.0ns/ctxsw) >>> >>> >>> If you test this after disabling the adaptive halt-polling in kvm? >>> What's the performance data of w/ this patchset and w/o the adaptive >>> halt-polling in kvm, and w/o this patchset and w/ the adaptive >>> halt-polling in kvm? In addition, both linux and windows guests can >>> get benefit as we have already done this in kvm. >> >> >> I will provide more data in next version. But it doesn't conflict with > > Another case I can think of is w/ both this patchset and the adaptive > halt-polling in kvm. > >> current halt polling inside kvm. This is just another enhancement. > > I didn't look close to the patchset, however, maybe there is another > poll in the kvm part again sometimes if you fails the poll in the > guest. In addition, the adaptive halt-polling in kvm has performance > penalty when the pCPU is heavily overcommitted though there is a > single_task_running() in my testing, it is hard to accurately aware > whether there are other tasks waiting on the pCPU in the guest which > will make it worser. Depending on vcpu_is_preempted() or steal time > maybe not accurately or directly. > > So I'm not sure how much sense it makes by adaptive halt-polling in > both guest and kvm. I prefer to just keep adaptive halt-polling in > kvm(then both linux/windows or other guests can get benefit) and avoid > to churn the core x86 path. This mechanism is not specific to KVM. It is a kernel feature which can benefit guest when running inside X86 virtualization environment. The guest includes KVM,Xen,VMWARE,Hyper-v. Administrator can control KVM to use adaptive halt poll but he cannot control the user to use halt polling inside guest. Lots of user set idle=poll inside guest to improve performance which occupy more CPU cycles. This mechanism is a enhancement to it not to KVM halt polling. > > Regards, > Wanpeng Li > -- Yang Alibaba Cloud Computing