Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752064AbdGDCTt (ORCPT ); Mon, 3 Jul 2017 22:19:49 -0400 Received: from mail-oi0-f67.google.com ([209.85.218.67]:35978 "EHLO mail-oi0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750849AbdGDCTq (ORCPT ); Mon, 3 Jul 2017 22:19:46 -0400 Subject: Re: [PATCH 2/2] x86/idle: use dynamic halt poll To: Thomas Gleixner References: <1498130534-26568-1-git-send-email-root@ip-172-31-39-62.us-west-2.compute.internal> <1498130534-26568-3-git-send-email-root@ip-172-31-39-62.us-west-2.compute.internal> <4444ffc8-9e7b-5bd2-20da-af422fe834cc@redhat.com> <2245bef7-b668-9265-f3f8-3b63d71b1033@gmail.com> <7d085956-2573-212f-44f4-86104beba9bb@gmail.com> <05ec7efc-fb9c-ae24-5770-66fc472545a4@redhat.com> <20170627134043.GA1487@potion> <2771f905-d1b0-b118-9ae9-db5fb87f877c@redhat.com> <20170627142251.GB1487@potion> Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Paolo Bonzini , Wanpeng Li , Ingo Molnar , "H. Peter Anvin" , 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: <22bd985d-739e-0a6e-7b16-9f6e3cd39445@gmail.com> Date: Tue, 4 Jul 2017 10:19:28 +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=windows-1252; 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: 1954 Lines: 47 On 2017/7/3 18:06, Thomas Gleixner wrote: > On Mon, 3 Jul 2017, Yang Zhang wrote: >> The background is that we(Alibaba Cloud) do get more and more complaints from >> our customers in both KVM and Xen compare to bare-mental.After investigations, >> the root cause is known to us: big cost in message passing workload(David show >> it in KVM forum 2015) >> >> A typical message workload like below: >> vcpu 0 vcpu 1 >> 1. send ipi 2. doing hlt >> 3. go into idle 4. receive ipi and wake up from hlt >> 5. write APIC time twice 6. write APIC time twice to >> to stop sched timer reprogram sched timer >> 7. doing hlt 8. handle task and send ipi to >> vcpu 0 >> 9. same to 4. 10. same to 3 >> >> One transaction will introduce about 12 vmexits(2 hlt and 10 msr write). The >> cost of such vmexits will degrades performance severely. Linux kernel already >> provide idle=poll to mitigate the trend. But it only eliminates the IPI and >> hlt vmexit. It has nothing to do with start/stop sched timer. A compromise >> would be to turn off NOHZ kernel, but it is not the default config for new >> distributions. > > You still can turn if off on the kernel command line via nohz=off You are right. Senior users will turn off it manually. But it only solve the sched timer. They still have the IPI/hlt problem. Another point is we release the distribution image to customer without any extra configuration to avoid mismatch between VM and bare-metal. To change such configuration needs reboot, but some customer's business cannot be interrupted after they start the service(like online gaming). It would be better if we can provide the sysctl interface to allow run-time modification. By the way, idle=poll seems too heavy to use. > > Thanks, > > tglx > -- Yang Alibaba Cloud Computing