Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754202AbdFWEgE (ORCPT ); Fri, 23 Jun 2017 00:36:04 -0400 Received: from mail-oi0-f65.google.com ([209.85.218.65]:35864 "EHLO mail-oi0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751168AbdFWEgB (ORCPT ); Fri, 23 Jun 2017 00:36:01 -0400 MIME-Version: 1.0 In-Reply-To: References: <1498130534-26568-1-git-send-email-root@ip-172-31-39-62.us-west-2.compute.internal> From: Wanpeng Li Date: Fri, 23 Jun 2017 12:35:59 +0800 Message-ID: Subject: Re: [PATCH 0/2] x86/idle: add halt poll support To: Yang Zhang 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 Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2276 Lines: 53 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. Regards, Wanpeng Li