From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932131AbcFGKjx (ORCPT ); Tue, 7 Jun 2016 06:39:53 -0400 Received: from mx1.redhat.com ([209.132.183.28]:45474 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753378AbcFGKjv (ORCPT ); Tue, 7 Jun 2016 06:39:51 -0400 Subject: Re: [PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug To: Wanpeng Li , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" References: Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Ingo Molnar , "Peter Zijlstra (Intel)" , Rik van Riel , Thomas Gleixner , Frederic Weisbecker , John Stultz From: Paolo Bonzini Message-ID: Date: Tue, 7 Jun 2016 12:39:42 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Tue, 07 Jun 2016 10:39:50 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/06/2016 09:59, Wanpeng Li wrote: > From: Wanpeng Li > > I observed that sometimes st is 100% instantaneous, then idle is 100% > even if there is a cpu hog on the guest cpu after the cpu hotplug comes > back(N.B. this can not always be readily reproduced). I add trace to > capture it as below: > > cpuhp/1-12 [001] d.h1 167.461657: account_process_tick: steal = 1291385514, prev_steal_time = 0 > cpuhp/1-12 [001] d.h1 167.461659: account_process_tick: steal_jiffies = 1291 > -0 [001] d.h1 167.462663: account_process_tick: steal = 18732255, prev_steal_time = 1291000000 > -0 [001] d.h1 167.462664: account_process_tick: steal_jiffies = 18446744072437 > > The steal clock warp and then steal_jiffies underflow. > > Rik also pointed out to me: > > | I have seen stuff like that with live migration too, in the past > > The root cause of steal clock warp during hotplug is kvm_steal_time reset > to 0 after cpu hotplug comes back which should be preexiting guest value. > This patch fix it by don't reset kvm_steal_time during guest cpu hotplug. Improved commit message: Sometimes, after CPU hotplug you can observe a spike in stolen time (100%) followed by the CPU being marked as 100% idle when it's actually busy with a CPU hog task. The trace looks like the following: cpuhp/1-12 [001] d.h1 167.461657: account_process_tick: steal = 1291385514, prev_steal_time = 0 cpuhp/1-12 [001] d.h1 167.461659: account_process_tick: steal_jiffies = 1291 -0 [001] d.h1 167.462663: account_process_tick: steal = 18732255, prev_steal_time = 1291000000 -0 [001] d.h1 167.462664: account_process_tick: steal_jiffies = 18446744072437 The sudden decrease of "steal" causes steal_jiffies to underflow. The root cause is kvm_steal_time being reset to 0 after hot-plugging back in a CPU. Instead, the preexisting value can be used, which is what the core scheduler code expects. John Stultz also reported a similar issue after guest S3. ------ Please also add Cc: John Stultz Thanks, Paolo