From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1945957AbcHRMnh (ORCPT ); Thu, 18 Aug 2016 08:43:37 -0400 Received: from mail-wm0-f54.google.com ([74.125.82.54]:38399 "EHLO mail-wm0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752557AbcHRMnG (ORCPT ); Thu, 18 Aug 2016 08:43:06 -0400 Date: Thu, 18 Aug 2016 14:42:31 +0200 From: Frederic Weisbecker To: mingo@kernel.org, wanpeng.li@hotmail.com, linux-kernel@vger.kernel.org, peterz@infradead.org, efault@gmx.de, tglx@linutronix.de, rkrcmar@redhat.com, torvalds@linux-foundation.org, hpa@zytor.com, pbonzini@redhat.com, riel@redhat.com Cc: linux-tip-commits@vger.kernel.org Subject: Re: [tip:sched/core] sched/cputime: Resync steal time when guest & host lose sync Message-ID: <20160818124229.GC22490@lerouge> References: <1471399546-4069-1-git-send-email-wanpeng.li@hotmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 18, 2016 at 03:55:39AM -0700, tip-bot for Wanpeng Li wrote: > Commit-ID: 03cbc732639ddcad15218c4b2046d255851ff1e3 > Gitweb: http://git.kernel.org/tip/03cbc732639ddcad15218c4b2046d255851ff1e3 > Author: Wanpeng Li > AuthorDate: Wed, 17 Aug 2016 10:05:46 +0800 > Committer: Ingo Molnar > CommitDate: Thu, 18 Aug 2016 11:19:48 +0200 > > sched/cputime: Resync steal time when guest & host lose sync > > Commit: > > 57430218317e ("sched/cputime: Count actually elapsed irq & softirq time") > > ... fixed a bug but also triggered a regression: > > On an i5 laptop, 4 pCPUs, 4vCPUs for one full dynticks guest, there are four > CPU hog processes(for loop) running in the guest, I hot-unplug the pCPUs > on host one by one until there is only one left, then observe CPU utilization > via 'top' in the guest, it shows: > > 100% st for cpu0(housekeeping) > 75% st for other CPUs (nohz full mode) > > However, w/o this commit it shows the correct 75% for all four CPUs. > > When a guest is interrupted for a longer amount of time, missed clock ticks > are not redelivered later. Because of that, we should not limit the amount > of steal time accounted to the amount of time that the calling functions > think have passed. > > However, the interval returned by account_other_time() is NOT rounded down > to the nearest jiffy, while the base interval in get_vtime_delta() it is > subtracted from is, so the max cputime limit is required to avoid underflow. > > This patch fixes the regression by limiting the account_other_time() from > get_vtime_delta() to avoid underflow, and lets the other three call sites > (in account_other_time() and steal_account_process_time()) account however > much steal time the host told us elapsed. > > Suggested-by: Rik van Riel > Suggested-by: Paolo Bonzini > Signed-off-by: Wanpeng Li > Reviewed-by: Rik van Riel > Cc: Frederic Weisbecker ACK, thanks Wanpeng Li!