From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759036Ab3KMLjo (ORCPT ); Wed, 13 Nov 2013 06:39:44 -0500 Received: from mail-qc0-f170.google.com ([209.85.216.170]:55016 "EHLO mail-qc0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756729Ab3KMLjm (ORCPT ); Wed, 13 Nov 2013 06:39:42 -0500 Date: Wed, 13 Nov 2013 06:39:27 -0500 From: Matthew Whitehead To: john.stultz@linaro.org, tglx@linutronix.de, linux-kernel@vger.kernel.org Cc: rostedt@goodmis.org, mwhitehe@redhat.com Subject: nohz problem with idle time on old hardware Message-ID: <20131113113927.GA13875@mwhitehe.csb> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.20 (2009-12-10) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I was testing the 3.12 kernel on some _old_ hardware and I uncovered a bug. It arises when nohz=on and goes away with nohz=off. On a crusty dual Pentium-1 system that is completely idle, the sar utility reports 0% idle time on cpu0 and 100% idle on cpu1. Cpu0 _should_ also be reporting 100% idle, but instead it reports around 75% system time and 25% user time. The problem was diagnosed by Steve Rostedt with help from John Stultz. The old system declares the dual TSCs unstable, and backs down to a timesource of refined-jiffies. Apparently refined-jiffies and jiffies are not a usable timesourcefor nohz, but we don't check for that case because most modern systems have several reliable hardware timesources. John suggested that we turn off nohz unless a usable hardware timesource is present. - Matthew Whitehead