From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932676AbZHQSQO (ORCPT ); Mon, 17 Aug 2009 14:16:14 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757896AbZHQSQN (ORCPT ); Mon, 17 Aug 2009 14:16:13 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:38291 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756844AbZHQSQN (ORCPT ); Mon, 17 Aug 2009 14:16:13 -0400 Subject: Re: clocksource changes in 2.6.31 - possible regression From: john stultz To: Stephen Hemminger Cc: Andrew Morton , Thomas Gleixner , linux-kernel@vger.kernel.org In-Reply-To: <20090817110127.40ee5c29@nehalam> References: <20090817090319.20979986@nehalam> <1250531337.26171.12.camel@work-vm> <20090817110127.40ee5c29@nehalam> Content-Type: text/plain Date: Mon, 17 Aug 2009 11:15:54 -0700 Message-Id: <1250532954.26171.35.camel@work-vm> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2009-08-17 at 11:01 -0700, Stephen Hemminger wrote: > On Mon, 17 Aug 2009 10:48:57 -0700 > john stultz wrote: > > > On Mon, 2009-08-17 at 09:03 -0700, Stephen Hemminger wrote: > > > The following commit causes a change for kernels built with HRT but > > > not actually using HRT. I typically use the generic kernel we ship > > > on test machines, and that kernel has NOHZ and HRT (for power savings/virt > > > and HRT for QoS), but I want to be able to enable TSC as a clock source > > > when doing performance tests with pktgen. > > > > > > The machine in question is a several year old Opteron box, that > > > normally reports clocksources: acpi_pm jiffies tsc > > > but now with 2.6.31-rc6, it only has acpi_pm. > > > > I might need to review the patch again, but I believe we just don't > > allow you to switch to non HRT compatible clocksources (like jiffies) if > > we're already in HRT mode (and thus would hang when switched). > > > > > > The behavior you describe where you can't switch to the TSC, may be due > > to the TSC disqualification code marking it as non HRT compatible > > (again, I need to double check). While I'm not sure that's really > > correct, as the TSC is fine for HRT, in this case on your box, the TSC > > has been marked as unstable (likely due to being unsynced on old AMD SMP > > systems). There is a real chance that the timekeeping code on your > > system could see the TSC go backwards, calculate a negative time > > interval, and then end up hanging. > > > > TSC was alway stable on this box, and worked fine. There was no > message in log about TSC instability. The change was bisected > down to that one commit. But just to clarify, the TSC was never selected as the default clocksource on the box either, right? thanks -john