From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750994AbZHSBFm (ORCPT ); Tue, 18 Aug 2009 21:05:42 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750800AbZHSBFl (ORCPT ); Tue, 18 Aug 2009 21:05:41 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:60899 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750769AbZHSBFk (ORCPT ); Tue, 18 Aug 2009 21:05:40 -0400 Subject: Re: [PATCH] make tsc=reliable override boot time stability checks From: john stultz To: akataria@vmware.com Cc: Stephen Hemminger , Andrew Morton , Thomas Gleixner , "linux-kernel@vger.kernel.org" In-Reply-To: <1250559540.32479.45.camel@ank32.eng.vmware.com> References: <20090817090319.20979986@nehalam> <1250531337.26171.12.camel@work-vm> <20090817110127.40ee5c29@nehalam> <1250532954.26171.35.camel@work-vm> <20090817112704.2b4b2987@nehalam> <1250543459.7212.41.camel@localhost.localdomain> <1250545077.7212.49.camel@localhost.localdomain> <20090817144546.7f1d6572@nehalam> <1250547802.7212.82.camel@localhost.localdomain> <20090817160212.029b3e42@nehalam> <1250551074.7212.84.camel@localhost.localdomain> <20090817162729.451613b4@nehalam> <1250552447.7212.92.camel@localhost.localdomain> <1250559540.32479.45.camel@ank32.eng.vmware.com> Content-Type: text/plain Date: Tue, 18 Aug 2009 18:04:31 -0700 Message-Id: <1250643871.7002.24.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.24.3 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 18:39 -0700, Alok Kataria wrote: > Hi John, > > On Mon, 2009-08-17 at 16:40 -0700, john stultz wrote: > > On Mon, 2009-08-17 at 16:27 -0700, Stephen Hemminger wrote: > > > On Mon, 17 Aug 2009 16:17:54 -0700 > > > john stultz wrote: > > > > > > > On Mon, 2009-08-17 at 16:02 -0700, Stephen Hemminger wrote: > > > > > This adds tsc, but makes it first? it is reliable, but do I want > > > > > to make it most important? > > > > > > > > > > $ cat /sys/devices/system/clocksource/clocksource0/available_clocksource > > > > > tsc acpi_pm > > > > > > > > > > > > Well, if you're overriding the system saying that its safe, then sure, > > > > its better then anything else, why wouldn't we? > > > > > > > > > > That's acceptable, maybe add change to Documentation/kernel-parameters.txt > > > > > > tsc= Disable clocksource-must-verify flag for TSC. > > > Format: > > > [x86] reliable: mark tsc clocksource as reliable and > > > makes tsc the default clocksource; this > > > disables clocksource verification at runtime. > > > Used to enable high-resolution timer mode on older > > > hardware, and in virtualized environment. > > > > > > > Sounds good. Thanks so much for the bug report and testing! > > > > > > This patch makes the tsc=reliable option disable the boot time stability > > checks. Currently the option only disables the runtime watchdog checks. > > This change allows folks who want to override the boot time TSC > > stability checks and use the TSC when the system would otherwise > > disqualify it. > > > > There still are some situations that the TSC will be disqualified, such > > as cpufreq scaling. But these are situations where the box will hang if > > allowed. > > > > I had purposefully kept the tsc=reliable separate from the TSC > synchronous checks. > With this patch TSC is marked as usable though the hardware below > doesn't export a CONSTANT_TSC, it might not be a problem generally, but > since TSC has the highest rating, don't you think that timekeeping might > be wayward on such systems ? Oh yea, there's a risk of that, but we are telling the kernel to override its runtime checking of the clocksource, so it seems reasonable to also include the boot time checks. I worry otherwise the option becomes too subtle to be really useful to users. > Having said that, I don't think I have a particular problem with the > patch as far as we are explicitly mentioning the fact that TSC=reliable > means TSC is blindly trusted on this system, and time might be little > off on some systems. I think the explicit boot option, along with the kernel-parameters text makes it clear enough, but if you have a specific wording in mind that works better, please send a patch and I'll ack it. thanks -john