From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932428Ab0FDOZv (ORCPT ); Fri, 4 Jun 2010 10:25:51 -0400 Received: from ksp.mff.cuni.cz ([195.113.26.206]:39948 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932293Ab0FDOZu (ORCPT ); Fri, 4 Jun 2010 10:25:50 -0400 Date: Fri, 4 Jun 2010 16:24:39 +0200 From: Pavel Machek To: Andi Kleen Cc: Dan Magenheimer , Arjan van de Ven , Venkatesh Pallipadi , Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , chris.mason@oracle.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] x86: Export tsc related information in sysfs Message-ID: <20100604142439.GC1760@ucw.cz> References: <1273887635-27610-1-git-send-email-venki@google.com> <87tyq9mqrz.fsf@basil.nowhere.org> <20100517102635.GB20761@basil.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100517102635.GB20761@basil.fritz.box> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi! > > Yes, understood. But the kernel doesn't expose a "gettimeofday > > performance sucks" flag either. If it did (or in the case of > > the patch, if tsc_reliable is zero) the application could at least > > choose to turn off the 10000-100000 timestamps/second and log > > a message saying "you are running on old hardware so you get > > fewer features". > > I don't think anyone would object to exporting such a flag if > it's cleanly designed. > > Getting the semantics right for that might be somewhat tricky > though. How is "slow" defined? Well... if you want to know how fast gettimeofday is, perhaps doing gettimeofday(); gettimeofday(); is good enough? If not, perhaps you can export 'how many clocks is gettimeofday expected to take' variable somewhere, but... Pavel > > A CPU-hotplugable system is a good example of a case where > > the kernel should expose that tsc_reliable is 0. (I've heard > > That would mean that a large class of systems which > are always hotplug capable (even if it's not used) > would never get fast TSC time. > > Wasn't the goal here to be faster? > > > anecdotally that CPU hotplug into a QPI or Hypertransport system > > will have some other interesting challenges, so may require some > > special kernel parameters anyway.) Even if tsc_reliable were > > only enabled if a "no-cpu_hotplug" kernel parameter is set, > > that is still useful. And with cores-per-socket (and even > > nodes-per-socket) going up seemingly every day, multi-socket > > systems will likely be an ever smaller percentage of new > > systems. > > Still the people running them will expect as good performance > as possible. > > -Andi > -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html