From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030215Ab2B1TJS (ORCPT ); Tue, 28 Feb 2012 14:09:18 -0500 Received: from smtp-outbound-2.vmware.com ([208.91.2.13]:32957 "EHLO smtp-outbound-2.vmware.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966016Ab2B1TJR (ORCPT ); Tue, 28 Feb 2012 14:09:17 -0500 Subject: Re: [PATCH] x86, tsc: Skip refined tsc calibration on systems with reliable TSC. From: Alok Kataria To: John Stultz Cc: Thomas Gleixner , the arch/x86 maintainers , dirk.brandewie@gmail.com, alan@linux.intel.com, stable@kernel.org, Dan Hecht , LKML In-Reply-To: <1329877195.10380.33.camel@ank32.eng.vmware.com> References: <1329876964.10380.28.camel@ank32.eng.vmware.com> <1329877195.10380.33.camel@ank32.eng.vmware.com> Content-Type: text/plain Date: Tue, 28 Feb 2012 11:09:16 -0800 Message-Id: <1330456156.4500.1.camel@ank32.eng.vmware.com> Mime-Version: 1.0 X-Mailer: Evolution 2.12.3 (2.12.3-8.el5_2.3) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Ping...any comments on this one. Thanks, Alok On Tue, 2012-02-21 at 18:19 -0800, Alok Kataria wrote: > [Oops forgot to copy LKML, now it is, sorry for the duplicates] > > While running the latest Linux as guest under VMware in highly > over-committed situations, we have seen cases when the refined TSC > algorithm fails to get a valid tsc_start value in > tsc_refine_calibration_work from multiple attempts. As a result the > kernel keeps on scheduling the tsc_irqwork task for later. Subsequently > after several attempts when it gets a valid start value it goes through > the refined calibration and either bails out or uses the new results. > Given that the kernel originally read the TSC frequency from the > platform, which is the best it can get, I don't think there is much > value in refining it. > > So IMO, for systems which get the TSC frequency from the platform we > should skip the refined tsc algorithm. > > We can use the TSC_RELIABLE cpu cap flag to detect this, right now it is > set only on VMware and for Moorestown Penwell both of which have there > own TSC calibration methods. > > Thanks, > Alok > > -- > > From: Alok N Kataria > > For systems which get the TSC frequency directly from the platform > and don't go through the native TSC calibration algorithm, we should > trust those values and not try to refine those. > > This patch is applicable for kernel from v2.6.38 to current mainline. > > Signed-off-by: Alok N Kataria > Cc: John Stultz > Cc: Dirk Brandewie > Cc: Alan Cox > Cc: stable@kernel.org > > > Index: linux-2.6/arch/x86/kernel/tsc.c > =================================================================== > --- linux-2.6.orig/arch/x86/kernel/tsc.c 2012-02-21 17:31:01.000000000 -0800 > +++ linux-2.6/arch/x86/kernel/tsc.c 2012-02-21 17:39:05.000000000 -0800 > @@ -874,6 +874,13 @@ static void tsc_refine_calibration_work( > goto out; > > /* > + * Trust the results of the earlier calibration on systems > + * exporting a reliable TSC. > + */ > + if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) > + goto out; > + > + /* > * Since the work is started early in boot, we may be > * delayed the first time we expire. So set the workqueue > * again once we know timers are working. >