From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262439AbVHCUtZ (ORCPT ); Wed, 3 Aug 2005 16:49:25 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262448AbVHCUtZ (ORCPT ); Wed, 3 Aug 2005 16:49:25 -0400 Received: from omx1-ext.sgi.com ([192.48.179.11]:8880 "EHLO omx1.americas.sgi.com") by vger.kernel.org with ESMTP id S262439AbVHCUtQ (ORCPT ); Wed, 3 Aug 2005 16:49:16 -0400 Date: Wed, 3 Aug 2005 13:49:08 -0700 (PDT) From: Christoph Lameter To: Alex Williamson cc: tony.luck@intel.com, linux-kernel@vger.kernel.org, akpm@osdl.org Subject: Re: [PATCH] optimize writer path in time_interpolator_get_counter() In-Reply-To: <1123086733.5193.69.camel@tdi> Message-ID: References: <1122911571.5243.23.camel@tdi> <200508021837.j72Ib7T9020681@agluck-lia64.sc.intel.com> <1123080155.5193.15.camel@tdi> <1123086733.5193.69.camel@tdi> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 3 Aug 2005, Alex Williamson wrote: > Ok, I can see the scenario where that could produce jitter. However, > that implies than any exit through that path could produce jitter as it > is. For instance: Well what is the difference of this approach from booting with "nojitter"? The ITC offsets are fixed right? Just check that the ITC difference is less than what you want to risk and switch on "nojitter" during bootup if less than the limit. Same effect without changes to the critical timer code patchs. The main problem occurs as far as I know when execution continues on another processor during time measurement. In that case you may get an earlier time later because the ITC of the processor you migrated to is not there yet.... Similar issues may occur if time information is communicated between threads running on different processor. You have an awful nest of rats here so if you do this then please at least do a printk that warns people of what is going on.