Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751246AbVLPCBL (ORCPT ); Thu, 15 Dec 2005 21:01:11 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751284AbVLPCBL (ORCPT ); Thu, 15 Dec 2005 21:01:11 -0500 Received: from viper.oldcity.dca.net ([216.158.38.4]:5806 "HELO viper.oldcity.dca.net") by vger.kernel.org with SMTP id S1751246AbVLPCBK (ORCPT ); Thu, 15 Dec 2005 21:01:10 -0500 Subject: Re: [PATCH] ia64: disable preemption in udelay() From: Lee Revell To: Zwane Mwaikambo Cc: "Luck, Tony" , hawkes@sgi.com, Tony Luck , Andrew Morton , linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, Jack Steiner , Keith Owens , Dimitri Sivanich In-Reply-To: References: <20051214232526.9039.15753.sendpatchset@tomahawk.engr.sgi.com> <20051215225040.GA9086@agluck-lia64.sc.intel.com> Content-Type: text/plain Date: Thu, 15 Dec 2005 21:03:55 -0500 Message-Id: <1134698636.12086.222.camel@mindpipe> Mime-Version: 1.0 X-Mailer: Evolution 2.4.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1852 Lines: 41 On Thu, 2005-12-15 at 17:52 -0800, Zwane Mwaikambo wrote: > On Thu, 15 Dec 2005, Luck, Tony wrote: > > > On Wed, Dec 14, 2005 at 03:25:26PM -0800, hawkes@sgi.com wrote: > > > Sending this to a wider audience: > > > > > > The udelay() inline for ia64 uses the ITC. If CONFIG_PREEMPT is enabled > > > and the platform has unsynchronized ITCs and the calling task migrates > > > to another CPU while doing the udelay loop, then the effective delay may > > > be too short or very, very long. > > > > > > The most simple fix is to disable preemption around the udelay looping. > > > The downside is that this inhibits realtime preemption for cases of long > > > udelays. One datapoint: an SGI realtime engineer reports that if > > > CONFIG_PREEMPT is turned off, that no significant holdoffs are > > > are attributed to udelay(). > > > > > > I am reluctant to propose a much more complicated patch (that disables > > > preemption only for "short" delays, and uses the global RTC as the time > > > base for longer, preemptible delays) unless this patch introduces > > > significant and unacceptable preemption delays. > > > > Stuck between a rock and the proverbial hard place. > > > > I think that the more complex patch is needed though. If some crazy > > driver has a pre-emptible udelay(10000), then you really don't want > > to spin for that long without allowing preemption. > > If it's a preemptible sleep period it should just use msleep. There are 10 drivers that udelay(10000) or more and a TON that udelay(1000). Turning those all into 1ms+ non preemptible sections will be very bad. Lee - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/