From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757050AbXKPIJU (ORCPT ); Fri, 16 Nov 2007 03:09:20 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751962AbXKPIJJ (ORCPT ); Fri, 16 Nov 2007 03:09:09 -0500 Received: from il.qumranet.com ([82.166.9.18]:46041 "EHLO il.qumranet.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751964AbXKPIJI (ORCPT ); Fri, 16 Nov 2007 03:09:08 -0500 Message-ID: <473D5016.4000105@qumranet.com> Date: Fri, 16 Nov 2007 10:08:54 +0200 From: Avi Kivity User-Agent: Thunderbird 2.0.0.6 (X11/20070926) MIME-Version: 1.0 To: Arjan van de Ven CC: Linux Kernel Mailing List , akpm@linux-foundation.org, mingo@elte.hu Subject: Re: x86: disable preemption in delay_tsc() References: <200711150400.lAF40lIr020160@hera.kernel.org> <20071115194116.12c7a0f6@laptopd505.fenrus.org> In-Reply-To: <20071115194116.12c7a0f6@laptopd505.fenrus.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.0rc1 (firebolt.argo.co.il [0.0.0.0]); Fri, 16 Nov 2007 10:08:55 +0200 (IST) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Arjan van de Ven wrote: > On Thu, 15 Nov 2007 04:00:47 GMT > Linux Kernel Mailing List wrote: > > >> Gitweb: >> http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=35d5d08a085c56f153458c3f5d8ce24123617faf >> Commit: 35d5d08a085c56f153458c3f5d8ce24123617faf Parent: >> 7eea436433b7b18045f272562e256976f593f7c0 Author: Andrew Morton >> AuthorDate: Wed Nov 14 17:00:41 2007 -0800 >> Committer: Linus Torvalds >> CommitDate: Wed Nov 14 18:45:44 2007 -0800 >> >> x86: disable preemption in delay_tsc() >> >> Marin Mitov points out that delay_tsc() can misbehave if it is >> preempted and rescheduled on a different CPU which has a skewed TSC. >> Fix it by disabling preemption. >> >> > > this worries me.. this appears to effectively disable preemption during > udelay() and mdelay() loops... which are very obvious latency inducers. > > Now you can argue that if you're preemptible you should have used > msleep() and co, and I'll totally buy that. > > > Maybe we should just check if we're still on the same cpu or something, > or have a cheap way to pin a process to a cpu.... but both are longer > term solutions. > > You can use preemption notifiers to get a callback when you are preempted. Not sure what you'd to with that callback, though. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.