From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753680AbYIZHvw (ORCPT ); Fri, 26 Sep 2008 03:51:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751496AbYIZHvn (ORCPT ); Fri, 26 Sep 2008 03:51:43 -0400 Received: from nebensachen.de ([195.34.83.29]:58676 "EHLO mail.nebensachen.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751486AbYIZHvm (ORCPT ); Fri, 26 Sep 2008 03:51:42 -0400 X-Hashcash: 1:20:080926:mingo@elte.hu::KEsbN1nnK34xOP7q:00000Rve X-Hashcash: 1:20:080926:tglx@linutronix.de::cWfsl48JTcr8H26H:000000000000000000000000000000000000000000052Lz X-Hashcash: 1:20:080926:linux-kernel@vger.kernel.org::ZcECN6ba9083U4m9:00000000000000000000000000000000014V1 From: Elias Oltmanns To: Ingo Molnar , Thomas Gleixner Cc: linux-kernel@vger.kernel.org Subject: udelay and timers Date: Fri, 26 Sep 2008 09:51:27 +0200 Message-ID: <87skrnz6o0.fsf@denkblock.local> User-Agent: Gnus/5.110007 (No Gnus v0.7) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi all, finally, I have managed to identify the cause of some odd symptoms on my system, but I need your help to understand what is really going on and what should be done to fix things. My problem is this: I have written a small test module which, in due course, does the following: ... mod_timer(&timera, jiffies + msecs_to_jiffies(10000)); mod_timer(&timerb, jiffies + HZ/50); ... The callbacks for those timers are defined as follows: void timera.function(unsigned long data) { udelay(120); mod_timer(&timera, jiffies + msecs_to_jiffies(10000)); } void timerb.function(unsigned long data) { ... mod_timer(&timerb, jiffies + HZ/50); } Now, I can observe that because of the call to udelay() in the callback of timera, timerb rather frequently fires *much* too early, i.e. after less than 1 msec rather than 20 msecs. This means that an udelay in a timer callback heavily affects the precision of other timers running at the time. The effect if particularly grave on a tickless system, but even when NO_HZ was not set, I have observed this behaviour. As I understand, udelay() is meant to be usable in softirq context. What can I do to find out what exactly causes the problem? Thank you very much for your assistance, Elias