From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753205AbYLRKss (ORCPT ); Thu, 18 Dec 2008 05:48:48 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751300AbYLRKsi (ORCPT ); Thu, 18 Dec 2008 05:48:38 -0500 Received: from www.tglx.de ([62.245.132.106]:41205 "EHLO www.tglx.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751026AbYLRKsh (ORCPT ); Thu, 18 Dec 2008 05:48:37 -0500 Date: Thu, 18 Dec 2008 11:48:13 +0100 (CET) From: Thomas Gleixner To: Ingo Molnar cc: Frederic Weisbecker , Steven Rostedt , Andrew Morton , Linux Kernel Subject: Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop In-Reply-To: <20081218103459.GD10513@elte.hu> Message-ID: References: <4949A2CC.6040209@gmail.com> <20081218103459.GD10513@elte.hu> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) 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 On Thu, 18 Dec 2008, Ingo Molnar wrote: > * Frederic Weisbecker wrote: > > > Impact: fix a system hang on slow systems > > > > While testing the function graph tracer on VirtualBox, I had a system hang > > immediatly after enabling the tracer. > > > > If hrtimer is enabled on kernel, a slow system can spend too much time > > during tracing the hrtimer_interrupt which will do eternal loops, > > assuming it always have to retry its process because too much time > > elapsed during its time update. Now we provide a feature which lurks at > > the number of retries on hrtimer_interrupt. After 10 retries, the > > function graph tracer will definetly stop its tracing. > > hm, i dont really like this solution - it just works around the problem by > 'speeding up' the system. If we have a _real_ slow system, there's no such > way for us to speed it up. > > Thomas, what do you think - would you expect this lockup to happen on > really slow systems? If yes, is there a way we could avoid it from > happening - by driving some sort of 'mandatory interval', that is doubled > in size every time we detect such a bad hrtimer loop? In reality I have not seen such a problem yet, even on an old real slow P1 which I tricked to do highres, but of course if we add such time consuming debugs and make it slow enough the system will spend all the time running the tick timer :) We should at least warn once about such a loop. I'm not sure about the mandatory interval though: Try the same test with HZ=1000 periodic mode (HIGHRES/NOHZ=off) and I bet you see the same problem, just not in hrtimer_interrupt(). Thanks, tglx