From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752459AbYLRKf2 (ORCPT ); Thu, 18 Dec 2008 05:35:28 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750776AbYLRKfM (ORCPT ); Thu, 18 Dec 2008 05:35:12 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:51942 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751015AbYLRKfK (ORCPT ); Thu, 18 Dec 2008 05:35:10 -0500 Date: Thu, 18 Dec 2008 11:34:59 +0100 From: Ingo Molnar To: Frederic Weisbecker , Thomas Gleixner Cc: Steven Rostedt , Andrew Morton , Linux Kernel Subject: Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop Message-ID: <20081218103459.GD10513@elte.hu> References: <4949A2CC.6040209@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4949A2CC.6040209@gmail.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * 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? Ingo