From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752356AbYLRLQl (ORCPT ); Thu, 18 Dec 2008 06:16:41 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751554AbYLRLQ3 (ORCPT ); Thu, 18 Dec 2008 06:16:29 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:46015 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751341AbYLRLQ2 (ORCPT ); Thu, 18 Dec 2008 06:16:28 -0500 Date: Thu, 18 Dec 2008 12:16:12 +0100 From: Ingo Molnar To: =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker Cc: Thomas Gleixner , Steven Rostedt , Andrew Morton , Linux Kernel Subject: Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop Message-ID: <20081218111612.GD14332@elte.hu> References: <4949A2CC.6040209@gmail.com> <20081218103459.GD10513@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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 * Frédéric Weisbecker wrote: > 2008/12/18 Ingo Molnar : > > > > * 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. > > > It doesn' speed up the system actually. That's not the goal of this > patch. i meant that abstractly. The "solution" here is that your patch turns off the function-graph-tracer. The practical effect of that is: the system gets much faster at processing hrtimer IRQs and effectively "speeds up". ok? Ingo