From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932595AbcFOREk (ORCPT ); Wed, 15 Jun 2016 13:04:40 -0400 Received: from www.linutronix.de ([62.245.132.108]:41081 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753541AbcFOREi (ORCPT ); Wed, 15 Jun 2016 13:04:38 -0400 Date: Wed, 15 Jun 2016 19:02:38 +0200 (CEST) From: Thomas Gleixner To: "Paul E. McKenney" cc: LKML , Ingo Molnar , Peter Zijlstra , Eric Dumazet , Frederic Weisbecker , Chris Mason , Arjan van de Ven , rt@linutronix.de Subject: Re: [patch 00/20] timer: Refactor the timer wheel In-Reply-To: <20160615151518.GA11383@linux.vnet.ibm.com> Message-ID: References: <20160613070440.950649741@linutronix.de> <20160613161514.GA3923@linux.vnet.ibm.com> <20160615151518.GA11383@linux.vnet.ibm.com> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 15 Jun 2016, Paul E. McKenney wrote: > On Mon, Jun 13, 2016 at 09:15:14AM -0700, Paul E. McKenney wrote: > > On Mon, Jun 13, 2016 at 08:40:50AM -0000, Thomas Gleixner wrote: > > > The current timer wheel has some drawbacks: > > > > > > 1) Cascading > > > > > > Cascading can be an unbound operation and is completely pointless in most > > > cases because the vast majority of the timer wheel timers are canceled or > > > rearmed before expiration. > > > > > > 2) No fast lookup of the next expiring timer > > > > > > In NOHZ scenarios the first timer soft interrupt after a long NOHZ period > > > must fast forward the base time to current jiffies. As we have no way to > > > find the next expiring timer fast, the code loops and increments the base > > > time by one and checks for expired timers in each step. I've observed loops > > > lasting 1 ms! > > > > > > There are some other issues caused by the above, but they are minor compare to > > > those. > > > > For SMP configurations, this passes light rcutorture testing. For UP > > builds, it complains about undefined symbols. Builds succeed with > > the following kneejerk patch. Am retesting rcutorture. > > And with the patch below, testing goes as well with your patch stack as > it does without it. So, with that patch (or equivalent): > > Tested-by: Paul E. McKenney > > There were some complaints about increasing the size of the tiny > configuration, FYI. I know. The extra storage space for the deferrable stuff makes it larger along with the extra code for avoiding all the crap which the current wheel suffers from :) Do the tiny people need NOHZ? > So, just out of curiosity, does anyone still run -rt on single-CPU systems? Of course :) Thanks, tglx