From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753541AbcFOU1K (ORCPT ); Wed, 15 Jun 2016 16:27:10 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:51624 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752958AbcFOU1E (ORCPT ); Wed, 15 Jun 2016 16:27:04 -0400 X-IBM-Helo: d01dlp01.pok.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Wed, 15 Jun 2016 13:26:59 -0700 From: "Paul E. McKenney" To: Thomas Gleixner 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 Reply-To: paulmck@linux.vnet.ibm.com References: <20160613070440.950649741@linutronix.de> <20160613161514.GA3923@linux.vnet.ibm.com> <20160615151518.GA11383@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16061520-0040-0000-0000-000000931BDF X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16061520-0041-0000-0000-0000046CF9BD Message-Id: <20160615202659.GS3923@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-06-15_09:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1604210000 definitions=main-1606150217 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 15, 2016 at 07:02:38PM +0200, Thomas Gleixner wrote: > 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? If it makes the code bigger, I would hope that they don't need it. > > So, just out of curiosity, does anyone still run -rt on single-CPU systems? > > Of course :) Hey, had to ask! ;-) Thanx, Paul