From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933498AbbELNNA (ORCPT ); Tue, 12 May 2015 09:13:00 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:55451 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932657AbbELNM5 (ORCPT ); Tue, 12 May 2015 09:12:57 -0400 Date: Tue, 12 May 2015 06:12:50 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Chris Metcalf , Gilad Ben Yossef , Steven Rostedt , Ingo Molnar , Andrew Morton , Rik van Riel , Tejun Heo , Frederic Weisbecker , Thomas Gleixner , Christoph Lameter , "Srivatsa S. Bhat" , linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/6] nohz: dataplane: allow tick to be fully disabled for dataplane Message-ID: <20150512131250.GJ6776@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1431107927-13998-1-git-send-email-cmetcalf@ezchip.com> <1431107927-13998-3-git-send-email-cmetcalf@ezchip.com> <20150512092607.GE21418@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150512092607.GE21418@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15051213-0017-0000-0000-00000AB8F6E4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 12, 2015 at 11:26:07AM +0200, Peter Zijlstra wrote: > On Fri, May 08, 2015 at 01:58:43PM -0400, Chris Metcalf wrote: > > While the current fallback to 1-second tick is still helpful for > > maintaining completely correct kernel semantics, processes using > > prctl(PR_SET_DATAPLANE) semantics place a higher priority on running > > completely tickless, so don't bound the time_delta for such processes. > > > > This was previously discussed in > > > > https://lkml.org/lkml/2014/10/31/364 > > > > and Thomas Gleixner observed that vruntime, load balancing data, > > load accounting, and other things might be impacted. Frederic > > Weisbecker similarly observed that allowing the tick to be indefinitely > > deferred just meant that no one would ever fix the underlying bugs. > > However it's at least true that the mode proposed in this patch can > > only be enabled on an isolcpus core, which may limit how important > > it is to maintain scheduler data correctly, for example. > > So how is making this available going to help people fix the actual > problem? It will at least provide an environment where adding more of this problem might get punished. This would be an improvement over what we have today, namely that the 1HZ fallback timer silently forgives adding more problems of this sort. Thanx, Paul > There is nothing fundamentally impossible about fixing this proper, its > just a lot of hard work. > > NAK on this, do it right. >