Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751710AbeAPWvd (ORCPT + 1 other); Tue, 16 Jan 2018 17:51:33 -0500 Received: from mail.kernel.org ([198.145.29.99]:33050 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750892AbeAPWvc (ORCPT ); Tue, 16 Jan 2018 17:51:32 -0500 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7DA7220C0F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=frederic@kernel.org Date: Tue, 16 Jan 2018 23:51:29 +0100 From: Frederic Weisbecker To: Luiz Capitulino Cc: Ingo Molnar , LKML , Peter Zijlstra , Chris Metcalf , Thomas Gleixner , Christoph Lameter , "Paul E . McKenney" , Wanpeng Li , Mike Galbraith , Rik van Riel Subject: Re: [GIT PULL] isolation: 1Hz residual tick offloading v3 Message-ID: <20180116225126.GA32665@lerouge> References: <1515039937-367-1-git-send-email-frederic@kernel.org> <20180112141813.32dcc84d@redhat.com> <20180116154055.GA27042@lerouge> <20180116115211.7fd55c9a@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180116115211.7fd55c9a@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On Tue, Jan 16, 2018 at 11:52:11AM -0500, Luiz Capitulino wrote: > On Tue, 16 Jan 2018 16:41:00 +0100 > Frederic Weisbecker wrote: > > So isolcpus= is now the place where we control the isolation features > > and nohz is one of them. > > That's the part I'm not very sure about. We've been advising users to > move away from isolcpus= when possible, but this very wanted nohz_offload > feature will force everyone back to using isolcpus= again. Note "isolcpus=nohz" only implies nohz. You need to add "domain" to get the behaviour that you've been advising users against. We are simply reusing a kernel parameter that was abandoned to now control the isolation features that were disorganized and opaque behind nohz. > > I have the impression this series is trying to solve two problems: > > 1. How (and where) we control the various isolation features in the > kernel No, that has already been done in the previous merge window. We have a dedicated isolation subsystem now (kernel/sched/isolation.c) and an interface to control all these isolation features that were abusively implied by nohz. The initial plan was to introduce "cpu_isolation=" but it looked too much like "isolcpus=". Then in fact, why not using "isolcpus=" and give it a second life. And there we are. In the end the goal is to propagate what is passed to "isolcpus=" to cpusets. > > 2. Where we add the control for the tick offload feature > > I think item 1 is too complex to solve right now. IMHO, this series > should focus on item 2. And regarding item 2, I think we have two > choices to make: > > 1. Make tick offload a first class citizen by making it default to > nohz_full=. If there are regressions, we handle them That's a possible way to go. > > 2. Add a new option to nohz_full=, like nohz_full=tick_offload > > As an avid user of nohz_full I'm dying to see option 1 happening, > but I'm not totally sure what the consequences can be. "nohz_full=" parameter has been badly designed as it implies much more than just full dynticks. So I'm not really looking forward to expanding it. > Another idea is to add CONFIG_NOHZ_TICK_OFFLOAD as an experimental > feature. I fear it's way too distro-unfriendly. They will want to have it as a capability without necessarily running it. Just like they do with CONFIG_NO_HZ_FULL. > > > The complain about isolcpus is the immutable result. I'm thinking about > > making it modifiable to cpuset but I only see two possible solutions: > > > > - Make the root cpuset modifiable > > - Create a directory called "isolcpus" visible on the first cpuset mount > > and move all processes there. > > So, if we move the control of the tick offload to nohz_full= itself, > we can completely ditch any isolcpus= change in this series. > > I think this should give you a great relief :) Not at all :) What would be a great relief to me is that we can finally propagate isolcpus= to cpusets so that we can continue to expand it without a second thought.