Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752316AbbG3Oq1 (ORCPT ); Thu, 30 Jul 2015 10:46:27 -0400 Received: from mail-wi0-f181.google.com ([209.85.212.181]:36726 "EHLO mail-wi0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750868AbbG3OqZ (ORCPT ); Thu, 30 Jul 2015 10:46:25 -0400 Date: Thu, 30 Jul 2015 16:46:22 +0200 From: Frederic Weisbecker To: Luiz Capitulino Cc: Chris Metcalf , LKML , Peter Zijlstra , Thomas Gleixner , Preeti U Murthy , Christoph Lameter , Ingo Molnar , Viresh Kumar , Rik van Riel Subject: Re: [PATCH 08/10] posix-cpu-timers: Migrate to use new tick dependency mask model Message-ID: <20150730144620.GA21890@lerouge> References: <1437669735-8786-1-git-send-email-fweisbec@gmail.com> <1437669735-8786-9-git-send-email-fweisbec@gmail.com> <55B26E74.5040803@ezchip.com> <20150729132343.GC11554@lerouge> <55B90C40.5090000@ezchip.com> <20150730004444.GA14744@lerouge> <20150730103147.116f98ba@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150730103147.116f98ba@redhat.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2112 Lines: 43 On Thu, Jul 30, 2015 at 10:31:47AM -0400, Luiz Capitulino wrote: > On Thu, 30 Jul 2015 02:44:45 +0200 > Frederic Weisbecker wrote: > > On Wed, Jul 29, 2015 at 01:24:16PM -0400, Chris Metcalf wrote: > > > I really worry about this! The vision EZchip offers our customers is > > > that they can run whatever they want on the slow path housekeeping > > > cores, i.e. random control-plane code. Then, on the fast-path cores, > > > they run their nohz_full stuff without interruption. Often they don't > > > even know what the hell is running on their control plane cores - SNMP > > > or random third-party crap or god knows what. And there is a decent > > > likelihood that some posix cpu timer code might sneak in. > > I share this thinking. We do the exactly same thing for KVM-RT and I > wouldn't be surprised at all if a posix timer pops up in the > housekeeping CPUs. Ok you guys convinced me, I'll reiterate with a per-process mask. > > > I see. But note that installing a posix cpu timer ends up triggering an > > IPI to all nohz full CPUs. That's how nohz full has always behaved. > > So users running posix timers on nohz should already suffer issues anyway. > > I haven't checked how this would affect us, but seems a lot less serious > then not having nohz at all. Indeed. It's the difference between just receiving one IPI on every CPU and having one interrupt every 1/Hz. I'll just keep that global IPI to tell all CPUs that they may run a task with a posix cpu timer queued. It's necessary for the CPUs to restart the tick if needed. It's the current behavior and it makes things very simple. Besides, nobody complained about it yet. I can fix it if people request it but this will be in another patchset because it's a complicated issue on its own. To summarize: this patchset won't change the current upstream behaviour. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/