From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932625AbbLBOLR (ORCPT ); Wed, 2 Dec 2015 09:11:17 -0500 Received: from mail-wm0-f41.google.com ([74.125.82.41]:35759 "EHLO mail-wm0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932540AbbLBOLO (ORCPT ); Wed, 2 Dec 2015 09:11:14 -0500 Date: Wed, 2 Dec 2015 15:11:10 +0100 From: Frederic Weisbecker To: Peter Zijlstra Cc: LKML , Chris Metcalf , Thomas Gleixner , Luiz Capitulino , Christoph Lameter , Ingo Molnar , Viresh Kumar , Rik van Riel Subject: Re: [PATCH 2/7] nohz: New tick dependency mask Message-ID: <20151202141109.GD14766@lerouge> References: <1447424529-13671-1-git-send-email-fweisbec@gmail.com> <1447424529-13671-3-git-send-email-fweisbec@gmail.com> <20151201204109.GN17308@twins.programming.kicks-ass.net> <20151201222025.GA31973@lerouge> <20151202124806.GD9928@worktop.ger.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151202124806.GD9928@worktop.ger.corp.intel.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 On Wed, Dec 02, 2015 at 01:48:06PM +0100, Peter Zijlstra wrote: > On Tue, Dec 01, 2015 at 11:20:28PM +0100, Frederic Weisbecker wrote: > > On Tue, Dec 01, 2015 at 09:41:09PM +0100, Peter Zijlstra wrote: > > > On Fri, Nov 13, 2015 at 03:22:04PM +0100, Frederic Weisbecker wrote: > > > > The tick dependency is evaluated on every IRQ. This is a batch of checks > > > > which determine whether it is safe to stop the tick or not. These checks > > > > are often split in many details: posix cpu timers, scheduler, sched clock, > > > > perf events. Each of which are made of smaller details: posix cpu > > > > timer involves checking process wide timers then thread wide timers. Perf > > > > involves checking freq events then more per cpu details. > > > > > > > > Checking these details asynchronously every time we update the full > > > > dynticks state bring avoidable overhead and a messy layout. > > > > > > > > Lets introduce instead tick dependency masks: one for system wide > > > > dependency (unstable sched clock), one for CPU wide dependency (sched, > > > > perf), and task/signal level dependencies. The subsystems are responsible > > > > of setting and clearing their dependency through a set of APIs that will > > > > take care of concurrent dependency mask modifications and kick targets > > > > to restart the relevant CPU tick whenever needed. > > > > > > Maybe better explain why we need the per task and per signal thingy? > > > > I'll detail that some more in the changelog. The only user of the per > > task/per signal tick dependency is posix cpu timer. I've been first > > proposing a global tick dependency as soon as any posix cpu timer is > > armed. > > > It simplified everything but some reviewers complained (eg: > > some users might want to run posix timers on housekeepers without > > bothering full dynticks CPUs). I could remove the per signal > > dependency with dispatching it through all threads in the group each > > time there is an update but that's the best I can think of. > > Right, I remember some of that. Seems worth preserving these reasons. > Maybe even in code comments near the definition of these things. Agreed! I'll comment that some more. Thanks.