From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756296Ab3C3JKn (ORCPT ); Sat, 30 Mar 2013 05:10:43 -0400 Received: from mail-ea0-f175.google.com ([209.85.215.175]:42171 "EHLO mail-ea0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755285Ab3C3JKl (ORCPT ); Sat, 30 Mar 2013 05:10:41 -0400 Date: Sat, 30 Mar 2013 10:10:37 +0100 From: Ingo Molnar To: Frederic Weisbecker Cc: LKML , Andrew Morton , Chris Metcalf , Christoph Lameter , Geoff Levand , Gilad Ben Yossef , Hakan Akkan , Kevin Hilman , Li Zhong , Namhyung Kim , "Paul E. McKenney" , Paul Gortmaker , Peter Zijlstra , Steven Rostedt , Thomas Gleixner Subject: Re: [PATCH 4/4] nohz: New option to force all CPUs in full dynticks range Message-ID: <20130330091037.GA31743@gmail.com> References: <1364398359-21990-1-git-send-email-fweisbec@gmail.com> <1364398359-21990-5-git-send-email-fweisbec@gmail.com> <20130328074507.GC24433@gmail.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) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Frederic Weisbecker wrote: > 2013/3/28 Ingo Molnar : > > > > * Frederic Weisbecker wrote: > > > >> +config NO_HZ_EXTENDED_ALL > >> + bool "Full dynticks system on all CPUs" > >> + depends on NO_HZ_EXTENDED > >> + help > >> + Force all CPUs to be full dynticks. The range specified in the > >> + nohz_extended boot option will then be ignored. > >> + > >> + Note the boot CPU will still be kept outside the range to handle > >> + the timekeeping duty. > > > > In most cases this will be the first time users doing 'make oldconfig' > > will hear about this feature, so I'd not mention noh_extended in such a > > primary way at all. > > > > Instead I'd suggest to explain, in a few simple sentences, what 'full > > dynticks' _is_. Then maybe mention this near the end of the next: > > > > ( Alternatively this feature can also be enabled via the > > nohz_extended= boot option. If this kernel config option is > > enabled then the nohz_extended boot parameter is ignored. ) > > Ok. Hmm, concerning what full dynticks is, those who run make oldconfig > should have passed through CONFIG_NO_HZ_EXTENDED help first so they > should know what this is all about, right? I can improve its help text > if you think something need more details. Or you think I should add some > more reminder in the new config? Also nohz_extended is not mentioned > there so I indeed need to fix that. And then do a reminder in the new > config. I'll reuse a bit your above sentence. Kconfig option + boot parameter, or kconfig option + kconfig option, just to get a feature to activate, is usability madness. I think we need _one_ config knob to configure, which, if selected, activates all of this feature. Preferably this would be similar to the preempt options, like CONFIG_PREEMPT is the most advanced preemption model: ( ) No Forced Preemption (Server) (X) Voluntary Kernel Preemption (Desktop) ( ) Preemptible Kernel (Low-Latency Desktop) NO_HZ_EXTENDED or NO_HZ_FULL could be a third variant to: ( ) Periodic Timer Ticks (X) Tickless Idle (Dynamic Ticks) ( ) Full Tickless (Extended Dynamic Ticks) or so - instead of this somewhat confusing hierarchy of config and boot options. Good help text would explain the differences between then. Internally, the 'Tickless Idle' option would cause NO_HZ to be set, while 'Full Tickless' would also set NO_HZ, but would also set NO_HZ_EXTENDED. Furthermore, right now it's hard to enable NO_HZ_EXTENDED, because it depends on so many other options. The option is just hidden unless the user knows about it and enables 5 other kernel options. Instead the tick mode selected should enable (select) any required features - it should drive things, not the other way around. (After all that is done, the boot option is something that augments this existing, kconfig driven mechanism.) Thanks, Ingo