Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750866Ab3IFRa6 (ORCPT ); Fri, 6 Sep 2013 13:30:58 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:33952 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750843Ab3IFRaz (ORCPT ); Fri, 6 Sep 2013 13:30:55 -0400 Date: Fri, 6 Sep 2013 10:30:47 -0700 From: "Paul E. McKenney" To: Geert Uytterhoeven Cc: "linux-kernel@vger.kernel.org" , Ingo Molnar , laijs@cn.fujitsu.com, dipankar@in.ibm.com, Andrew Morton , Mathieu Desnoyers , josh@joshtriplett.org, niv@us.ibm.com, Thomas Gleixner , Peter Zijlstra , Steven Rostedt , David Howells , edumazet@google.com, darren@dvhart.com, =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker , sbw@mit.edu, Linux-Arch , linux-kbuild Subject: Re: [PATCH tip/core/rcu 8/9] nohz_full: Add full-system-idle state machine Message-ID: <20130906173047.GT3966@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20130820024700.GA31075@linux.vnet.ibm.com> <1376966841-31774-1-git-send-email-paulmck@linux.vnet.ibm.com> <1376966841-31774-8-git-send-email-paulmck@linux.vnet.ibm.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) X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13090617-8236-0000-0000-0000017FF934 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3438 Lines: 95 On Fri, Sep 06, 2013 at 10:08:22AM +0200, Geert Uytterhoeven wrote: > On Tue, Aug 20, 2013 at 4:47 AM, Paul E. McKenney > wrote: > > --- a/kernel/time/Kconfig > > +++ b/kernel/time/Kconfig > > @@ -157,6 +157,33 @@ config NO_HZ_FULL_SYSIDLE > > > > Say N if you are unsure. > > > > +config NO_HZ_FULL_SYSIDLE_SMALL > > + int "Number of CPUs above which large-system approach is used" > > + depends on NO_HZ_FULL_SYSIDLE > > + range 1 NR_CPUS > > This causes "kernel/time/Kconfig:162:warning: range is invalid" on m68k and > all other architectures that do not support SMP. > > How to reproduce: > make ARCH=m68k defconfig OK, this does complain, but seems to give a reasonable .config file. (From what I can tell.) It would clearly be good to get rid of the complaint. > Furthermore, it seems only hexagon, metag, mips, and x86 set NR_CPUS to 1 > if !SMP. On other architectures, NR_CPUS is not defined and presumed to be 0. Would it make sense to require that NR_CPUS=1 for !SMP? > Hence in non-interactive configs (e.g. "make defconfig"), > NO_HZ_FULL_SYSIDLE_SMALL will end up as 0. > In interactive configs (e.g. "make oldconfig") Kconfig suggest "0" as > the default, > but refuses to accept it as it doesn't fall within the range 1..0. > > How to reproduce: > Remove the "depends on NO_HZ_FULL_SYSIDLE" > make ARCH=powerpc mpc83xx_defconfig > grep NO_HZ_FULL_SYSIDLE_SMALL .config > -> CONFIG_NO_HZ_FULL_SYSIDLE_SMALL=0 > sed 's/CONFIG_NO_HZ_FULL_SYSIDLE_SMALL=0//g' -i .config > make ARCH=powerpc oldconfig > -> no value is accepted If it turns out that there is some reason by NR_CPUS=1 is impossible, there are a few things that I could do: I could just leave the range off, which would allow people to give nonsense values. This would be harmless in the code, for example, a negative value would simply disable small-system handling, while a too-large value would similarly disable large-system handling. Might be a bit obnoxious for the guy who typoed and then wasted a kernel build/boot/test cycle, but it is an option. I could use a small fixed range (say from 1 to 64), which would provide at least some checking. In the unlikely event that someone really wants more than 64 CPUs handled with small-system handling, we could revisit at that point. I tried creating a NR_CPUS_REALLY as follows: config NR_CPUS_REALLY int "Fixed version of NR_CPUS" default NR_CPUS if NR_CPUS default 1 if !NR_CPUS But this still gave a warning on the first "default" even though it was not in effect. I also tried using Kconfig "if": if SMP config NR_CPUS_REALLY int "Fixed version of NR_CPUS" default NR_CPUS endif if !SMP config NR_CPUS_REALLY int "Fixed version of NR_CPUS" default 1 if !SMP endif However, Kconfig complained about the use of NR_CPUS even though this was under an "if" whose condition was not set. Perhaps someone with better Kconfig-fu than I have can come up with something. Defining NR_CPUS=1 if !SMP is looking pretty good to me just now. This would probably have other benefits -- I cannot be the only person who ever wanted this. ;-) Thanx, Paul -- 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/