From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756739AbYLKNn2 (ORCPT ); Thu, 11 Dec 2008 08:43:28 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755569AbYLKNnU (ORCPT ); Thu, 11 Dec 2008 08:43:20 -0500 Received: from mtagate6.de.ibm.com ([195.212.29.155]:36388 "EHLO mtagate6.de.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755438AbYLKNnT (ORCPT ); Thu, 11 Dec 2008 08:43:19 -0500 Date: Thu, 11 Dec 2008 14:41:31 +0100 From: Heiko Carstens To: Mike Travis Cc: Ingo Molnar , Rusty Russell , "H. Peter Anvin" , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/4] cpumask: use maxcpus=NUM to extend the cpu limit as well as restrict the limit Message-ID: <20081211134131.GB6264@osiris.boeblingen.de.ibm.com> References: <20081211112806.499831000@polaris-admin.engr.sgi.com> <20081211112807.004085000@polaris-admin.engr.sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20081211112807.004085000@polaris-admin.engr.sgi.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 11, 2008 at 03:28:09AM -0800, Mike Travis wrote: > Impact: allow adding additional cpus. > > Use maxcpus=NUM kernel parameter to extend the number of possible cpus as well > as (currently) limit them. Any cpus >= number of present cpus will disabled. > > The ability to HOTPLUG ON cpus that are "possible" but not "present" is > dealt with in a later patch. Hm.. documentation/kernel-parameters.txt says: maxcpus= [SMP] Maximum number of processors that an SMP kernel should make use of. maxcpus=n : n >= 0 limits the kernel to using 'n' processors. n=0 is a special case, it is equivalent to "nosmp", which also disables the IO APIC. but documentation/cpu-hotplug.txt says: maxcpus=n Restrict boot time cpus to n. Say if you have 4 cpus, using maxcpus=2 will only boot 2. You can choose to bring the other cpus later online, read FAQ's for more info. It used to be (implementation wise) that maxcpus doesn't influence the number of possible cpus but just indicated how many cpus were brought online at startup of the kernel. Which is what cpu-hotplug.txt describes. Other present cpus would appear offline and could be brought online later. For s390 I added the possible_cpus kernel parameter back then, since my understanding back then was that maxcpus doesn't and shouldn't influence the number of possible cpus: possible_cpus=n [s390 only] use this to set hotpluggable cpus. This option sets possible_cpus bits in cpu_possible_map. Thus keeping the numbers of bits set constant even if the machine gets rebooted. Dunno... it all looks like a mess ;)