Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756337Ab2FZJl4 (ORCPT ); Tue, 26 Jun 2012 05:41:56 -0400 Received: from mail-ey0-f174.google.com ([209.85.215.174]:45940 "EHLO mail-ey0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753645Ab2FZJly (ORCPT ); Tue, 26 Jun 2012 05:41:54 -0400 Message-ID: <4FE983E0.2050802@linaro.org> Date: Tue, 26 Jun 2012 11:41:52 +0200 From: Daniel Lezcano User-Agent: Mozilla/5.0 (X11; Linux i686; rv:12.0) Gecko/20120430 Thunderbird/12.0.1 MIME-Version: 1.0 To: Thomas Renninger CC: "Srivatsa S. Bhat" , Deepthi Dharwar , linux-acpi@vger.kernel.org, linux-pm@lists.linux-foundation.org, Linux PM mailing list , lenb@kernel.org, "Rafael J. Wysocki" , x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] acpi, cpuidle: Register with cpuidle even if cpu is onlined after boot (beyond maxcpus) References: <4FDB549F.1020002@linaro.org> <201206251553.55867.trenn@suse.de> <4FE88BDE.1050406@linux.vnet.ibm.com> <201206261129.45591.trenn@suse.de> In-Reply-To: <201206261129.45591.trenn@suse.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3747 Lines: 82 On 06/26/2012 11:29 AM, Thomas Renninger wrote: > On Monday, June 25, 2012 06:03:42 PM Srivatsa S. Bhat wrote: >> On 06/25/2012 07:23 PM, Thomas Renninger wrote: >> >>> On Monday, June 25, 2012 01:25:43 PM Srivatsa S. Bhat wrote: >>>> >>>> Daniel Lezcano noticed that after booting with maxcpus=X, if we online the >>>> remaining cpus by writing: echo 1 > /sys/devices/system/cpu/cpuY/online, then >>>> for the newly onlined cpus, the cpuidle directory is not found under >>>> /sys/devices/system/cpu/cpuY. >>>> >>>> Partly, the reason for this is that acpi restricts the initialization to cpus >>>> within the maxcpus limit. (See commit 75cbfb9 "ACPI: Do not try to set up acpi >>>> processor stuff on cores exceeding maxcpus="). The maxcpus= kernel parameter is >>>> used to restrict the number of cpus brought up during boot. That doesn't mean >>>> that we should hard restrict the bring up of the remaining cpus later on. >>> >>> Sorry, but IMO it exaclty does mean that (adding more general lists for >>> further comments). >>> >>> If you can online more cores than maxcpus= via sysfs, this sounds like a bug. >>> Not the other way around. >>> >>> Compare with Documentation/kernel-parameters.txt: >>> 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. >>> >>> Chances that you run into more problems are high. >> >> >> Right, I agree on that. So, IMHO, maxcpus=X doesn't mean that the kernel must and >> should forbid any new cpus from coming online, but in the interest of avoiding >> problems/complications in some obscure paths, I guess it makes sense to avoid >> onlining new cpus beyond maxcpus. > > Yep, for such reasons: > - That nobody realizes this to be useful and makes use of it in a productive > environment > - If I see maxcpus=X in a bugreport's dmesg command line, > I want to be sure that's true. > - To enforce that things work as documented > > > Wow, after looking a bit into this I found (Documentation/cpu-hotplug.txt): > > 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. > > Looks like someone already documented this (IMO broken) behavior. > I didn't find further info in the FAQs. > >> In any case, I was just trying to see why the simple removal of the setup_max_cpus >> check in acpi_processor_add() wasn't enough to expose the cpuidle directories under >> the new cpus.. and while debugging that, I came up with this patch. I don't mind >> if this doesn't get picked up. > >> Right, the usecase of why somebody would like to online new cpus beyond maxcpus >> doesn't look all that solid anyway. So I am OK with leaving the code as it is now. > > In the end this is a debug option, I expect everybody is aware of that. > Yep, let's just leave it... In this case, let's remove the intel_idle_cpu_init stuff in acpi_cpu_soft_notify, no ? -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog -- 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/