linux-kernel - Re: [PATCH] x86: Issue a warning if number of present CPUs > maxcpus and CONFIG

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Message-ID: <20140217134006.GA14786@khazad-dum.debian.net>
Date:	Mon, 17 Feb 2014 10:40:07 -0300
From:	Henrique de Moraes Holschuh <hmh@....eng.br>
To:	Petr Tesarik <ptesarik@...e.cz>
Cc:	Jan Beulich <JBeulich@...e.com>, Borislav Petkov <bp@...en8.de>,
	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: Issue a warning if number of present CPUs > maxcpus
 and CONFIG_HOTPLUG=n

On Mon, 17 Feb 2014, Petr Tesarik wrote:
> Well, if the user passes both nr_cpus and maxcpus parameters to the
> kernel, I think it's fair to issue two warnings. But if everyone agrees
> that only the maxcpus warning should be printed in that case, I can
> send a version 2 of my patch.

Please remember that the market is full of motherboards with the extremely
annoying behaviour of declaring ACPI objects for CPU cores that will never
be available.  This includes a large number of workstation and server boards
at the very least, from at least one rather large vendor.

As far as I know, we still don't have a way to realiably detect this and get
rid of the ghost processors which will *NEVER* become online.  Setting
maxcpus or nr_cpus manually is the current way to avoid wasting runtime
resources because of phantom cores that will never become reality.

So, when you fix the bug that always supress the warnings, you will at the
same time cause a regression on those boxes, which will now print undesired
warnings.  If the user has manually set nr_cpus or maxcpus, maybe it would
be best to not print any warnings or alternatively to downgrade them to
debug level?

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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