linux-kernel - Re: <PING> Re: [patch x86/core] x86: allow number of additional hotplug CPUs to be set at compile time

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Message-ID: <20081006132252.GF3180@one.firstfloor.org>
Date:	Mon, 6 Oct 2008 15:22:52 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Bodo Eggert <7eggert@....de>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Chuck Ebbert <cebbert@...hat.com>,
	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: <PING> Re: [patch x86/core] x86: allow number of additional hotplug  CPUs to be set at compile time

On Mon, Oct 06, 2008 at 12:59:18PM +0200, Bodo Eggert wrote:
> I don't have a machine with pluggable CPUs, but I'd imagine if you'd take out
> some CPUs, the number of additional CPUs you can plug in will increase by the
> same number (forcing me to change the kernel command lince if I do), while the

It only costs you some memory to have more hotpluggable CPUs (on my
kernel here around 40k/possible CPU), so there's very little "force" in 
practice unless you set excessive values.

Some other architectures invented a "possible_cpus=..." parameter to address
this. I presume this could be done on x86 too, but it doesn't seem
like a very pressing issue. Even if it was done it would be better
to keep additional_cpus for compatibility.

> number of slots (and the number of possible CPUs) stays the same unless
> somebody offers a new kind of CPU card. Therefore, I'd expect "maxcpus=" to be
> the only interface I want for this purpose. Put in a value bigger than the
> amount plugged in -> voila.

maxcpus=... doesn't affect the cpu_possible_mask.  It really is only 
good for limiting CPUs.

> OTOH, looking at Thomas' patch, I'd guess it would not work as expected ...
> and looking at the code seems to confirm this. Besides that, I'd possibly
> want a way to limit the number of online CPUs at boot saying something like
> "onlinecpus=", which would not limit the number of CPUs I can plug in.

That is exactly what maxcpus=... does.

Anyways I don't see the additional_cpus=... removal patch in tip
so hopefully things are fine now

-Andi

-- 
ak@...ux.intel.com
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