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

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Date:	Thu, 2 Oct 2008 10:12:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Chuck Ebbert <cebbert@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch x86/core] x86: allow number of additional hotplug CPUs
	to be set at compile time


* Chuck Ebbert <cebbert@...hat.com> wrote:

> From: Chuck Ebbert <cebbert@...hat.com>
> 
> x86: allow number of additional hotplug CPUs to be set at compile time
> 
> The default number of additional CPU IDs for hotplugging is determined 
> by asking ACPI or mptables how many "disabled" CPUs there are in the 
> system, but many systems get this wrong so that e.g. a uniprocessor 
> machine gets an extra CPU allocated and never switches to single CPU 
> mode.
> 
> And sometimes CPU hotplugging is enabled only for suspend/hibernate 
> anyway, so the additional CPU IDs are not wanted. Allow the number to 
> be set to zero at compile time.
> 
> Also, force the number of extra CPUs to zero if hotplugging is 
> disabled which allows removing some conditional code.
> 
> Tested on uniprocessor x86_64 that ACPI claims has a disabled 
> processor, with CPU hotplugging configured.
> 
> ("After" has the number of additional CPUs set to 0)
> Before: NR_CPUS: 512, nr_cpu_ids: 2, nr_node_ids 1
> After: NR_CPUS: 512, nr_cpu_ids: 1, nr_node_ids 1
> 
> Signed-off-by: Chuck Ebbert <cebbert@...hat.com>

hm, wouldnt this option kill 'real' hot-plug CPUs (how rare they might 
be) which are properly enumerated in the BIOS tables?

i dont mind having a facility to disable real CPU hotplug, but the 
CONFIG_HOTPLUG_DEFAULT_ADDITIONAL_CPUS does not spell that out clearly 
IMO. Something like CONFIG_HOTPLUG_RESTRICT_TO_BOOTUP_CPUS=y would be 
more appropriately named i think?

	Ingo
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