linux-kernel - Re: [patch x86/core] x86: allow number of additional hotplug CPUs to be set at compile time, V2

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Date:	Thu, 2 Oct 2008 21:42:15 +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>,
	Li Zefan <lizf@...fujitsu.com>
Subject: Re: [patch x86/core] x86: allow number of additional hotplug CPUs
	to be set at compile time, V2


* 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, V2
> 
> 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
> 
> [Changed the name of the option and the prompt according to Ingo's
>  suggestion.]

> +config HOTPLUG_RESTRICT_TO_BOOTUP_CPUS
> +	def_bool n
> +	prompt "Restrict CPU hotplugging to processors found during boot" if HOTPLUG_CPU

ok, that description and naming makes the purpose much clearer, and it's 
default-disabled as well.

Applied to tip/x86/core, thanks Chuck!

note that this chunk:

> @@ -1282,16 +1282,13 @@ __init void prefill_possible_map(void)
>  	if (!num_processors)
>  		num_processors = 1;
>  
> -#ifdef CONFIG_HOTPLUG_CPU
>  	if (additional_cpus == -1) {
>  		if (disabled_cpus > 0)
>  			additional_cpus = disabled_cpus;
>  		else
>  			additional_cpus = 0;
>  	}
> -#else
> -	additional_cpus = 0;
> -#endif
> +

was already in -tip, by virtue of:

| commit 2bd455dbfebfd632a8dcf1d3d1612737986fde0a
| Author: Li Zefan <lizf@...fujitsu.com>
| Date:   Mon Aug 4 11:26:38 2008 +0800
|
|     x86: remove nesting CONFIG_HOTPLUG_CPU

please double-check latest tip/master nevertheless:

  http://people.redhat.com/mingo/tip.git/README

to make sure i merged your patch correctly and that it plays well with 
other changes.

Thanks,

	Ingo
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