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: <alpine.LFD.2.00.0810052252520.3398@apollo>
Date:	Sun, 5 Oct 2008 23:49:27 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andi Kleen <andi@...stfloor.org>
cc:	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 Sun, 5 Oct 2008, Andi Kleen wrote:
> > 
> > Please lets get rid of all this.
> 
> The result is that there's no way to override a BIOS now that
> doesn't declare the number of hotplug CPUs with the Linux hotplug
> extension.
> 
> Completely trusting the BIOS seems like a bad idea.
> 
> So I think you still need the command line parameter back, otherwise there's no
> way to override the BIOS.
> 
> -Andi (who wished you guys would check with the original author
> before removing code you clearly don't understand)

Sigh, could you please start thinking first before you insult me ?

Chucks problem is that the BIOS advertises

1 present CPU and 1 disabled CPU

Now the current code does not switch to UP alternatives because the
check in alternatives.c is:

      if (num_possible_cpus() == 1 ...

      which evaluates to false, due to:

      num_possible_cpus = num_processors + additional_cpus

      where additional_cpus = disabled_cpus when no command line
      parameter is given, i.e. the default behaviour of the kernel.

      and where num_processors = num_present_cpus()

      so in Chucks case num_possible_cpus() == 2

Your proposed solution is:

     exporting additional_cpus  and change the check to:

     if ((num_possible_cpus() - additional_cpus) == 1 ...

This is simply stupid. We have the information already in
num_present_cpus() and that's the correct check in the alternatives
code.

     if (num_present_cpus() == 1 ...

This is not a question of trusting the BIOS: 

If we can not trust present_cpu_map then additional_cpus does not help
at all.

additional_cpus is useless ballast.

Thanks,

	tglx - who refrains from adding a nasty insulting comment
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