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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: rusty@rustcorp.com.au (Rusty Russell), anton@samba.org, davej@suse.de,
        marcelo@conectiva.com.br, linux-kernel@vger.kernel.org,
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Subject: Re: Linux 2.4.17-pre5
In-Reply-To: <E16DEVr-0008SW-00@the-village.bc.nu>
From: ebiederm@xmission.com (Eric W. Biederman)
Date: 11 Dec 2001 02:00:23 -0700
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Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

> > 	If you number each CPU so its two IDs are smp_num_cpus()/2
> > 	apart, you will NOT need to put some crappy hack in the
> > 	scheduler to pack your CPUs correctly.
> 
> Which is a major change to the x86 tree and an invasive one. Right now the
> X86 is doing a 1:1 mapping, and I can offer Marcelo no proof that somewhere
> buried in the x86 arch code there isnt something that assumes this or mixes
> a logical and physical cpu id wrongly in error. 

Actually we don't do a 1:1 physical to logical mapping.  I currently
have a board that has physical id's of:  0:6 and logical id's of 0:1
with no changes to the current x86 code. 
> 
> At best you are exploiting an obscure quirk of the current scheduler that is
> quite likely to break the moment someone factors power management into the
> idling equation (turning cpus off and on is more expensive so if you idle
> a cpu you want to keep it the idle one for PM). Congratulations on your
> zen like mastery of the scheduler algorithm. Now tell me it wont change in
> that property.

The idea of a cpu priority for filling sounds like a nice one.  Even
if we don't use the cpu id bits for it.

Eric

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