linux-kernel - Re: [rfc 08/45] cpu alloc: x86 support

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Date:	Wed, 21 Nov 2007 14:08:16 +0100
From:	Andi Kleen <ak@...e.de>
To:	Christoph Lameter <clameter@....com>
Cc:	akpm@...ux-foundation.org, travis@....com,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc 08/45] cpu alloc: x86 support


> > All you need is a 2MB area (16MB is too large if you really
> > want 16k CPUs someday) somewhere in the -2GB or probably better
> > in +2GB. Then the linker puts stuff in there and you use
> > the offsets for referencing relative to %gs.
> 
> 2MB * 16k = 32GB. Even with 4k cpus we will have 2M * 4k = 8GB both do
> not fit in the 2GB area.

I was referring here to the 16MB/CPU you proposed originally which will not fit 
into _any_  kernel area for 16k CPUs. 

The whole mapping for all CPUs cannot fit into 2GB of course, but the reference 
linker managed range can.

> The offset relative to %gs cannot be used if you have a loop and are 
> calculating the addresses for all instances. That is what we are talking 
> about. The CPU_xxx operations that are using the %gs register are fine and 
> are not affected by the changes we are discussing.

Sure it can -- you just get the base address from a global array
and then add the offset

> 
> > Then the reference data would be initdata and eventually freed.
> > That is similar to how the current per cpu data works.
> 
> Yes that is also how the current patchset works. I just do not understand 
> what you want changed.

Anyways i think your current scheme cannot work (too much VM, placed at the wrong 
place; some wrong assumptions).

But since I seem unable to communicate this to you I'll stop commenting
and let you find it out the hard way. Have fun.

-Andi
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