linux-kernel - Re: [patch 05/14] percpu: Use a Kconfig variable to configure arch specific percpu setup

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Message-ID: <Pine.LNX.4.64.0711281731410.19914@schroedinger.engr.sgi.com>
Date:	Wed, 28 Nov 2007 17:35:01 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Rusty Russell <rusty@...tcorp.com.au>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...e.de>
Subject: Re: [patch 05/14] percpu: Use a Kconfig variable to configure arch
 specific percpu setup

On Wed, 28 Nov 2007, Jeremy Fitzhardinge wrote:

> I don't see the problem.  The way i386 does it inherently supports
> per-cpu data very early on (it uses the prototype percpu section until
> the real percpu values are set up).

Ok so we could do that for x86_64 as well? There is more complicated 
bootstrap since i386 does not support NUMA aware placement of per cpu 
areas.

> > The i386 way of referring to per cpu data is not optimal because it is 
> > always offset by __per_cpu_start. per cpu data offsets need to be relative 
> > to the beginning of the per cpu area. per cpu data is less than 64k so 2 
> > byte offsets would be enough.
> >   
> 
> I don't see that's terribly important.  percpu references aren't all
> that common overall, and - at least on x86 - using a 16-bit offset
> (assuming its possible) would require a prefix anyway, so it would only
> save 1 byte per reference.  But I can't convince gas to generate a
> 16-bit offset anyway.

percpu references are quite frequent already (vm statistics) and will be 
more frequent after we have converted the per cpu arrays to per cpu 
allocations.


> > That way the __per_cpu_offset array and the registers that are used on 
> > various platforms are pointing to the actual data and can be loaded
> > directly into a register and then a load with a small offset to that 
> > register can be performed. On x86_64 this is gs, on i386 fs, on sparc g5, 
> > on ia64 a fixed address stands in for the register.
> 
> The asm used to generate these references is inherently arch-specific
> anyway, so the type and size of offset needed from the per-cpu base
> register to the data itself can be arch-dependent without loss of
> generality.  

Well yes that is already the case and made explicit by the percpu cleanup 
done so far. The offset of a base is used by multiple architectures.


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