linux-kernel - Re: [PATCH 0/3] x86: adapt CPU topology detection for AMD Magny-Cours

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Date:	Tue, 5 May 2009 11:22:38 +0200
From:	Andreas Herrmann <andreas.herrmann3@....com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] x86: adapt CPU topology detection for AMD
	Magny-Cours

On Mon, May 04, 2009 at 10:16:15PM +0200, Andi Kleen wrote:
> Andreas Herrmann <andreas.herrmann3@....com> writes:
> 
> > Following patches add support for AMD Magny-Cours CPU.
> >
> > I slightly change struct cpuinfo where I'd like to introduce
> > cpu_node_id to reflect CPU topology for AMD Magny-Cours CPU which
> > consists of two internal-nodes.
> 
> It's unclear to me why you need this special case versus 
> just using the normal NUMA distances to represent the internal
> nodes as "nearby nodes" versus sockets as "farer away".

NUMA memory allocation and scheduling based on SRAT works just fine
w/o that fix. But the patches are _not_ for NUMA node detection.

This is for CPU topology detection -- which cores are using the same
northbridge.

> Essentially it should be just two level NUMA which can be already
> described fine for the scheduler or VM using the existing SRAT
> mechanism.

It doesn't suffice to rely on SRAT here.

Best example is node interleaving. Usually you won't get a SRAT table
on such a system. Thus you see just one NUMA node in
/sys/devices/system/node.  But on such a configuration you still see
(and you want to see) the correct CPU topology information in
/sys/devices/system/cpu/cpuX/topology. Based on that you always can
figure out which cores are on the same physical package independent of
availability and contents of SRAT and even with kernels that are
compiled w/o NUMA support.

A big change with Magny-Cours in contrast to other AMD
CPUs is that instead of

   physical package == one northbridge (one node)

we have

   physical package == two northbridges (two nodes)

and this needs to be represented somehow in the kernel.

> Who needs this additional information?

The kernel needs to know this when accessing processor configuration
space, when accessing shared MSRs or for counting northbridge specific
events.

Regards,
Andreas 

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