linux-kernel - Re: RFC: /proc/cpuinfo confusion with AMD processors

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Message-ID: <20140630131309.GA5199@pd.tnic>
Date:	Mon, 30 Jun 2014 15:13:09 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: RFC: /proc/cpuinfo confusion with AMD processors

On Mon, Jun 30, 2014 at 08:50:47AM -0400, Prarit Bhargava wrote:
> AMD defines a "Package" as the hardware processor itself.  Each Package contains
> multiple Nodes, and each Node has multiple Compute Units.  Each Compute Unit can
> have up to 2 cores that [with the 62xx and 63xx] do not have multiple Threads.
> 
> That is, to determine the number of CPUs that Linux sees, multiply
> 
>      Package * Nodes * Compute Units * Cores
> 
> Note that Nodes and Compute Units are not indicated in /proc/cpuinfo directly
> (although it could be argued that they should be).
> 
> The output of /proc/cpuinfo is confusing at this point as ...
> 
> 
> processor       : 31
> vendor_id       : AuthenticAMD
> cpu family      : 21
> model           : 2
> model name      : AMD Opteron(tm) Processor 6386 SE
> stepping        : 0
> microcode       : 0x6000822
> cpu MHz         : 2800.000
> cache size      : 2048 KB
> physical id     : 1
> siblings        : 16  <<< this is number of threads per package
> core id         : 7   <<< this is the core id of this thread relative to node
> cpu cores       : 8   <<< this is the number of cores per node

siblings / cpu cores = threads per compute unit.

> which makes deciphering the system topology quite difficult as values are
> relative to both nodes and the entire package.  It is not possible using this
> information to uniquely identify a processor.

To do what with that information? What is the task you're trying to
accomplish?

> Thoughts/concerns?

BIOS does all kinds of hacks and renumbering to accomodate the
brainf*cked design of other OSes so this info you're trying to put in
cpuinfo might turn to be completely misleading utterly useless in some
cases.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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