linux-kernel - Re: [PATCH 3/3] x86: fix node_possible

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Date:	Mon, 11 May 2009 12:27:49 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Jack Steiner <steiner@....com>
cc:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] x86: fix node_possible_map logic -v2

On Mon, 11 May 2009, Jack Steiner wrote:

> Do we have a clear and unambiguous definition of what a node really is?
> In this case, is a board (socket) with cpus, a unique PXM but no memory
> considered a node. Even though it has no memory, it is a node (depending on the
> definition of "node") for purposes such as scheduling. The memoryless node also
> has local IO buses that want to direct interrupts to node-local cpus.
> 

In your example of two cpus (0-1) that are remote to the system's only 
memory and two cpus (2-3) that have affinity to that memory, it appears as 
though the kernel is considering cpus 2-3 and the memory to be a node and 
cpus 0-1 to be a memoryless node.

That's a pretty useless scenario for memoryless node support, actually, 
unless there's a third node with memory that cpus 0-1 have a different 
distance to.  cpus 0-1 have no memory that is local, so the "remote" 
memory should be considered local to them.

I don't know who has been pushing the memoryless node support, but it 
appears as though it hasn't been fully tested yet.  The NULL pglist_data 
here for node 0 seems appropriate since you don't need it unless you're 
describing memory, but the kernel implies that if a bit is set in 
node_online_map or node_possible_map that it has this associated data.

Added Andi Kleen to the cc list.
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