linux-kernel - Re: [PATCH v2] cpusets: randomize node rotor used in cpuset_mem_spread

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Message-ID: <alpine.DEB.2.00.1105271623410.9445@chino.kir.corp.google.com>
Date:	Fri, 27 May 2011 16:27:34 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jack Steiner <steiner@....com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Paul Menage <menage@...gle.com>, Robin Holt <holt@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org
Subject: Re: [PATCH v2] cpusets: randomize node rotor used in
 cpuset_mem_spread_node()

On Sat, 28 May 2011, Michal Hocko wrote:

> > CONFIG_NODES_SHIFT is used for UMA machines that are using DISCONTIGMEM 
> > usually because they have very large holes; such machines don't need 
> > things like mempolicies but do need the data structures that abstract 
> > ranges of memory in the physical address space.  This build breakage 
> > probably isn't restricted to only alpha, you could probably see it with at 
> > least ia64 and mips as well.
> 
> Hmmm. I just find strange that some UMA arch uses functions like
> {first,next}_online_node.
> 

They shouldn't, but they do use NUMA data structures like pg_data_t for 
DISCONTIGMEM.  The MAX_NUMNODES > 1 optimization in nodemask.h is to 
prevent doing things like node_weight() on a nodemask when we know that 
only one bit will ever be set, otherwise we could make it conditional on 
CONFIG_NEED_MULTIPLE_NODES.
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