linux-kernel - Re: [PATCH v2] Fix fake numa on ppc

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Date:	Tue, 1 Sep 2009 23:37:05 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Ankita Garg <ankita@...ibm.com>
cc:	LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...abs.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Balbir Singh <balbir@...ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] Fix fake numa on ppc

On Wed, 2 Sep 2009, Ankita Garg wrote:

> Hi,
> 
> Below is a patch to fix a couple of issues with fake numa node creation
> on ppc:
> 
> 1) Presently, fake nodes could be created such that real numa node
> boundaries are not respected. So a node could have lmbs that belong to
> different real nodes.
> 

On x86_64, we can use numa=off to completely disable NUMA so that all 
memory and all cpus are mapped to a single node 0.  That's an extreme 
example of the above and is totally permissible.

> 2) The cpu association is broken. On a JS22 blade for example, which is
> a 2-node numa machine, I get the following:
> 
> # cat /proc/cmdline
> root=/dev/sda6  numa=fake=2G,4G,,6G,8G,10G,12G,14G,16G
> # cat /sys/devices/system/node/node0/cpulist
> 0-3
> # cat /sys/devices/system/node/node1/cpulist
> 4-7
> # cat /sys/devices/system/node/node4/cpulist
> 
> #
> 

This doesn't show what the true NUMA topology of the machine is, could you 
please post the output of

	$ cat /sys/devices/system/node/node*/cpulist
	$ cat /sys/devices/system/node/node*/distance
	$ ls -d /sys/devices/system/node/node*/cpu[0-8]

from a normal boot without any numa=fake?

> So, though the cpus 4-7 should have been associated with node4, they
> still belong to node1. The patch works by recording a real numa node
> boundary and incrementing the fake node count. At the same time, a
> mapping is stored from the real numa node to the first fake node that
> gets created on it.
> 

If there are multiple fake nodes on a real physical node, all cpus in that 
node should appear in the cpulist for each fake node for which it has 
local distance.
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