linux-kernel - Re: [Bug fix PATCH v2] numa, cpu hotplug: Change links of CPU and node when changing node number by onlining CPU

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Message-ID: <5175D01E.5000302@jp.fujitsu.com>
Date:	Tue, 23 Apr 2013 09:04:46 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	<kosaki.motohiro@...il.com>, <mingo@...nel.org>, <hpa@...or.com>,
	<srivatsa.bhat@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, <linux-mm@...ck.org>
Subject: Re: [Bug fix PATCH v2] numa, cpu hotplug: Change links of CPU and
 node when changing node number by onlining CPU

2013/04/23 7:35, Andrew Morton wrote:
> On Fri, 19 Apr 2013 14:23:23 +0900 Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com> wrote:
>
>> When booting x86 system contains memoryless node, node numbers of CPUs
>> on memoryless node were changed to nearest online node number by
>> init_cpu_to_node() because the node is not online.
>>
>> ...
>>
>> If we hot add memory to memoryless node and offine/online all CPUs on
>> the node, node numbers of these CPUs are changed to correct node numbers
>> by srat_detect_node() because the node become online.
>
> OK, here's a dumb question.
>
> At boot time the CPUs are assigned to the "nearest online node" rather
> than to their real memoryless node.  The patch arranges for those CPUs
> to still be assigned to the "nearest online node" _after_ some memory
> is hot-added to their real node.  Correct?

Yes. For changing node number of CPUs safely, we should offline CPUs.

>
> Would it not be better to fix this by assigning those CPUs to their real,
> memoryless node right at the initial boot?  Or is there something in
> the kernel which makes cpus-on-a-memoryless-node not work correctly?
>

I think assigning CPUs to real node is better. But current Linux's node
strongly depend on memory. Thus if we just create cpus-on-a-memoryless-node,
the kernel cannot work correctly.

Thanks,
Yasuaki Ishimatsu

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