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: <5175E2D4.5030004@jp.fujitsu.com>
Date:	Tue, 23 Apr 2013 10:24:36 +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 9:34, Andrew Morton wrote:
> On Tue, 23 Apr 2013 09:04:46 +0900 Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com> wrote:
>
>> 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.
>
> hm, why.  I'd have thought that if we tell the kernel something like
> "this node has one zone, the size of which is zero bytes" then a
> surprising amount of the existing code will Just Work.
>
> What goes wrong?

Sorry I forgot detailed issue.
When I saw following issue, I tried to fix it and found that current
Linux's node strongly depend on memory.
https://lkml.org/lkml/2012/9/12/20

I'll try to fix it again.

Thanks,
Yasuaki Ishimatsu

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