linux-kernel - Re: [PATCH] Do not use cpu_to_node() to find an offlined cpu's node.

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Date:	Wed, 10 Oct 2012 18:10:09 +0800
From:	Wen Congyang <wency@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	David Rientjes <rientjes@...gle.com>,
	Tang Chen <tangchen@...fujitsu.com>, mingo@...hat.com,
	miaox@...fujitsu.com, linux-kernel@...r.kernel.org,
	linux-numa@...r.kernel.org
Subject: Re: [PATCH] Do not use cpu_to_node() to find an offlined cpu's node.

At 10/10/2012 05:51 PM, Peter Zijlstra Wrote:
> On Wed, 2012-10-10 at 17:33 +0800, Wen Congyang wrote:
>>
>> Hmm, if per-cpu memory is preserved, and we can't offline and remove
>> this memory. So we can't offline the node.
>>
>> But, if the node is hot added, and per-cpu memory doesn't use the
>> memory on this node. We can hotremove cpu/memory on this node, and then
>> offline this node.
>>
>> Before the cpu is hotadded, cpu's node is -1. We set cpu<->node mapping
>> when it is hotadded. So the entire cpu<->node mapping was not invariant
>> during hotplug.
>>
>> So it is why I try to clear it when the cpu is hot-removed.
>>
>> As we need the mapping to migrate a task to the cpu on the same node first,
>> I think we can clear the mapping when the node is offlined.
> 
> Hmm maybe, but hardware that can hot-add is rare and nobody has it so
> nobody cares ;-)

Yes, nobody cares it now. But we have a such hardware, so I care it now.

> 
> But by clearing cpu_to_node on every hotplug you change semantics for
> all hardware and everybody gets to feel the pain.
> 
> I'm not saying you cannot change things, I'm only saying you should be
> far more careful about it, not change it and wait for things to break.
> Put in some effort to find things that might break and warn people --
> sure, you'll always miss some, and that's ok.

I use ./scripts/get_maintainer.pl, and it doesn't tell me that I should cc
you when I post that patch.


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