From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wen Congyang Subject: Re: [PATCH] Do not use cpu_to_node() to find an offlined cpu's node. Date: Wed, 10 Oct 2012 18:10:09 +0800 Message-ID: <50754981.80900@cn.fujitsu.com> References: <1349665183-11718-1-git-send-email-tangchen@cn.fujitsu.com> <1349780256.7880.12.camel@twins> <1349815676.7880.85.camel@twins> <1349860216.7880.105.camel@twins> <507540F5.7040501@cn.fujitsu.com> <1349862694.7880.114.camel@twins> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1349862694.7880.114.camel@twins> Sender: linux-numa-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Peter Zijlstra Cc: David Rientjes , Tang Chen , mingo@redhat.com, miaox@cn.fujitsu.com, linux-kernel@vger.kernel.org, linux-numa@vger.kernel.org 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.