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Date: Wed, 8 Apr 2015 12:52:12 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Brice Goglin <Brice.Goglin@inria.fr>
Cc: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@linux.vnet.ibm.com>,
        Anshuman Khandual <khandual@linux.vnet.ibm.com>,
        linuxppc-dev@lists.ozlabs.org,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Anton Blanchard <anton@samba.org>
Subject: Re: Topology updates and NUMA-level sched domains
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On Wed, Apr 08, 2015 at 12:32:01PM +0200, Brice Goglin wrote:
> Le 07/04/2015 21:41, Peter Zijlstra a écrit :
> > No, that's very much not the same. Even if it were dealing with hotplug
> > it would still assume the cpu to return to the same node.
> >
> > But mostly people do not even bother to handle hotplug.
> >
> 
> You said userspace assumes the cpu<->node relation is a boot-time fixed
> one, and hotplug breaks this.

I said no such thing. Regular hotplug actually respects that relation.

> How do you expect userspace to handle hotplug?

Mostly not. Why would they? CPU hotplug is rare and mostly a case of:
don't do that then.

Its just that some of the virt wankers are using it for resource
management which is entirely misguided. Then again, most of virt is.

> Is there a convenient way to be notified when a CPU (or memory)
> is unplugged?

I think you can poll some sysfs file or other.