Date: Thu, 9 Apr 2015 15:40:40 -0700
From: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Brice Goglin <Brice.Goglin@inria.fr>, 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
Message-ID: <20150409224040.GG53918@linux.vnet.ibm.com>
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 <20150407171410.GA62529@linux.vnet.ibm.com>
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On 08.04.2015 [12:52:12 +0200], Peter Zijlstra wrote:
> On Wed, Apr 08, 2015 at 12:32:01PM +0200, Brice Goglin wrote:
> > Le 07/04/2015 21:41, Peter Zijlstra a ?crit :
> > > 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.

Wel, sort of. If you *just* hotplug a CPU out, your invariant of what
CPUs are currently available on what nodes is no longer held. Similarly
if you just add a CPU. And means that you could end up using cpumasks
that are incorrect if you don't make them at runtime, it seems?

> > 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.

I guess that is a matter of opinion.

-Nish

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