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Date:   Mon, 18 Mar 2019 16:17:30 +0530
From:   Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Laurent Vivier <lvivier@redhat.com>, linux-kernel@vger.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
        Borislav Petkov <bp@suse.de>,
        David Gibson <david@gibson.dropbear.id.au>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nathan Fontenot <nfont@linux.vnet.ibm.com>,
        Michael Bringmann <mwb@linux.vnet.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, Ingo Molnar <mingo@redhat.com>
Subject: Re: [RFC v3] sched/topology: fix kernel crash when a CPU is
 hotplugged in a memoryless node
Reply-To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
References: <20190304195952.16879-1-lvivier@redhat.com>
 <20190305115952.GH32477@hirez.programming.kicks-ass.net>
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> > node 0 (because firmware doesn't provide the distance information for
> > memoryless/cpuless nodes):
> > 
> >   node   0   1   2   3
> >     0:  10  40  10  10
> >     1:  40  10  40  40
> >     2:  10  40  10  10
> >     3:  10  40  10  10
> 
> *groan*... what does it do for things like percpu memory? ISTR the
> per-cpu chunks are all allocated early too. Having them all use memory
> out of node-0 would seem sub-optimal.

In the specific failing case, there is only one node with memory; all other
nodes are cpu only nodes.

However in the generic case since its just a cpu hotplug ops, the memory
allocated for per-cpu chunks allocated early would remain.

May be Michael Ellerman can correct me here.

> 
> > We should have:
> > 
> >   node   0   1   2   3
> >     0:  10  40  40  40
> >     1:  40  10  40  40
> >     2:  40  40  10  40
> >     3:  40  40  40  10
> 
> Can it happen that it introduces a new distance in the table? One that
> hasn't been seen before? This example only has 10 and 40, but suppose
> the new node lands at distance 20 (or 80); can such a thing happen?
> 
> If not; why not?

Yes distances can be 20, 40 or 80. There is nothing that makes the node
distance to be 40 always.

> So you're relying on sched_domain_numa_masks_set/clear() to fix this up,
> but that in turn relies on the sched_domain_numa_levels thing to stay
> accurate.
> 
> This all seems very fragile and unfortunate.
> 

Any reasons why this is fragile?

-- 
Thanks and Regards
Srikar Dronamraju