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Re: [PATCH 1/2] spapr: number of SMP sockets must be equal to NUMA nodes

From: Daniel Henrique Barboza
Subject: Re: [PATCH 1/2] spapr: number of SMP sockets must be equal to NUMA nodes
Date: Thu, 25 Mar 2021 07:15:01 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 3/25/21 5:56 AM, Cédric Le Goater wrote:
On 3/25/21 3:10 AM, David Gibson wrote:
On Tue, Mar 23, 2021 at 02:21:33PM -0300, Daniel Henrique Barboza wrote:

On 3/22/21 10:03 PM, David Gibson wrote:
On Fri, Mar 19, 2021 at 03:34:52PM -0300, Daniel Henrique Barboza wrote:
Kernel commit 4bce545903fa ("powerpc/topology: Update
topology_core_cpumask") cause a regression in the pseries machine when
defining certain SMP topologies [1]. The reasoning behind the change is
explained in kernel commit 4ca234a9cbd7 ("powerpc/smp: Stop updating
cpu_core_mask"). In short, cpu_core_mask logic was causing troubles with
large VMs with lots of CPUs and was changed by cpu_cpu_mask because, as
far as the kernel understanding of SMP topologies goes, both masks are

Further discussions in the kernel mailing list [2] shown that the
powerpc kernel always considered that the number of sockets were equal
to the number of NUMA nodes. The claim is that it doesn't make sense,
for Power hardware at least, 2+ sockets being in the same NUMA node. The
immediate conclusion is that all SMP topologies the pseries machine were
supplying to the kernel, with more than one socket in the same NUMA node
as in [1], happened to be correctly represented in the kernel by
accident during all these years.

There's a case to be made for virtual topologies being detached from
hardware constraints, allowing maximum flexibility to users. At the same
time, this freedom can't result in unrealistic hardware representations
being emulated. If the real hardware and the pseries kernel don't
support multiple chips/sockets in the same NUMA node, neither should we.

Starting in 6.0.0, all sockets must match an unique NUMA node in the
pseries machine. qtest changes were made to adapt to this new

Oof.  I really don't like this idea.  It means a bunch of fiddly work
for users to match these up, for no real gain.  I'm also concerned
that this will require follow on changes in libvirt to not make this a
really cryptic and irritating point of failure.

Haven't though about required Libvirt changes, although I can say that there
will be some amount to be mande and it will probably annoy existing users
(everyone that has a multiple socket per NUMA node topology).

There is not much we can do from the QEMU layer aside from what I've proposed
here. The other alternative is to keep interacting with the kernel folks to
see if there is a way to keep our use case untouched.

Right.  Well.. not necessarily untouched, but I'm hoping for more
replies from Cédric to my objections and mpe's.  Even with sockets
being a kinda meaningless concept in PAPR, I don't think tying it to
NUMA nodes makes sense.

I did a couple of replies in different email threads but maybe not
to all. I felt it was going nowhere :/ Couple of thoughts,

Shouldn't we get rid of the socket concept, die also, under pseries
since they don't exist under PAPR ? We only have numa nodes, cores,
threads AFAICT.

I don't think we work with 'die'.

Getting rid of the 'socket' representation is sensible regarding PAPR,
but the effect for pseries will be similar to what this patch is already
doing: users could have multiple sockets in the same NUMA node, and then
they won't. Either because we got rid of the 'socket' representation or
because socket == NUMA node.

Should we diverged from PAPR and add extra DT properties "qemu,..." ?
There are a couple of places where Linux checks for the underlying
hypervisor already.

This also means that
'ibm,chip-id' will probably remain in use since it's the only place where
we inform cores per socket information to the kernel.

Well.. unless we can find some other sensible way to convey that
information.  I haven't given up hope for that yet.

Well, we could start by fixing the value in QEMU. It is broken today.

I'll look into it. It makes more sense to talk about keeping it when
it's working properly.


This is all coming from some work we did last year to evaluate our HW
(mostly for XIVE) on 2s, 4s, 16s systems on baremetal, KVM and PowerVM.
We saw some real problems because Linux did not have a clear view of the
topology. See the figures here :


The node id is a key parameter for system resource management, memory
allocation, interrupt affinity, etc. Linux scales much better if used


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