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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: Mon, 29 Mar 2021 15:32:37 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 3/29/21 12:32 PM, Cédric Le Goater wrote:
On 3/29/21 6:20 AM, David Gibson wrote:
On Thu, Mar 25, 2021 at 09:56:04AM +0100, 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,

I think I saw some of those, but maybe not all.

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.

Theoretically, yes.  I'm not sure it's really practical, though, since
AFAICT, both qemu and the kernel have the notion of sockets (though
not dies) built into generic code.

Yes. But, AFAICT, these topology notions have not reached "arch/powerpc"
and PPC Linux only has a NUMA node id, on pseries and powernv.

It does mean that one possible approach here - maybe the best one - is
to simply declare that sockets are meaningless under, so we simply
don't expect what the guest kernel reports to match what's given to

It'd be nice to avoid that if we can: in a sense it's just cosmetic,
but it is likely to surprise and confuse people.

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

Fixing what value, exactly?

The value of the "ibm,chip-id" since we are keeping the property under

David, I believe this has to do with the discussing we had last Friday.

I mentioned that the ibm,chip-id property is being calculated in a way that
promotes the same ibm,chip-id in CPUs that belongs to different NUMA nodes,

-smp 4,cores=4,maxcpus=8,threads=1 \
-numa node,nodeid=0,cpus=0-1,cpus=4-5,memdev=ram-node0 \
-numa node,nodeid=1,cpus=2-3,cpus=6-7,memdev=ram-node1

$ dtc -I dtb -O dts fdt.dtb | grep -B2 ibm,chip-id
                        ibm,associativity = <0x05 0x00 0x00 0x00 0x00 0x00>;
                        ibm,pft-size = <0x00 0x19>;
                        ibm,chip-id = <0x00>;
                        ibm,associativity = <0x05 0x00 0x00 0x00 0x00 0x01>;
                        ibm,pft-size = <0x00 0x19>;
                        ibm,chip-id = <0x00>;
                        ibm,associativity = <0x05 0x01 0x01 0x01 0x01 0x02>;
                        ibm,pft-size = <0x00 0x19>;
                        ibm,chip-id = <0x00>;
                        ibm,associativity = <0x05 0x01 0x01 0x01 0x01 0x03>;
                        ibm,pft-size = <0x00 0x19>;
                        ibm,chip-id = <0x00>;

We assign ibm,chip-id=0x0 to CPUs 0-3, but CPUs 2-3 are located in a different
NUMA node than 0-1. This would mean that the same socket would belong to
different NUMA nodes at the same time.

I believe this is what Cedric wants to be addressed. Given that the property is
called after the OPAL property ibm,chip-id, the kernel expects that the property
will have the same semantics as in OPAL.



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

Well, sure.  And we have all the ibm,associativity stuff to convey the
node ids to the guest (which has its own problems, but not that are
relevant here).  What's throwing me is why getting node IDs correct
has anything to do with socket numbers.

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