Re: [PATCH v2 0/4] NUMA: Apply socket-NUMA-node boundary for aarch64 and RiscV machines

[Gavin Shan <gshan@redhat.com>]

On 2/24/23 12:18 AM, Daniel Henrique Barboza wrote:
> On 2/23/23 05:13, Gavin Shan wrote:
>> For arm64 and RiscV architecture, the driver (/base/arch_topology.c) is
>> used to populate the CPU topology in the Linux guest. It's required that
>> the CPUs in one socket can't span mutiple NUMA nodes. Otherwise, the Linux
>> scheduling domain can't be sorted out, as the following warning message
>> indicates. To avoid the unexpected confusion, this series attempts to
>> rejects such kind of insane configurations.
>>
>>     -smp 6,maxcpus=6,sockets=2,clusters=1,cores=3,threads=1 \
>>     -numa node,nodeid=0,cpus=0-1,memdev=ram0                \
>>     -numa node,nodeid=1,cpus=2-3,memdev=ram1                \
>>     -numa node,nodeid=2,cpus=4-5,memdev=ram2                \
> 
> 
> And why is this a QEMU problem? This doesn't hurt ACPI.
> 
> Also, this restriction impacts breaks ARM guests in the wild that are running
> non-Linux OSes. I don't see why we should impact use cases that has nothing to
> do with Linux Kernel feelings about sockets - NUMA nodes exclusivity.
> 

With above configuration, CPU-0/1/2 are put into socket-0-cluster-0 while CPU-3/4/5
are put into socket-1-cluster-0, meaning CPU-2/3 belong to different socket and
cluster. However, CPU-2/3 are associated with NUMA node-1. In summary, multiple
CPUs in different clusters and sockets have been associated with one NUMA node.

If I'm correct, the configuration isn't sensible in a baremetal environment and
same Linux kernel is supposed to work well for baremetal and virtualized machine.
So I think QEMU needs to emulate the topology as much as we can to match with the
baremetal environment. It's the reason why I think it's a QEMU problem even it
doesn't hurt ACPI. As I said in the reply to Daniel P. Berrangé <berrange@redhat.com>
in another thread, we may need to gurantee that the CPUs in one cluster can't be
split to multiple NUMA nodes, which matches with the baremetal environment, as I
can understand.

Right, the restriction to have socket-NUMA-node or cluster-NUMA-node boundary will
definitely break the configurations running in the wild.

Thanks,
Gavin

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