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

[Daniel Henrique Barboza <danielhb413@gmail.com>]

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
>>>>>>> equivalent.
>>>>>>>
>>>>>>> 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
>>>>>>> condition.
>>>>>>
>>>>>> 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
>> qemu.
>>
>> 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
>>> today.
>>
>> Fixing what value, exactly?
> 
> The value of the "ibm,chip-id" since we are keeping the property under
> QEMU.

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,
e.g.:

-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.



Thanks,


DHB




> 
>>> 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 :
>>>
>>> http://patchwork.ozlabs.org/project/linuxppc-dev/patch/20210303174857.1760393-9-clg@kaod.org/
>>>
>>> The node id is a key parameter for system resource management, memory
>>> allocation, interrupt affinity, etc. Linux scales much better if used
>>> correctly.
>>
>> 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.
> 
>