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

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Mar 29, 2021 at 03:32:37PM -0300, Daniel Henrique Barboza wrote:
> 
> 
> 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.

Right... and I'm still not seeing why that's a problem.  AFAICT that's
a possible, if unexpected, situation under real hardware - though
maybe not for POWER9 specifically.

> 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.

Even on powernv, I'm not clear why chip-id is tied into the NUMA
configuration, rather than getting all the NUMA info from
associativity properties.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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