On Wed, 10 Nov 2021 12:01:11 +0100 David Hildenbrand <da...@redhat.com> wrote:
> On 10.11.21 11:33, Igor Mammedov wrote: > > On Fri, 5 Nov 2021 23:47:37 +1100 > > Gavin Shan <gs...@redhat.com> wrote: > > > >> Hi Drew and Igor, > >> > >> On 11/2/21 6:39 PM, Andrew Jones wrote: > >>> On Tue, Nov 02, 2021 at 10:44:08AM +1100, Gavin Shan wrote: > >>>> > >>>> Yeah, I agree. I don't have strong sense to expose these empty nodes > >>>> for now. Please ignore the patch. > >>>> > >>> > >>> So were describing empty numa nodes on the command line ever a reasonable > >>> thing to do? What happens on x86 machine types when describing empty numa > >>> nodes? I'm starting to think that the solution all along was just to > >>> error out when a numa node has memory size = 0... > > > > memory less nodes are fine as long as there is another type of device > > that describes a node (apic/gic/...). > > But there is no way in spec to describe completely empty nodes, > > and I dislike adding out of spec entries just to fake an empty node. > > > > There are reasonable *upcoming* use cases for initially completely empty > NUMA nodes with virtio-mem: being able to expose a dynamic amount of > performance-differentiated memory to a VM. I don't know of any existing > use cases that would require that as of now. > > Examples include exposing HBM or PMEM to the VM. Just like on real HW, > this memory is exposed via cpu-less, special nodes. In contrast to real > HW, the memory is hotplugged later (I don't think HW supports hotplug > like that yet, but it might just be a matter of time). I suppose some of that maybe covered by GENERIC_AFFINITY entries in SRAT some by MEMORY entries. Or nodes created dynamically like with normal hotplug memory. > The same should be true when using DIMMs instead of virtio-mem in this > example. > > > > >> Sorry for the delay as I spent a few days looking into linux virtio-mem > >> driver. I'm afraid we still need this patch for ARM64. I don't think x86 > > > > does it behave the same way is using pc-dimm hotplug instead of virtio-mem? > > > > CCing David > > as it might be virtio-mem issue. > > Can someone share the details why it's a problem on arm64 but not on > x86-64? I assume this really only applies when having a dedicated, empty > node -- correct? > > > > > PS: > > maybe for virtio-mem-pci, we need to add GENERIC_AFFINITY entry into SRAT > > and describe it as PCI device (we don't do that yet if I'm no mistaken). > > virtio-mem exposes the PXM itself, and avoids exposing it memory via any > kind of platform specific firmware maps. The PXM gets translated in the > guest accordingly. For now there was no need to expose this in SRAT -- > the SRAT is really only used to expose the maximum possible PFN to the > VM, just like it would have to be used to expose "this is a possible node". > > Of course, we could use any other paravirtualized interface to expose > both information. For example, on s390x, I'll have to introduce a new > hypercall to query the "device memory region" to detect the maximum > possible PFN, because existing interfaces don't allow for that. For now > we're ruinning SRAT to expose "maximum possible PFN" simply because it's > easy to re-use. > > But I assume that hotplugging a DIMM to an empty node will have similar > issues on arm64. > > > > >> has this issue even though I didn't experiment on X86. For example, I > >> have the following command lines. The hot added memory is put into node#0 > >> instead of node#2, which is wrong. > > I assume Linux will always fallback to node 0 if node X is not possible > when translating the PXM. I tested how x86 behaves, with pc-dimm, and it seems that fc43 guest works only sometimes. cmd: -numa node,memdev=mem,cpus=0 -numa node,cpus=1 -numa node -numa node 1: hotplug into the empty last node creates a new node dynamically 2: hotplug into intermediate empty node (last-1) is broken, memory goes into the first node We should check if it possible to fix guest instead of adding bogus SRAT entries.
