On Mon, Mar 16, 2026 at 02:07:42PM -0700, Long Li wrote:
> When hv_pci_assign_numa_node() processes a device that does not have
> HV_PCI_DEVICE_FLAG_NUMA_AFFINITY set or has an out-of-range
> virtual_numa_node, the device NUMA node is left unset. On x86_64,
> the uninitialized default happens to be 0, but on ARM64 it is
> NUMA_NO_NODE (-1).
>
> Tests show that when no NUMA information is available from the Hyper-V
> host, devices perform best when assigned to node 0. With NUMA_NO_NODE
> the kernel may spread work across NUMA nodes, which degrades
> performance on Hyper-V, particularly for high-throughput devices like
> MANA.
>
> Always set the device NUMA node to 0 before the conditional NUMA
> affinity check, so that devices get a performant default when the host
> provides no NUMA information, and behavior is consistent on both
> x86_64 and ARM64.
>
> Fixes: 999dd956d838 ("PCI: hv: Add support for protocol 1.3 and support
> PCI_BUS_RELATIONS2")
> Signed-off-by: Long Li <[email protected]>
I can pick this up next week. PCI maintainers, if you want this to go
through your tree instead, please let me know.
Wei
> ---
> Changes in v2:
> - Rewrite commit message to focus on performance as the primary
> motivation: NUMA_NO_NODE causes the kernel to spread work across
> NUMA nodes, degrading performance on Hyper-V
>
> drivers/pci/controller/pci-hyperv.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/drivers/pci/controller/pci-hyperv.c
> b/drivers/pci/controller/pci-hyperv.c
> index 2c7a406b4ba8..38a790f642a1 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -2485,6 +2485,14 @@ static void hv_pci_assign_numa_node(struct
> hv_pcibus_device *hbus)
> if (!hv_dev)
> continue;
>
> + /*
> + * If the Hyper-V host doesn't provide a NUMA node for the
> + * device, default to node 0. With NUMA_NO_NODE the kernel
> + * may spread work across NUMA nodes, which degrades
> + * performance on Hyper-V.
> + */
> + set_dev_node(&dev->dev, 0);
> +
> if (hv_dev->desc.flags & HV_PCI_DEVICE_FLAG_NUMA_AFFINITY &&
> hv_dev->desc.virtual_numa_node < num_possible_nodes())
> /*
> --
> 2.43.0
>
