Re: [PATCH v2] PCI: hv: Set default NUMA node to 0 for devices without affinity info

[Wei Liu <wei.liu@kernel.org>]

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 <longli@microsoft.com>

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
>