Re: [RFC][PATCH 5/6] x86/topo: Fix SNC topology mess

[Chen, Yu C]

Hi Peter,

On 2/26/2026 6:49 PM, Peter Zijlstra wrote:
> +	int u = __num_nodes_per_package;

Yes, this is much simpler, thanks for the patch!

> +	long d = 0;
> +	int x, y;
> +
> +	/*
> +	 * Is this a unit cluster on the trace?
> +	 */
> +	if ((i / u) == (j / u))
> +		return node_distance(i, j);

If the number of nodes per package is 3, we assume that
every 3 consecutive nodes are SNC siblings (on the same
trace):node0, node1, and node2 are SNC siblings, while
node3, node4, and node5 form another group of SNC siblings.

I have a curious thought: could it be possible that
node0, node2, and node4 are SNC siblings, and node1,
node3, and node5 are another set of SNC siblings instead?

Then I studied the code a little more, node ids are dynamically
allocated via the acpi_map_pxm_to_node, so the assignment of node
ids depends on the order in which each processor affinity structure
is listed in the SRAT table. For example, suppose CPU0 belongs to
package0 and CPU1 belongs to package1, but their entries are placed
consecutively in the SRAT. In this case, the Proximity Domain of
CPU0 would be mapped to node0 via acpi_map_pxm_to_node, and CPU1’s
Proximity Domain would be assigned node1. The logic above would
then treat them as belonging to the same package, even though they
are physically in different packages. However, I believe such a
scenario is unlikely to occur in practice in the BIOS and if it
happens it should be a BIOS bug if I understand correctly.

thanks,
Chenyu