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

[Tim Chen]

On Fri, 2026-02-27 at 01:07 +0800, Chen, Yu C wrote:
> 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.
> 
> 

May be a good idea to sanity check that the nodes in the first unit cluster
has the same package id and give a WARNING if that's not the case.

Tim