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

[Tim Chen]

On Fri, 2026-02-27 at 14:01 +0100, Peter Zijlstra wrote:
> On Thu, Feb 26, 2026 at 11:00:38AM -0800, Tim Chen wrote:
> > 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.
> 
> But then we'd also have check the second cluster is another package. And
> if we're checking that, we might as well check they're symmetric.
> 
> Is this sufficiently paranoid for you? :-)
> 

Thanks. Looks pretty good and hopefully those warnings will
never be triggered.

Tim

> 
> ---
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -61,6 +61,7 @@
>  #include <linux/cpuhotplug.h>
>  #include <linux/mc146818rtc.h>
>  #include <linux/acpi.h>
> +#include <linux/once_lite.h>
>  
>  #include <asm/acpi.h>
>  #include <asm/cacheinfo.h>
> @@ -506,12 +507,58 @@ static void __init build_sched_topology(
>  }
>  
>  #ifdef CONFIG_NUMA
> +static bool slit_cluster_symmetric(int N)
> +{
> +	for (int k = 0; k < __num_nodes_per_package; k++) {
> +		for (int l = k; l < __num_nodes_per_package; l++) {
> +			if (node_distance(N + k, N + l) != 
> +			    node_distance(N + l, N + k))
> +				return false;
> +		}
> +	}
> +
> +	return true;
> +}
> +
> +static u32 slit_cluster_package(int N)
> +{
> +	u32 pkg_id = ~0;
> +
> +	for (int n = 0; n < __num_nodes_per_package; n++) {
> +		const struct cpumask *cpus = cpumask_of_node(N + n);
> +		int cpu;
> +
> +		for_each_cpu(cpu, cpus) {
> +			u32 id = topology_logical_package_id(cpu);
> +			if (pkg_id == ~0)
> +				pkg_id = id;
> +			if (pkg_id != id)
> +				return ~0;
> +		}
> +	}
> +
> +	return pkg_id;
> +}
> +
> +/* If you NUMA_EMU on top of SNC, you get to keep the pieces */
> +static void slit_validate(void)
> +{
> +	u32 pkg1 = slit_cluster_package(0);
> +	u32 pkg2 = slit_cluster_package(__num_nodes_per_package);
> +	WARN_ON(pkg1 == ~0 || pkg2 == ~0 || pkg1 == pkg2);
> +
> +	WARN_ON(!slit_cluster_symmetric(0));
> +	WARN_ON(!slit_cluster_symmetric(__num_nodes_per_package));
> +}
> +
>  static int slit_cluster_distance(int i, int j)
>  {
>  	int u = __num_nodes_per_package;
>  	long d = 0;
>  	int x, y;
>  
> +	DO_ONCE_LITE(slit_validate);
> +
>  	/*
>  	 * Is this a unit cluster on the trace?
>  	 */
>