Re: [PATCH v2] sched/topology: Check average distances to remote packages

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Feb 25, 2026 at 04:44:09PM +0100, Peter Zijlstra wrote:

> Yes, so this assumes that all u sized clusters on the trace are similar
> and 'sane' without verification.

That gave me an idea; how's this then?

---
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 5cd6950ab672..b1e464fd98c0 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -513,33 +513,99 @@ static void __init build_sched_topology(void)
 }
 
 #ifdef CONFIG_NUMA
-static int sched_avg_remote_distance;
-static int avg_remote_numa_distance(void)
+
+static bool slit_cluster_symmetric(int i, int j, int n)
 {
-	int i, j;
-	int distance, nr_remote, total_distance;
+	WARN_ON_ONCE((i % n) || (j % n));
 
-	if (sched_avg_remote_distance > 0)
-		return sched_avg_remote_distance;
-
-	nr_remote = 0;
-	total_distance = 0;
-	for_each_node_state(i, N_CPU) {
-		for_each_node_state(j, N_CPU) {
-			distance = node_distance(i, j);
-
-			if (distance >= REMOTE_DISTANCE) {
-				nr_remote++;
-				total_distance += distance;
-			}
+	for (int k = i; k < i + n; k++) {
+		for (int l = k; l < j + n; l++) {
+			if (node_distance(k, l) != node_distance(k, l))
+				return false;
 		}
 	}
-	if (nr_remote)
-		sched_avg_remote_distance = total_distance / nr_remote;
-	else
-		sched_avg_remote_distance = REMOTE_DISTANCE;
 
-	return sched_avg_remote_distance;
+	return true;
+}
+
+static bool slit_cluster_match(int i, int j, int x, int y, int n)
+{
+	WARN_ON_ONCE((i % n) || (j % n) || (x % n) || (y % n));
+
+	for (int k = 0; k < n; k++) {
+		for (int l = k; l < n; l++) {
+			if (node_distance(i + k, j + l) != node_distance(x + k, y + l))
+				return false;
+		}
+	}
+
+	return true;
+}
+
+/*
+ * Find the largest symmetric,repeating cluster in an attempt to identify the
+ * unit size.
+ */
+static int slit_cluster_size(void)
+{
+	int nodes = num_possible_nodes();
+
+	/*
+	 * There are at least 2 packages; so half-nodes is the largest
+	 * possible unit, go down from that.
+	 */
+	for (int u = nodes / 2; u; u--) {
+		/*
+		 * If u doesn't divide nodes, it can't be a unit.
+		 */
+		if (nodes % u)
+			continue;
+
+		/*
+		 * Unit must be symmetric,
+		 */
+		if (!slit_cluster_symmetric(0, 0, u))
+			continue;
+
+		/*
+		 * and repeating.
+		 */
+		if (slit_cluster_match(0, 0, u, u, u))
+			return u;
+	}
+
+	return nodes;
+}
+
+static int slit_cluster_distance(int i, int j)
+{
+	static int u = 0;
+	long d = 0;
+	int x, y;
+
+	if (!u)
+		u = slit_cluster_size();
+
+	/*
+	 * Is this a unit cluster on the trace?
+	 */
+	if ((i / u) == (j / u))
+		return node_distance(i, j);
+
+	/*
+	 * Off-trace cluster, return average of the cluster to force symmetry.
+	 */
+	x = i - (i % u);
+	y = j - (j % u);
+
+	for (i = x; i < x + u; i++) {
+		for (j = y; j < y + u; j++) {
+			d += node_distance(i, j);
+			d += node_distance(j, i);
+		}
+	}
+
+	return d / (2*u*u);
 }
 
 int arch_sched_node_distance(int from, int to)
@@ -550,8 +616,7 @@ int arch_sched_node_distance(int from, int to)
 	case INTEL_GRANITERAPIDS_X:
 	case INTEL_ATOM_DARKMONT_X:
 
-		if (!x86_has_numa_in_package || topology_max_packages() == 1 ||
-		    d < REMOTE_DISTANCE)
+		if (!x86_has_numa_in_package || topology_max_packages() == 1)
 			return d;
 
 		/*
@@ -564,19 +629,8 @@ int arch_sched_node_distance(int from, int to)
 		 * in the remote package in the same sched group.
 		 * Simplify NUMA domains and avoid extra NUMA levels including
 		 * different remote NUMA nodes and local nodes.
-		 *
-		 * GNR and CWF don't expect systems with more than 2 packages
-		 * and more than 2 hops between packages. Single average remote
-		 * distance won't be appropriate if there are more than 2
-		 * packages as average distance to different remote packages
-		 * could be different.
 		 */
-		WARN_ONCE(topology_max_packages() > 2,
-			  "sched: Expect only up to 2 packages for GNR or CWF, "
-			  "but saw %d packages when building sched domains.",
-			  topology_max_packages());
-
-		d = avg_remote_numa_distance();
+		return slit_cluster_distance(from, to);
 	}
 	return d;
 }