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

Peter Zijlstra
2026-02-25T16:32:58Z

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;
}