Re: [PATCH -V3 1/2] NUMA balancing: fix NUMA topology for systems with CPU-less nodes

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 14, 2022 at 08:15:52PM +0800, Huang Ying wrote:

> This isn't a practical problem now yet.  Because the PMEM nodes (node
> 2 and node 3 in example system) are offlined by default during system
> boot.  So init_numa_topology_type() called during system boot will
> ignore them and set sched_numa_topology_type to NUMA_DIRECT.  And
> init_numa_topology_type() is only called at runtime when a CPU of a
> never-onlined-before node gets plugged in.  And there's no CPU in the
> PMEM nodes.  But it appears better to fix this to make the code more
> robust.

IIRC there are pre-existing issues with this; namely the distance_map is
created for all nodes, online or not, therefore the levels and
max_distance include the pmem stuff.

At the same time, the numa_topolog_type() uses those values, and the
only reason it 'worked' is because the combination of arguments fails to
hit any of the existing types and exits without setting a type,
defaulting to NUMA_DIRECT by 'accident' of that being type 0 and
bss/data being 0 initialized.

Also, Power (and possibly other architectures) already have CPU-less
nodes and are similarly suffering issues.

Anyway, aside from this the patches look like they should do.

There's a few niggles, like using READ_ONCE() on sched_max_numa_distance
without using WRITE_ONCE() (see below) and having
sched_domains_numa_distance and sched_domains_numa_masks separate RCU
variables (that could go side-ways if there were a function using both,
afaict there isn't and I couldn't be bothered changing that, but it's
something to keep in mind).

I'll go queue these, thanks!

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1259,11 +1259,10 @@ static bool numa_is_active_node(int nid,
 
 /* Handle placement on systems where not all nodes are directly connected. */
 static unsigned long score_nearby_nodes(struct task_struct *p, int nid,
-					int maxdist, bool task)
+					int lim_dist, bool task)
 {
 	unsigned long score = 0;
-	int node;
-	int sys_max_dist;
+	int node, max_dist;
 
 	/*
 	 * All nodes are directly connected, and the same distance
@@ -1273,7 +1272,7 @@ static unsigned long score_nearby_nodes(
 		return 0;
 
 	/* sched_max_numa_distance may be changed in parallel. */
-	sys_max_dist = READ_ONCE(sched_max_numa_distance);
+	max_dist = READ_ONCE(sched_max_numa_distance);
 	/*
 	 * This code is called for each node, introducing N^2 complexity,
 	 * which should be ok given the number of nodes rarely exceeds 8.
@@ -1286,7 +1285,7 @@ static unsigned long score_nearby_nodes(
 		 * The furthest away nodes in the system are not interesting
 		 * for placement; nid was already counted.
 		 */
-		if (dist >= sys_max_dist || node == nid)
+		if (dist >= max_dist || node == nid)
 			continue;
 
 		/*
@@ -1296,8 +1295,7 @@ static unsigned long score_nearby_nodes(
 		 * "hoplimit", only nodes closer by than "hoplimit" are part
 		 * of each group. Skip other nodes.
 		 */
-		if (sched_numa_topology_type == NUMA_BACKPLANE &&
-					dist >= maxdist)
+		if (sched_numa_topology_type == NUMA_BACKPLANE && dist >= lim_dist)
 			continue;
 
 		/* Add up the faults from nearby nodes. */
@@ -1315,8 +1313,8 @@ static unsigned long score_nearby_nodes(
 		 * This seems to result in good task placement.
 		 */
 		if (sched_numa_topology_type == NUMA_GLUELESS_MESH) {
-			faults *= (sys_max_dist - dist);
-			faults /= (sys_max_dist - LOCAL_DISTANCE);
+			faults *= (max_dist - dist);
+			faults /= (max_dist - LOCAL_DISTANCE);
 		}
 
 		score += faults;
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1927,7 +1927,7 @@ void sched_init_numa(int offline_node)
 	sched_domain_topology = tl;
 
 	sched_domains_numa_levels = nr_levels;
-	sched_max_numa_distance = sched_domains_numa_distance[nr_levels - 1];
+	WRITE_ONCE(sched_max_numa_distance, sched_domains_numa_distance[nr_levels - 1]);
 
 	init_numa_topology_type(offline_node);
 }