[PATCH v2 1/2] sched/topology: Skip updating masks for non-online nodes

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

Currently scheduler doesn't check if node is online before adding CPUs
to the node mask. However on some architectures, node distance is only
available for nodes that are online. Its not sure how much to rely on
the node distance, when one of the nodes is offline.

If said node distance is fake (since one of the nodes is offline) and
the actual node distance is different, then the cpumask of such nodes
when the nodes become becomes online will be wrong.

This can cause topology_span_sane to throw up a warning message and the
rest of the topology being not updated properly.

Resolve this by skipping update of cpumask for nodes that are not
online.

However by skipping, relevant CPUs may not be set when nodes are
onlined. i.e when coming up with NUMA masks at a certain NUMA distance,
CPUs that are part of other nodes, which are already online will not be
part of the NUMA mask. Hence the first time, a CPU is added to the newly
onlined node, add the other CPUs to the numa_mask.

Cc: LKML <linux-kernel@vger.kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Nathan Lynch <nathanl@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Geetika Moolchandani <Geetika.Moolchandani1@ibm.com>
Cc: Laurent Dufour <ldufour@linux.ibm.com>
Reported-by: Geetika Moolchandani <Geetika.Moolchandani1@ibm.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
Changelog v1->v2:
v1 link: http://lore.kernel.org/lkml/20210520154427.1041031-4-srikar@linux.vnet.ibm.com/t/#u
Update the NUMA masks, whenever 1st CPU is added to cpuless node

 kernel/sched/topology.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index b77ad49dc14f..f25dbcab4fd2 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1833,6 +1833,9 @@ void sched_init_numa(void)
 			sched_domains_numa_masks[i][j] = mask;
 
 			for_each_node(k) {
+				if (!node_online(j))
+					continue;
+
 				if (sched_debug() && (node_distance(j, k) != node_distance(k, j)))
 					sched_numa_warn("Node-distance not symmetric");
 
@@ -1891,12 +1894,30 @@ void sched_init_numa(void)
 void sched_domains_numa_masks_set(unsigned int cpu)
 {
 	int node = cpu_to_node(cpu);
-	int i, j;
+	int i, j, empty;
 
+	empty = cpumask_empty(sched_domains_numa_masks[0][node]);
 	for (i = 0; i < sched_domains_numa_levels; i++) {
 		for (j = 0; j < nr_node_ids; j++) {
-			if (node_distance(j, node) <= sched_domains_numa_distance[i])
+			if (!node_online(j))
+				continue;
+
+			if (node_distance(j, node) <= sched_domains_numa_distance[i]) {
 				cpumask_set_cpu(cpu, sched_domains_numa_masks[i][j]);
+
+				/*
+				 * We skip updating numa_masks for offline
+				 * nodes. However now that the node is
+				 * finally online, CPUs that were added
+				 * earlier, should now be accommodated into
+				 * newly oneline node's numa mask.
+				 */
+				if (node != j && empty) {
+					cpumask_or(sched_domains_numa_masks[i][node],
+							sched_domains_numa_masks[i][node],
+							sched_domains_numa_masks[0][j]);
+				}
+			}
 		}
 	}
 }
-- 
2.27.0