powerpc/numa: Use cpu node map of first sibling thread

All the sibling threads of a core have to be part of the same node.
To ensure that all the sibling threads map to the same node, always
lookup/update the cpu-to-node map of the first thread in the core.
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200129135301.24739-4-srikar@linux.vnet.ibm.com

arch/powerpc/mm/numa.c

@@ -467,15 +467,20 @@ static int of_drconf_to_nid_single(struct drmem_lmb *lmb)
  */
 static int numa_setup_cpu(unsigned long lcpu)
 {
-	int nid = NUMA_NO_NODE;
 	struct device_node *cpu;
+	int fcpu = cpu_first_thread_sibling(lcpu);
+	int nid = NUMA_NO_NODE;
 
 	/*
 	 * If a valid cpu-to-node mapping is already available, use it
 	 * directly instead of querying the firmware, since it represents
 	 * the most recent mapping notified to us by the platform (eg: VPHN).
+	 * Since cpu_to_node binding remains the same for all threads in the
+	 * core. If a valid cpu-to-node mapping is already available, for
+	 * the first thread in the core, use it.
 	 */
-	if ((nid = numa_cpu_lookup_table[lcpu]) >= 0) {
+	nid = numa_cpu_lookup_table[fcpu];
+	if (nid >= 0) {
 		map_cpu_to_node(lcpu, nid);
 		return nid;
 	}
@@ -496,6 +501,19 @@ static int numa_setup_cpu(unsigned long lcpu)
 	if (nid < 0 || !node_possible(nid))
 		nid = first_online_node;
 
+	/*
+	 * Update for the first thread of the core. All threads of a core
+	 * have to be part of the same node. This not only avoids querying
+	 * for every other thread in the core, but always avoids a case
+	 * where virtual node associativity change causes subsequent threads
+	 * of a core to be associated with different nid. However if first
+	 * thread is already online, expect it to have a valid mapping.
+	 */
+	if (fcpu != lcpu) {
+		WARN_ON(cpu_online(fcpu));
+		map_cpu_to_node(fcpu, nid);
+	}
+
 	map_cpu_to_node(lcpu, nid);
 	of_node_put(cpu);
 out: