x86-64, NUMA: Don't call numa_set_distanc() for all possible node com…

…binations during emulation

The distance transforming in numa_emulation() used to call
numa_set_distance() for all MAX_NUMNODES * MAX_NUMNODES node
combinations regardless of which are enabled.  As numa_set_distance()
ignores all out-of-bound distance settings, this doesn't cause any
problem other than looping unnecessarily many times during boot.

However, as MAX_NUMNODES * MAX_NUMNODES can be pretty high, update the
code such that it iterates through only the enabled combinations.

Yinghai Lu identified the issue and provided an initial patch to
address the issue; however, the patch was incorrect in that it didn't
build emulated distance table when there's no physical distance table
and unnecessarily complex.

  http://thread.gmane.org/gmane.linux.kernel/1107986/focus=1107988

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Yinghai Lu <yinghai@kernel.org>

arch/x86/mm/numa_emulation.c

@@ -301,7 +301,7 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
 	const u64 max_addr = max_pfn << PAGE_SHIFT;
 	u8 *phys_dist = NULL;
 	size_t phys_size = numa_dist_cnt * numa_dist_cnt * sizeof(phys_dist[0]);
-	int dfl_phys_nid;
+	int max_emu_nid, dfl_phys_nid;
 	int i, j, ret;
 
 	if (!emu_cmdline)
@@ -358,12 +358,17 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
 					node_distance(i, j);
 	}
 
-	/* determine the default phys nid to use for unmapped nodes */
+	/*
+	 * Determine the max emulated nid and the default phys nid to use
+	 * for unmapped nodes.
+	 */
+	max_emu_nid = 0;
 	dfl_phys_nid = NUMA_NO_NODE;
 	for (i = 0; i < ARRAY_SIZE(emu_nid_to_phys); i++) {
 		if (emu_nid_to_phys[i] != NUMA_NO_NODE) {
-			dfl_phys_nid = emu_nid_to_phys[i];
-			break;
+			max_emu_nid = i;
+			if (dfl_phys_nid == NUMA_NO_NODE)
+				dfl_phys_nid = emu_nid_to_phys[i];
 		}
 	}
 	if (dfl_phys_nid == NUMA_NO_NODE) {
@@ -393,14 +398,10 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
 		if (emu_nid_to_phys[i] == NUMA_NO_NODE)
 			emu_nid_to_phys[i] = dfl_phys_nid;
 
-	/*
-	 * Transform distance table.  numa_set_distance() ignores all
-	 * out-of-bound distances.  Just call it for every possible node
-	 * combination.
-	 */
+	/* transform distance table */
 	numa_reset_distance();
-	for (i = 0; i < MAX_NUMNODES; i++) {
-		for (j = 0; j < MAX_NUMNODES; j++) {
+	for (i = 0; i < max_emu_nid + 1; i++) {
+		for (j = 0; j < max_emu_nid + 1; j++) {
 			int physi = emu_nid_to_phys[i];
 			int physj = emu_nid_to_phys[j];
 			int dist;