New comment explains it all -- previous code was failing to find the

Nth core, so it fell over to try to find the Nth PU.

-----

hwloc isn't able to find cores on all platforms.  Example: PPC64
running RHEL 5.4 (linux kernel 2.6.18) only reports NUMA nodes and
PU's.  Fine.

However, note that hwloc_get_obj_by_type() will return NULL in 2
(effectively) different cases:

- no objects of the requested type were found
- the Nth object of the requested type was not found

So first we have to see if we can find *any* cores by looking for the
0th core.  If we find it, then try to find the Nth core.  Otherwise,
try to find the Nth PU.

This commit was SVN r24632.

opal/mca/paffinity/hwloc/paffinity_hwloc_module.c

@@ -412,19 +412,32 @@ static int module_get_core_info(int socket, int *num_cores)
 static int module_get_physical_processor_id(int logical_processor_id,
                                             int *physical_processor_id)
 {
+    int obj_type = HWLOC_OBJ_CORE;
     hwloc_obj_t obj;
     hwloc_bitmap_t good;
     hwloc_topology_t *t = &mca_paffinity_hwloc_component.topology;
 
     /* hwloc isn't able to find cores on all platforms.  Example:
        PPC64 running RHEL 5.4 (linux kernel 2.6.18) only reports NUMA
-       nodes and PU's.  Fine.  So just use PUs in that situation. */
-    obj = hwloc_get_obj_by_type(*t, HWLOC_OBJ_CORE, logical_processor_id);
+       nodes and PU's.  Fine.  
+
+       However, note that hwloc_get_obj_by_type() will return NULL in
+       2 (effectively) different cases:
+
+       - no objects of the requested type were found
+       - the Nth object of the requested type was not found
+
+       So first we have to see if we can find *any* cores by looking
+       for the 0th core.  If we find it, then try to find the Nth
+       core.  Otherwise, try to find the Nth PU. */
+    if (NULL == hwloc_get_obj_by_type(*t, HWLOC_OBJ_CORE, 0)) {
+        obj_type = HWLOC_OBJ_PU;
+    }
+
+    /* Now do the actual lookup. */
+    obj = hwloc_get_obj_by_type(*t, obj_type, logical_processor_id);
     if (NULL == obj) {
-        obj = hwloc_get_obj_by_type(*t, HWLOC_OBJ_PU, logical_processor_id);
-        if (NULL == obj) {
-            return OPAL_ERR_NOT_FOUND;
-        }
+        return OPAL_ERR_NOT_FOUND;
     }
 
     /* Found the right core (or PU).  Now find the processor ID of the