[PATCH] fix sched_domains hotplug bootstrap ordering vs. cpu_online_map issue

cpu_online_map is not set up at the time of sched domain initialization
when hotplug cpu paths are used for SMP booting.

At this phase of bootstrapping, cpu_possible_map can be used by the
various architectures using cpu hotplugging for SMP bootstrap, but the
manipulations of cpu_online_map done on behalf of NUMA architectures,
done indirectly via node_to_cpumask(), can't, because cpu_online_map
starts depopulated and hasn't yet been populated.

On true NUMA architectures this is a distinct cpumask_t from
cpu_online_map and so the unpatched code works on NUMA; on non-NUMA
architectures the definition of node_to_cpumask() this way breaks and
would require an invasive sweeping of users of node_to_cpumask() to
change it to e.g.  cpu_possible_map, as cpu_possible_map is not suitable
for use at runtime as a substitute for cpu_online_map.
Signed-off-by: William Irwin <wli@holomorphy.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -4468,7 +4468,11 @@ __init static void arch_init_sched_domains(void)
 		sd = &per_cpu(phys_domains, i);
 		group = cpu_to_phys_group(i);
 		*sd = SD_CPU_INIT;
+#ifdef CONFIG_NUMA
 		sd->span = nodemask;
+#else
+		sd->span = cpu_possible_map;
+#endif
 		sd->parent = p;
 		sd->groups = &sched_group_phys[group];
 
@@ -4506,6 +4510,7 @@ __init static void arch_init_sched_domains(void)
 						&cpu_to_isolated_group);
 	}
 
+#ifdef CONFIG_NUMA
 	/* Set up physical groups */
 	for (i = 0; i < MAX_NUMNODES; i++) {
 		cpumask_t nodemask = node_to_cpumask(i);
@@ -4517,6 +4522,10 @@ __init static void arch_init_sched_domains(void)
 		init_sched_build_groups(sched_group_phys, nodemask,
 						&cpu_to_phys_group);
 	}
+#else
+	init_sched_build_groups(sched_group_phys, cpu_possible_map,
+							&cpu_to_phys_group);
+#endif
 
 #ifdef CONFIG_NUMA
 	/* Set up node groups */