[patch 2/5] sched: NULL domains

[Nick Piggin]

2/5
The previous patch fixed the last 2 places that directly access a
runqueue's sched-domain and assume it cannot be NULL.

We can now use a NULL domain instead of a dummy domain to signify
no balancing is to happen. No functional changes.

Signed-off-by: Nick Piggin <nickpiggin@xxxxxxxxxxxx>

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c	2005-04-05 16:38:40.000000000 +1000
+++ linux-2.6/kernel/sched.c	2005-04-05 18:39:08.000000000 +1000
@@ -4887,7 +4887,7 @@ void __devinit cpu_attach_domain(struct 
 			tmp->parent = parent->parent;
 	}
 
-	if (sd_degenerate(sd))
+	if (sd && sd_degenerate(sd))
 		sd = sd->parent;
 
 	sched_domain_debug(sd, cpu);
@@ -5054,7 +5054,7 @@ static void __devinit arch_init_sched_do
 	cpus_and(cpu_default_map, cpu_default_map, cpu_online_map);
 
 	/*
-	 * Set up domains. Isolated domains just stay on the dummy domain.
+	 * Set up domains. Isolated domains just stay on the NULL domain.
 	 */
 	for_each_cpu_mask(i, cpu_default_map) {
 		int group;
@@ -5167,18 +5167,11 @@ static void __devinit arch_destroy_sched
 
 #endif /* ARCH_HAS_SCHED_DOMAIN */
 
-/*
- * Initial dummy domain for early boot and for hotplug cpu. Being static,
- * it is initialized to zero, so all balancing flags are cleared which is
- * what we want.
- */
-static struct sched_domain sched_domain_dummy;
-
 #ifdef CONFIG_HOTPLUG_CPU
 /*
  * Force a reinitialization of the sched domains hierarchy.  The domains
  * and groups cannot be updated in place without racing with the balancing
- * code, so we temporarily attach all running cpus to a "dummy" domain
+ * code, so we temporarily attach all running cpus to the NULL domain
  * which will prevent rebalancing while the sched domains are recalculated.
  */
 static int update_sched_domains(struct notifier_block *nfb,
@@ -5190,7 +5183,7 @@ static int update_sched_domains(struct n
 	case CPU_UP_PREPARE:
 	case CPU_DOWN_PREPARE:
 		for_each_online_cpu(i)
-			cpu_attach_domain(&sched_domain_dummy, i);
+			cpu_attach_domain(NULL, i);
 		arch_destroy_sched_domains();
 		return NOTIFY_OK;
 
@@ -5253,7 +5246,7 @@ void __init sched_init(void)
 		rq->best_expired_prio = MAX_PRIO;
 
 #ifdef CONFIG_SMP
-		rq->sd = &sched_domain_dummy;
+		rq->sd = NULL;
 		for (j = 1; j < 3; j++)
 			rq->cpu_load[j] = 0;
 		rq->active_balance = 0;