[PATCH] sched: null domains

Fix the last 2 places that directly access a runqueue's sched-domain and
assume it cannot be NULL.

That allows the use of NULL for domain, instead of a dummy domain, to signify
no balancing is to happen.  No functional changes.

Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 21 insertions, 15 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index b1410577f9a8..77c07c2928b9 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2579,11 +2579,15 @@ out:
 #ifdef CONFIG_SCHED_SMT
 static inline void wake_sleeping_dependent(int this_cpu, runqueue_t *this_rq)
 {
-        struct sched_domain *sd = this_rq->sd;
+        struct sched_domain *tmp, *sd = NULL;
         cpumask_t sibling_map;
         int i;
 
-        if (!(sd->flags & SD_SHARE_CPUPOWER))
+        for_each_domain(this_cpu, tmp)
+                if (tmp->flags & SD_SHARE_CPUPOWER)
+                        sd = tmp;
+
+        if (!sd)
                 return;
 
         /*
@@ -2624,13 +2628,17 @@ static inline void wake_sleeping_dependent(int this_cpu, runqueue_t *this_rq)
 
 static inline int dependent_sleeper(int this_cpu, runqueue_t *this_rq)
 {
-        struct sched_domain *sd = this_rq->sd;
+        struct sched_domain *tmp, *sd = NULL;
         cpumask_t sibling_map;
         prio_array_t *array;
         int ret = 0, i;
         task_t *p;
 
-        if (!(sd->flags & SD_SHARE_CPUPOWER))
+        for_each_domain(this_cpu, tmp)
+                if (tmp->flags & SD_SHARE_CPUPOWER)
+                        sd = tmp;
+
+        if (!sd)
                 return 0;
 
         /*
@@ -4617,6 +4625,11 @@ static void sched_domain_debug(struct sched_domain *sd, int cpu)
 {
         int level = 0;
 
+        if (!sd) {
+                printk(KERN_DEBUG "CPU%d attaching NULL sched-domain.\n", cpu);
+                return;
+        }
+
         printk(KERN_DEBUG "CPU%d attaching sched-domain:\n", cpu);
 
         do {
@@ -4874,7 +4887,7 @@ static void __devinit arch_init_sched_domains(void)
         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;
@@ -4987,18 +5000,11 @@ static void __devinit arch_destroy_sched_domains(void)
 
 #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,
@@ -5010,7 +5016,7 @@ static int update_sched_domains(struct notifier_block *nfb,
         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;
 
@@ -5072,7 +5078,7 @@ 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;