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Subject[patch 2/5] sched: NULL domains
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@yahoo.com.au>

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;
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