sched_rt: avoid ASSERT()ing on runq dump if there are no domains

being serviced by the RTDS scheduler, as that is a
legit situation to be in: think, for instance, of a
newly created RTDS cpupool, with no domains migrated
to it yet.

While there:
 - move the spinlock acquisition up, to effectively
   protect the domain list and avoid races;
 - the mask of online pCPUs was being retrieved
   but then not used anywhere in the function: get
   rid of that.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Meng Xu <mengxu@cis.upenn.edu>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>

xen/common/sched_rt.c

@@ -264,18 +264,17 @@ rt_dump(const struct scheduler *ops)
     struct list_head *iter_sdom, *iter_svc, *runq, *depletedq, *iter;
     struct rt_private *prv = rt_priv(ops);
     struct rt_vcpu *svc;
-    cpumask_t *online;
     struct rt_dom *sdom;
     unsigned long flags;
 
-    ASSERT(!list_empty(&prv->sdom));
+    spin_lock_irqsave(&prv->lock, flags);
+
+    if ( list_empty(&prv->sdom) )
+        goto out;
 
-    sdom = list_entry(prv->sdom.next, struct rt_dom, sdom_elem);
-    online = cpupool_scheduler_cpumask(sdom->dom->cpupool);
     runq = rt_runq(ops);
     depletedq = rt_depletedq(ops);
 
-    spin_lock_irqsave(&prv->lock, flags);
     printk("Global RunQueue info:\n");
     list_for_each( iter, runq )
     {
@@ -303,6 +302,7 @@ rt_dump(const struct scheduler *ops)
         }
     }
 
+ out:
     spin_unlock_irqrestore(&prv->lock, flags);
 }