xen: credit1: simplify csched_runq_steal() a little bit.

Since we're holding the lock on the pCPU from which we
are trying to steal, it can't have disappeared, so we
can drop the check for that (and convert it in an
ASSERT()).

And since we try to steal only from busy pCPUs, it's
unlikely for such pCPU to be idle, so we can:
 - tell the compiler this is actually unlikely,
 - bail early if the pCPU, unfortunately, turns out
   to really be idle.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>

xen/common/sched_credit.c

@@ -1593,64 +1593,65 @@ static struct csched_vcpu *
 csched_runq_steal(int peer_cpu, int cpu, int pri, int balance_step)
 {
     const struct csched_pcpu * const peer_pcpu = CSCHED_PCPU(peer_cpu);
-    const struct vcpu * const peer_vcpu = curr_on_cpu(peer_cpu);
     struct csched_vcpu *speer;
     struct list_head *iter;
     struct vcpu *vc;
 
+    ASSERT(peer_pcpu != NULL);
+
     /*
      * Don't steal from an idle CPU's runq because it's about to
      * pick up work from it itself.
      */
-    if ( peer_pcpu != NULL && !is_idle_vcpu(peer_vcpu) )
+    if ( unlikely(is_idle_vcpu(curr_on_cpu(peer_cpu))) )
+        goto out;
+
+    list_for_each( iter, &peer_pcpu->runq )
     {
-        list_for_each( iter, &peer_pcpu->runq )
+        speer = __runq_elem(iter);
+
+        /*
+         * If next available VCPU here is not of strictly higher
+         * priority than ours, this PCPU is useless to us.
+         */
+        if ( speer->pri <= pri )
+            break;
+
+        /* Is this VCPU runnable on our PCPU? */
+        vc = speer->vcpu;
+        BUG_ON( is_idle_vcpu(vc) );
+
+        /*
+         * If the vcpu has no useful soft affinity, skip this vcpu.
+         * In fact, what we want is to check if we have any "soft-affine
+         * work" to steal, before starting to look at "hard-affine work".
+         *
+         * Notice that, if not even one vCPU on this runq has a useful
+         * soft affinity, we could have avoid considering this runq for
+         * a soft balancing step in the first place. This, for instance,
+         * can be implemented by taking note of on what runq there are
+         * vCPUs with useful soft affinities in some sort of bitmap
+         * or counter.
+         */
+        if ( balance_step == CSCHED_BALANCE_SOFT_AFFINITY
+             && !__vcpu_has_soft_affinity(vc, vc->cpu_hard_affinity) )
+            continue;
+
+        csched_balance_cpumask(vc, balance_step, cpumask_scratch);
+        if ( __csched_vcpu_is_migrateable(vc, cpu, cpumask_scratch) )
         {
-            speer = __runq_elem(iter);
-
-            /*
-             * If next available VCPU here is not of strictly higher
-             * priority than ours, this PCPU is useless to us.
-             */
-            if ( speer->pri <= pri )
-                break;
-
-            /* Is this VCPU runnable on our PCPU? */
-            vc = speer->vcpu;
-            BUG_ON( is_idle_vcpu(vc) );
-
-            /*
-             * If the vcpu has no useful soft affinity, skip this vcpu.
-             * In fact, what we want is to check if we have any "soft-affine
-             * work" to steal, before starting to look at "hard-affine work".
-             *
-             * Notice that, if not even one vCPU on this runq has a useful
-             * soft affinity, we could have avoid considering this runq for
-             * a soft balancing step in the first place. This, for instance,
-             * can be implemented by taking note of on what runq there are
-             * vCPUs with useful soft affinities in some sort of bitmap
-             * or counter.
-             */
-            if ( balance_step == CSCHED_BALANCE_SOFT_AFFINITY
-                 && !__vcpu_has_soft_affinity(vc, vc->cpu_hard_affinity) )
-                continue;
-
-            csched_balance_cpumask(vc, balance_step, cpumask_scratch);
-            if ( __csched_vcpu_is_migrateable(vc, cpu, cpumask_scratch) )
-            {
-                /* We got a candidate. Grab it! */
-                TRACE_3D(TRC_CSCHED_STOLEN_VCPU, peer_cpu,
-                         vc->domain->domain_id, vc->vcpu_id);
-                SCHED_VCPU_STAT_CRANK(speer, migrate_q);
-                SCHED_STAT_CRANK(migrate_queued);
-                WARN_ON(vc->is_urgent);
-                __runq_remove(speer);
-                vc->processor = cpu;
-                return speer;
-            }
+            /* We got a candidate. Grab it! */
+            TRACE_3D(TRC_CSCHED_STOLEN_VCPU, peer_cpu,
+                     vc->domain->domain_id, vc->vcpu_id);
+            SCHED_VCPU_STAT_CRANK(speer, migrate_q);
+            SCHED_STAT_CRANK(migrate_queued);
+            WARN_ON(vc->is_urgent);
+            __runq_remove(speer);
+            vc->processor = cpu;
+            return speer;
         }
     }
-
+ out:
     SCHED_STAT_CRANK(steal_peer_idle);
     return NULL;
 }