_csched_cpu_pick(): don't write idle bias more than once

For the bias to be really meaningful, it should be updated only when
the CPU selected will indeed be returned (and hence used for placing
the vCPU in question).

Signed-off-by: Jan Beulich <jbeulich@novell.com>

xen/common/sched_credit.c

@@ -464,6 +464,7 @@ _csched_cpu_pick(const struct scheduler *ops, struct vcpu *vc, bool_t commit)
     cpumask_t cpus;
     cpumask_t idlers;
     cpumask_t *online;
+    struct csched_pcpu *spc = NULL;
     int cpu;
 
     /*
@@ -531,9 +532,8 @@ _csched_cpu_pick(const struct scheduler *ops, struct vcpu *vc, bool_t commit)
                   && (weight_cpu * migrate_factor < weight_nxt) ) )
         {
             cpus_and(nxt_idlers, cpus, nxt_idlers);
-            cpu = cycle_cpu(CSCHED_PCPU(nxt)->idle_bias, nxt_idlers);
-            if ( commit )
-               CSCHED_PCPU(nxt)->idle_bias = cpu;
+            spc = CSCHED_PCPU(nxt);
+            cpu = cycle_cpu(spc->idle_bias, nxt_idlers);
             cpus_andnot(cpus, cpus, per_cpu(cpu_sibling_map, cpu));
         }
         else
@@ -542,6 +542,9 @@ _csched_cpu_pick(const struct scheduler *ops, struct vcpu *vc, bool_t commit)
         }
     }
 
+    if ( commit && spc )
+       spc->idle_bias = cpu;
+
     return cpu;
 }