When csched_cpu_pick() decides to move a vCPU to a different pCPU, so
far in the vast majority of cases it selected the first core/thread of
the most idle socket/core. When there are many short executing
entities, this will generally lead to them not getting evenly
distributed (since primary cores/threads will be preferred), making the
need for subsequent migration more likely. Instead, candidate
cores/threads should get treated as symmetrically as possible, and
hence this changes the selection logic to cycle through all candidates.

Further, since csched_cpu_pick() will never move a vCPU between threads
of the same core (and since the weights calculated for individual
threads of the same core are always identical), rather than removing
just the selected pCPU from the mask that still needs looking at, all
siblings of the chosen pCPU can be removed at once without affecting
the outcome.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

--- 2009-10-27.orig/xen/common/sched_credit.c	2009-10-27 12:07:32.000000000 +0100
+++ 2009-10-27/xen/common/sched_credit.c	2009-10-27 14:27:28.000000000 +0100
@@ -110,6 +110,7 @@ struct csched_pcpu {
     uint32_t runq_sort_last;
     struct timer ticker;
     unsigned int tick;
+    unsigned int idle_bias;
 };
 
 /*
@@ -298,6 +299,7 @@ csched_pcpu_init(int cpu)
     init_timer(&spc->ticker, csched_tick, (void *)(unsigned long)cpu, cpu);
     INIT_LIST_HEAD(&spc->runq);
     spc->runq_sort_last = csched_priv.runq_sort;
+    spc->idle_bias = NR_CPUS - 1;
     per_cpu(schedule_data, cpu).sched_priv = spc;
 
     /* Start off idling... */
@@ -379,7 +381,7 @@ __csched_vcpu_is_migrateable(struct vcpu
 }
 
 static int
-csched_cpu_pick(struct vcpu *vc)
+_csched_cpu_pick(struct vcpu *vc, bool_t commit)
 {
     cpumask_t cpus;
     cpumask_t idlers;
@@ -438,8 +440,10 @@ csched_cpu_pick(struct vcpu *vc)
         if ( ( (weight_cpu < weight_nxt) ^ sched_smt_power_savings )
                 && (weight_cpu != weight_nxt) )
         {
-            cpu = nxt;
-            cpu_clear(cpu, cpus);
+            cpu = cycle_cpu(CSCHED_PCPU(nxt)->idle_bias, nxt_idlers);
+            if ( commit )
+               CSCHED_PCPU(nxt)->idle_bias = cpu;
+            cpus_andnot(cpus, cpus, per_cpu(cpu_sibling_map, cpu));
         }
         else
         {
@@ -450,6 +454,12 @@ csched_cpu_pick(struct vcpu *vc)
     return cpu;
 }
 
+static int
+csched_cpu_pick(struct vcpu *vc)
+{
+    return _csched_cpu_pick(vc, 1);
+}
+
 static inline void
 __csched_vcpu_acct_start(struct csched_vcpu *svc)
 {
@@ -529,7 +539,7 @@ csched_vcpu_acct(unsigned int cpu)
     {
         __csched_vcpu_acct_start(svc);
     }
-    else if ( csched_cpu_pick(current) != cpu )
+    else if ( _csched_cpu_pick(current, 0) != cpu )
     {
         CSCHED_VCPU_STAT_CRANK(svc, migrate_r);
         CSCHED_STAT_CRANK(migrate_running);