commit fd8fe6d8526cc9d6abe510aae7a654d1b72d4305
Author: George Dunlap <george.dunlap@xxxxxxxxxx>
Commit: George Dunlap <george.dunlap@xxxxxxxxxx>

    George's mods

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 6ccc6f0..3e1720c 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -353,8 +353,8 @@ struct csched2_runqueue_data {
     struct list_head svc;  /* List of all vcpus assigned to this runqueue */
     unsigned int max_weight;
 
-    cpumask_t idle,        /* Currently idle */
-        smt_idle,          /* Fully idle cores (as in all the siblings are idle) */
+    cpumask_t idle,        /* Currently idle pcpus */
+        smt_idle,          /* Fully idle-and-untickled cores (see below) */
         tickled;           /* Have been asked to go through schedule */
     int load;              /* Instantaneous load: Length of queue  + num non-idle threads */
     s_time_t load_last_update;  /* Last time average was updated */
@@ -454,17 +454,20 @@ struct csched2_dom {
  */
 
 /*
- * If all the siblings of cpu (including cpu itself) are in idlers,
- * set all their bits in mask.
- *
- * In order to properly take into account tickling, idlers needs to be
- * set qeual to something like:
- *
- *  rqd->idle & (~rqd->tickled)
- *
- * This is because cpus that have been tickled will very likely pick up some
- * work as soon as the manage to schedule, and hence we should really consider
- * them as busy.
+ * If all the siblings of cpu (including cpu itself) are both idle and
+ * untickled, set all their bits in mask.
+ *
+ * NB that rqd->smt_idle is different than rqd->idle.  rqd->idle
+ * records pcpus that at are merely idle (i.e., at the moment do not
+ * have a vcpu running on them).  But you have to manually filter out
+ * which pcpus have been tickled in order to find cores that are not
+ * going to be busy soon.  Filtering out tickled cpus pairwise is a
+ * lot of extra pain; so for rqd->smt_idle, we explicitly make so that
+ * the bits of a pcpu are set only if all the threads on its core are
+ * both idle *and* untickled.
+ *
+ * This means changing the mask when either rqd->idle or rqd->tickled
+ * changes.
  */
 static inline
 void smt_idle_mask_set(unsigned int cpu, const cpumask_t *idlers,