credit: generalize __vcpu_has_soft_affinity()

As pointed out in the discussion of the patch at
http://lists.xenproject.org/archives/html/xen-devel/2015-02/msg03256.html
generalizing the conditions here means code elsewhere doesn't need to
take into consideration internals of how load balancing in the credit
scheduler works.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
---
v3: Swap soft and hard affinities in the 2nd of the 3 checks. Alter
    comment as requested by George (which made us notice the need for
    the code change, therefore retaining his review tag).
v2: Use VCPU2ONLINE(vc) (or really an open coded variant thereof)
    instead of cpu_online_map (suggested by Dario).

--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -279,24 +279,23 @@ __runq_remove(struct csched_vcpu *svc)
 
 /*
  * Hard affinity balancing is always necessary and must never be skipped.
- * OTOH, if the vcpu's soft affinity is full (it spans all the possible
- * pcpus) we can safely avoid dealing with it entirely.
+ * But soft affinity need only be considered when it has a functionally
+ * different effect than other constraints (such as hard affinity, cpus
+ * online, or cpupools).
  *
- * A vcpu's soft affinity is also deemed meaningless in case it has empty
- * intersection with mask, to cover the cases where using the soft affinity
- * mask seems legit, but would instead led to trying to schedule the vcpu
- * on _no_ pcpu! Typical use cases are for mask to be equal to the vcpu's
- * hard affinity, or to the && of hard affinity and the set of online cpus
- * in the domain's cpupool.
+ * Soft affinity only needs to be considered if:
+ * * The cpus in the cpupool are not a subset of soft affinity
+ * * The hard affinity is not a subset of soft affinity
+ * * There is an overlap between the soft affinity and the mask which is
+ *   currently being considered.
  */
 static inline int __vcpu_has_soft_affinity(const struct vcpu *vc,
                                            const cpumask_t *mask)
 {
-    if ( cpumask_full(vc->cpu_soft_affinity)
-         || !cpumask_intersects(vc->cpu_soft_affinity, mask) )
-        return 0;
-
-    return 1;
+    return !cpumask_subset(cpupool_online_cpumask(vc->domain->cpupool),
+                           vc->cpu_soft_affinity) &&
+           !cpumask_subset(vc->cpu_hard_affinity, vc->cpu_soft_affinity) &&
+           cpumask_intersects(vc->cpu_soft_affinity, mask);
 }
 
 /*