Credit1: Tweak reset condition

VMs that don't use their full timeslice are guaranteed to flip back
and forth between "active" and "inactive".  If we set credit to 0
when setting "inactive", then when the VM comes back to "active"
again, it will effectively be behind most other vcpus in credit.
This causes the credit1 to effectively discriminate *against*
VMs which use less than their full timeslice.

Instead of setting credit to 0, divide it in half.  This gets rid of
some of the system credit while allowing non-cpu-bound VMs to keep
some priority advantage.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>

xen/common/sched_credit.c

@@ -1100,7 +1100,9 @@ csched_acct(void* dummy)
                 if ( credit > CSCHED_CREDITS_PER_TSLICE )
                 {
                     __csched_vcpu_acct_stop_locked(prv, svc);
-                    credit = 0;
+                    /* Divide credits in half, so that when it starts
+                     * accounting again, it starts a little bit "ahead" */
+                    credit /= 2;
                     atomic_set(&svc->credit, credit);
                 }
             }