[PATCH] Credit1: Tweak reset condition

[George Dunlap <george.dunlap@eu.citrix.com>]

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 (shifting
by one bit for more efficiency).  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>

diff -r 6f07d9ac1e7c -r 1a29b22ef2e9 xen/common/sched_credit.c
--- a/xen/common/sched_credit.c	Thu Aug 05 14:41:14 2010 +0100
+++ b/xen/common/sched_credit.c	Mon Aug 09 12:38:12 2010 +0100
@@ -1069,7 +1069,9 @@
                 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 >>= 1;
                     atomic_set(&svc->credit, credit);
                 }
             }