Add context_saved scheduler callback.

Because credit2 shares a runqueue between several cpus, it needs
to know when a scheduled-out process has finally been context-switched
away so that it can be added to the runqueue again.  (Otherwise it may
be grabbed by another processor before the context has been properly
saved.)

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

diff -r c44b7b9b6306 xen/common/schedule.c
--- a/xen/common/schedule.c	Wed Jan 13 13:33:57 2010 +0000
+++ b/xen/common/schedule.c	Wed Jan 13 13:36:37 2010 +0000
@@ -877,6 +877,8 @@
     /* Check for migration request /after/ clearing running flag. */
     smp_mb();
 
+    SCHED_OP(context_saved, prev);
+
     if ( unlikely(test_bit(_VPF_migrating, &prev->pause_flags)) )
         vcpu_migrate(prev);
 }
diff -r c44b7b9b6306 xen/include/xen/sched-if.h
--- a/xen/include/xen/sched-if.h	Wed Jan 13 13:33:57 2010 +0000
+++ b/xen/include/xen/sched-if.h	Wed Jan 13 13:36:37 2010 +0000
@@ -69,6 +69,7 @@
 
     void         (*sleep)          (struct vcpu *);
     void         (*wake)           (struct vcpu *);
+    void         (*context_saved)  (struct vcpu *);
 
     struct task_slice (*do_schedule) (s_time_t);