credit2: add a callback to migrate to a new cpu

In credit2, there needs to be a strong correlation between
v->processor and the runqueue to which a vcpu is assigned;
much of the code relies on this invariant.  Allow credit2
to manage the actual migration itself.

This fixes the most recent credit2 bug reported on the list
(Xen BUG at sched_credit2.c:1606) in Xen 4.1, as well as
the bug at sched_credit2.c:811 in -unstable (which catches the
same condition earlier).

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

xen/common/sched_credit2.c

@@ -1352,32 +1352,29 @@ out:
 static int
 csched_cpu_pick(const struct scheduler *ops, struct vcpu *vc)
 {
-    struct csched_vcpu * const svc = CSCHED_VCPU(vc);
     int new_cpu;
 
-    /* The scheduler interface doesn't have an explicit mechanism to
-     * involve the choosable scheduler in the migrate process, so we
-     * infer that a change may happen by the call to cpu_pick, and
-     * remove it from the old runqueue while the lock for the old
-     * runqueue is held.  It can't be actively waiting to run.  It
-     * will be added to the new runqueue when it next wakes.
-     *
-     * If we want to be able to call pick() separately, we need to add
-     * a mechansim to remove a vcpu from an old processor / runqueue
-     * before releasing the lock. */
-    BUG_ON(__vcpu_on_runq(svc));
-
     new_cpu = choose_cpu(ops, vc);
-    /* If we're suggesting moving to a different runqueue, remove it
-     * from the old runqueue while we have the lock.  It will be added
-     * to the new one when it wakes. */
-    if ( svc->rqd != NULL
-         && RQD(ops, new_cpu) != svc->rqd )
-        runq_deassign(ops, vc);
 
     return new_cpu;
 }
 
+static void
+csched_vcpu_migrate(
+    const struct scheduler *ops, struct vcpu *vc, unsigned int new_cpu)
+{
+    struct csched_vcpu * const svc = CSCHED_VCPU(vc);
+    struct csched_runqueue_data *trqd;
+
+    /* Check if new_cpu is valid */
+    BUG_ON(!cpu_isset(new_cpu, CSCHED_PRIV(ops)->initialized));
+
+    trqd = RQD(ops, new_cpu);
+
+    if ( trqd != svc->rqd )
+        migrate(ops, svc, trqd, NOW());
+}
+
 static int
 csched_dom_cntl(
     const struct scheduler *ops,
@@ -2121,6 +2118,7 @@ const struct scheduler sched_credit2_def = {
     .adjust         = csched_dom_cntl,
 
     .pick_cpu       = csched_cpu_pick,
+    .migrate        = csched_vcpu_migrate,
     .do_schedule    = csched_schedule,
     .context_saved  = csched_context_saved,
 

xen/common/schedule.c

@@ -489,7 +489,11 @@ static void vcpu_migrate(struct vcpu *v)
      * Switch to new CPU, then unlock new and old CPU.  This is safe because
      * the lock pointer cant' change while the current lock is held.
      */
-    v->processor = new_cpu;
+    if ( VCPU2OP(v)->migrate )
+        SCHED_OP(VCPU2OP(v), migrate, v, new_cpu);
+    else
+        v->processor = new_cpu;
+
 
     if ( old_lock != new_lock )
         spin_unlock(new_lock);

xen/include/xen/sched-if.h

@@ -170,6 +170,8 @@ struct scheduler {
                                       bool_t tasklet_work_scheduled);
 
     int          (*pick_cpu)       (const struct scheduler *, struct vcpu *);
+    void         (*migrate)        (const struct scheduler *, struct vcpu *,
+                                    unsigned int);
     int          (*adjust)         (const struct scheduler *, struct domain *,
                                     struct xen_domctl_scheduler_op *);
     int          (*adjust_global)  (const struct scheduler *,