xen: Have schedulers revise initial placement

The generic domain creation logic in
xen/common/domctl.c:default_vcpu0_location() attempts to try to do
initial placement load-balancing by placing vcpu 0 on the least-busy
non-primary hyperthread available.  Unfortunately, the logic can end
up picking a pcpu that's not in the online mask.  When this is passed
to a scheduler such which assumes that the initial assignment is
valid, it causes a null pointer dereference looking up the runqueue.

Furthermore, this initial placement doesn't take into account hard or
soft affinity, or any scheduler-specific knowledge (such as historic
runqueue load, as in credit2).

To solve this, when inserting a vcpu, always call the per-scheduler
"pick" function to revise the initial placement.  This will
automatically take all knowledge the scheduler has into account.

csched2_cpu_pick ASSERTs that the vcpu's pcpu scheduler lock has been
taken.  Grab and release the lock to minimize time spend with irqs
disabled.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Meng Xu <mengxu@cis.upenn.edu>
Reviwed-by: Dario Faggioli <dario.faggioli@citrix.com>

xen/common/sched_credit.c

@@ -998,6 +998,9 @@ csched_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
 
     BUG_ON( is_idle_vcpu(vc) );
 
+    /* This is safe because vc isn't yet being scheduled */
+    vc->processor = csched_cpu_pick(ops, vc);
+
     lock = vcpu_schedule_lock_irq(vc);
 
     if ( !__vcpu_on_runq(svc) && vcpu_runnable(vc) && !vc->is_running )

xen/common/sched_credit2.c

@@ -2055,9 +2055,16 @@ csched2_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
     ASSERT(!is_idle_vcpu(vc));
     ASSERT(list_empty(&svc->runq_elem));
 
-    /* Add vcpu to runqueue of initial processor */
+    /* csched2_cpu_pick() expects the pcpu lock to be held */
     lock = vcpu_schedule_lock_irq(vc);
 
+    vc->processor = csched2_cpu_pick(ops, vc);
+
+    spin_unlock_irq(lock);
+
+    lock = vcpu_schedule_lock_irq(vc);
+
+    /* Add vcpu to runqueue of initial processor */
     runq_assign(ops, vc);
 
     vcpu_schedule_unlock_irq(lock, vc);

xen/common/sched_rt.c

@@ -874,6 +874,9 @@ rt_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
 
     BUG_ON( is_idle_vcpu(vc) );
 
+    /* This is safe because vc isn't yet being scheduled */
+    vc->processor = rt_cpu_pick(ops, vc);
+
     lock = vcpu_schedule_lock_irq(vc);
 
     now = NOW();