From: George Dunlap <george.dunlap@citrix.com>
Date: Fri, 5 Aug 2016 12:07:27 +0000 (+0200)
Subject: xen: Have schedulers revise initial placement
X-Git-Tag: RELEASE-4.5.4~26
X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=c421378a8d14c811e5467d535bc71adc0328a316;p=xen.git

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>
master commit: 9f358ddd69463fa8fb65cf67beb5f6f0d3350e32
master date: 2016-07-26 10:42:49 +0100
---

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 54fd830efb..2e11aa27f9 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -892,6 +892,9 @@ csched_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
     spinlock_t *lock;
     unsigned long flags;
 
+    /* This is safe because vc isn't yet being scheduled */
+    vc->processor = csched_cpu_pick(ops, vc);
+
     lock = vcpu_schedule_lock_irqsave(vc, &flags);
 
     if ( !__vcpu_on_runq(svc) && vcpu_runnable(vc) && !vc->is_running )
diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index db9e1c4a10..7a81db509a 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -269,6 +269,7 @@ struct csched2_dom {
     uint16_t nr_vcpus;
 };
 
+static int csched2_cpu_pick(const struct scheduler *ops, struct vcpu *vc);
 
 /*
  * Time-to-credit, credit-to-time.
@@ -874,9 +875,16 @@ csched2_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
         /* FIXME: Do we need the private lock here? */
         list_add_tail(&svc->sdom_elem, &svc->sdom->vcpu);
 
-        /* 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);
diff --git a/xen/common/sched_rt.c b/xen/common/sched_rt.c
index a77e80800a..bb972fa8f9 100644
--- a/xen/common/sched_rt.c
+++ b/xen/common/sched_rt.c
@@ -169,6 +169,8 @@ struct rt_dom {
     struct domain *dom;         /* pointer to upper domain */
 };
 
+static int rt_cpu_pick(const struct scheduler *ops, struct vcpu *vc);
+
 /*
  * Useful inline functions
  */
@@ -554,6 +556,9 @@ rt_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
     if ( is_idle_vcpu(vc) )
         return;
 
+    /* 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();