[PATCH 3/7] xen: sched: better handle (not) inserting idle vCPUs in runqueues

[Dario Faggioli <dario.faggioli@citrix.com>]

Idle vCPUs should never really be explicitly inserted
in any of the schedulers' runqueue. In fact, they are
just put in execution immediately (either on boot, or
when a pCPU is assigned to a cpupool), and it will be
the first scheduling decision that --by preempting
them-- will 'park' them in the queue.

In fact, avoiding inserting those vCPUs in runqueues
is something that Credit2 and RTDS intercept and
forbid already (when implementing insert_vcpu()).
Credit1 does that implicitly, as the idle vCPU will
always be already running when insert_vcpu() is called,
and hence not put in the runqueue.

Let's make it *always* explicit, as that simplifies
things by quite a bit. For instance, we can now
introduce some BUG_ON() guards, with the twofold
objective of making this assumption clear, and of
catching misuse and bugs.

The check for whether we'd be inserting an idle vCPU
in a queue, now, happens, once and for all schedulers,
in generic code, at vCPU initialization time, while
we can just avoid trying (and always failing!) doing
so when doing cpupools manipulations.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
---
Cc: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Juergen Gross <jgross@suse.com>
---
 xen/common/sched_credit.c  |    2 ++
 xen/common/sched_credit2.c |   25 ++++++++++---------------
 xen/common/sched_rt.c      |    4 +---
 xen/common/schedule.c      |   21 +++++++++++----------
 4 files changed, 24 insertions(+), 28 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index fccb368..fc447a7 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -905,6 +905,8 @@ csched_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
     struct csched_vcpu *svc = vc->sched_priv;
     spinlock_t *lock;
 
+    BUG_ON( is_idle_vcpu(vc) );
+
     lock = vcpu_schedule_lock_irq(vc);
 
     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 912e1a2..234f798 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -870,30 +870,25 @@ csched2_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
 {
     struct csched2_vcpu *svc = vc->sched_priv;
     struct csched2_dom * const sdom = svc->sdom;
+    spinlock_t *lock;
 
     printk("%s: Inserting %pv\n", __func__, vc);
 
-    /* NB: On boot, idle vcpus are inserted before alloc_pdata() has
-     * been called for that cpu.
-     */
-    if ( ! is_idle_vcpu(vc) )
-    {
-        spinlock_t *lock;
+    BUG_ON(is_idle_vcpu(vc));
 
-        /* FIXME: Do we need the private lock here? */
-        list_add_tail(&svc->sdom_elem, &svc->sdom->vcpu);
+    /* FIXME: Do we need the private lock here? */
+    list_add_tail(&svc->sdom_elem, &svc->sdom->vcpu);
 
-        /* Add vcpu to runqueue of initial processor */
-        lock = vcpu_schedule_lock_irq(vc);
+    /* Add vcpu to runqueue of initial processor */
+    lock = vcpu_schedule_lock_irq(vc);
 
-        runq_assign(ops, vc);
+    runq_assign(ops, vc);
 
-        vcpu_schedule_unlock_irq(lock, vc);
+    vcpu_schedule_unlock_irq(lock, vc);
 
-        sdom->nr_vcpus++;
+    sdom->nr_vcpus++;
 
-        SCHED_STAT_CRANK(vcpu_insert);
-    }
+    SCHED_STAT_CRANK(vcpu_insert);
 
     CSCHED2_VCPU_CHECK(vc);
 }
diff --git a/xen/common/sched_rt.c b/xen/common/sched_rt.c
index 1086399..37a32a4 100644
--- a/xen/common/sched_rt.c
+++ b/xen/common/sched_rt.c
@@ -624,9 +624,7 @@ rt_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
     s_time_t now = NOW();
     spinlock_t *lock;
 
-    /* not addlocate idle vcpu to dom vcpu list */
-    if ( is_idle_vcpu(vc) )
-        return;
+    BUG_ON( is_idle_vcpu(vc) );
 
     lock = vcpu_schedule_lock_irq(vc);
     if ( now >= svc->cur_deadline )
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index 9aa209d..55daf73 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -240,20 +240,22 @@ int sched_init_vcpu(struct vcpu *v, unsigned int processor)
     init_timer(&v->poll_timer, poll_timer_fn,
                v, v->processor);
 
-    /* Idle VCPUs are scheduled immediately. */
+    v->sched_priv = SCHED_OP(DOM2OP(d), alloc_vdata, v, d->sched_priv);
+    if ( v->sched_priv == NULL )
+        return 1;
+
+    TRACE_2D(TRC_SCHED_DOM_ADD, v->domain->domain_id, v->vcpu_id);
+
+    /* Idle VCPUs are scheduled immediately, not inserting them in runqueue. */
     if ( is_idle_domain(d) )
     {
         per_cpu(schedule_data, v->processor).curr = v;
         v->is_running = 1;
     }
-
-    TRACE_2D(TRC_SCHED_DOM_ADD, v->domain->domain_id, v->vcpu_id);
-
-    v->sched_priv = SCHED_OP(DOM2OP(d), alloc_vdata, v, d->sched_priv);
-    if ( v->sched_priv == NULL )
-        return 1;
-
-    SCHED_OP(DOM2OP(d), insert_vcpu, v);
+    else
+    {
+        SCHED_OP(DOM2OP(d), insert_vcpu, v);
+    }
 
     return 0;
 }
@@ -1514,7 +1516,6 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
     ppriv_old = per_cpu(schedule_data, cpu).sched_priv;
     per_cpu(schedule_data, cpu).sched_priv = ppriv;
     SCHED_OP(new_ops, tick_resume, cpu);
-    SCHED_OP(new_ops, insert_vcpu, idle);
 
     SCHED_OP(old_ops, free_vdata, vpriv_old);
     SCHED_OP(old_ops, free_pdata, ppriv_old, cpu);