sched: fix cpu offlining with core scheduling

Offlining a cpu with core scheduling active can result in a hanging
system. Reason is the scheduling resource and unit of the to be removed
cpus needs to be split in order to remove the cpu from its cpupool and
move it to the idle scheduler. In case one of the involved cpus happens
to have received a sched slave event due to a vcpu former having been
running on that cpu being woken up again, it can happen that this cpu
will enter sched_wait_rendezvous_in() while its scheduling resource is
just about to be split. It might wait for ever for the other sibling
to join, which will never happen due to the resources already being
modified.

This can easily be avoided by:
- resetting the rendezvous counters of the idle unit which is kept
- checking for a new scheduling resource in sched_wait_rendezvous_in()
  after reacquiring the scheduling lock and resetting the counters in
  that case without scheduling another vcpu
- moving schedule resource modifications (in schedule_cpu_rm()) and
  retrieving (schedule(), sched_slave() is fine already, others are not
  critical) into locked regions

Reported-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>

xen/common/sched/core.c

@@ -2299,6 +2299,10 @@ void sched_context_switched(struct vcpu *vprev, struct vcpu *vnext)
     rcu_read_unlock(&sched_res_rculock);
 }
 
+/*
+ * Switch to a new context or keep the current one running.
+ * On x86 it won't return, so it needs to drop the still held sched_res_rculock.
+ */
 static void sched_context_switch(struct vcpu *vprev, struct vcpu *vnext,
                                  bool reset_idle_unit, s_time_t now)
 {
@@ -2408,6 +2412,9 @@ static struct vcpu *sched_force_context_switch(struct vcpu *vprev,
  * zero do_schedule() is called and the rendezvous counter for leaving
  * context_switch() is set. All other members will wait until the counter is
  * becoming zero, dropping the schedule lock in between.
+ * Either returns the new unit to run, or NULL if no context switch is
+ * required or (on Arm) has already been performed. If NULL is returned
+ * sched_res_rculock has been dropped.
  */
 static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev,
                                                    spinlock_t **lock, int cpu,
@@ -2415,7 +2422,8 @@ static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev,
 {
     struct sched_unit *next;
     struct vcpu *v;
-    unsigned int gran = get_sched_res(cpu)->granularity;
+    struct sched_resource *sr = get_sched_res(cpu);
+    unsigned int gran = sr->granularity;
 
     if ( !--prev->rendezvous_in_cnt )
     {
@@ -2482,6 +2490,21 @@ static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev,
             atomic_set(&prev->next_task->rendezvous_out_cnt, 0);
             prev->rendezvous_in_cnt = 0;
         }
+
+        /*
+         * Check for scheduling resource switched. This happens when we are
+         * moved away from our cpupool and cpus are subject of the idle
+         * scheduler now.
+         */
+        if ( unlikely(sr != get_sched_res(cpu)) )
+        {
+            ASSERT(is_idle_unit(prev));
+            atomic_set(&prev->next_task->rendezvous_out_cnt, 0);
+            prev->rendezvous_in_cnt = 0;
+            pcpu_schedule_unlock_irq(*lock, cpu);
+            rcu_read_unlock(&sched_res_rculock);
+            return NULL;
+        }
     }
 
     return prev->next_task;
@@ -2567,11 +2590,11 @@ static void schedule(void)
 
     rcu_read_lock(&sched_res_rculock);
 
+    lock = pcpu_schedule_lock_irq(cpu);
+
     sr = get_sched_res(cpu);
     gran = sr->granularity;
 
-    lock = pcpu_schedule_lock_irq(cpu);
-
     if ( prev->rendezvous_in_cnt )
     {
         /*
@@ -3151,7 +3174,10 @@ int schedule_cpu_rm(unsigned int cpu)
         per_cpu(sched_res_idx, cpu_iter) = 0;
         if ( cpu_iter == cpu )
         {
-            idle_vcpu[cpu_iter]->sched_unit->priv = NULL;
+            unit = idle_vcpu[cpu_iter]->sched_unit;
+            unit->priv = NULL;
+            atomic_set(&unit->next_task->rendezvous_out_cnt, 0);
+            unit->rendezvous_in_cnt = 0;
         }
         else
         {
@@ -3182,6 +3208,8 @@ int schedule_cpu_rm(unsigned int cpu)
     }
     sr->scheduler = &sched_idle_ops;
     sr->sched_priv = NULL;
+    sr->granularity = 1;
+    sr->cpupool = NULL;
 
     smp_mb();
     sr->schedule_lock = &sched_free_cpu_lock;
@@ -3194,9 +3222,6 @@ int schedule_cpu_rm(unsigned int cpu)
     sched_free_udata(old_ops, vpriv_old);
     sched_free_pdata(old_ops, ppriv_old, cpu);
 
-    sr->granularity = 1;
-    sr->cpupool = NULL;
-
 out:
     rcu_read_unlock(&sched_res_rculock);
     xfree(sr_new);