sched: Fix select_task_rq() vs hotplug issues

Since select_task_rq() is now responsible for guaranteeing
->cpus_allowed and cpu_active_mask, we need to verify this.

select_task_rq_rt() can blindly return
smp_processor_id()/task_cpu() without checking the valid masks,
select_task_rq_fair() can do the same in the rare case that all
SD_flags are disabled.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20091216170517.961475466@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -2317,6 +2317,43 @@ void task_oncpu_function_call(struct task_struct *p,
 }
 
 #ifdef CONFIG_SMP
+static int select_fallback_rq(int cpu, struct task_struct *p)
+{
+	int dest_cpu;
+	const struct cpumask *nodemask = cpumask_of_node(cpu_to_node(cpu));
+
+	/* Look for allowed, online CPU in same node. */
+	for_each_cpu_and(dest_cpu, nodemask, cpu_active_mask)
+		if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
+			return dest_cpu;
+
+	/* Any allowed, online CPU? */
+	dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
+	if (dest_cpu < nr_cpu_ids)
+		return dest_cpu;
+
+	/* No more Mr. Nice Guy. */
+	if (dest_cpu >= nr_cpu_ids) {
+		rcu_read_lock();
+		cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
+		rcu_read_unlock();
+		dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed);
+
+		/*
+		 * Don't tell them about moving exiting tasks or
+		 * kernel threads (both mm NULL), since they never
+		 * leave kernel.
+		 */
+		if (p->mm && printk_ratelimit()) {
+			printk(KERN_INFO "process %d (%s) no "
+			       "longer affine to cpu%d\n",
+			       task_pid_nr(p), p->comm, cpu);
+		}
+	}
+
+	return dest_cpu;
+}
+
 /*
  * Called from:
  *
@@ -2343,14 +2380,8 @@ int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
 	 *   not worry about this generic constraint ]
 	 */
 	if (unlikely(!cpumask_test_cpu(cpu, &p->cpus_allowed) ||
-		     !cpu_active(cpu))) {
-
-		cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
-		/*
-		 * XXX: race against hot-plug modifying cpu_active_mask
-		 */
-		BUG_ON(cpu >= nr_cpu_ids);
-	}
+		     !cpu_active(cpu)))
+		cpu = select_fallback_rq(task_cpu(p), p);
 
 	return cpu;
 }
@@ -7319,36 +7350,10 @@ static int __migrate_task_irq(struct task_struct *p, int src_cpu, int dest_cpu)
 static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
 {
 	int dest_cpu;
-	const struct cpumask *nodemask = cpumask_of_node(cpu_to_node(dead_cpu));
 
 again:
-	/* Look for allowed, online CPU in same node. */
-	for_each_cpu_and(dest_cpu, nodemask, cpu_active_mask)
-		if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
-			goto move;
-
-	/* Any allowed, online CPU? */
-	dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
-	if (dest_cpu < nr_cpu_ids)
-		goto move;
-
-	/* No more Mr. Nice Guy. */
-	if (dest_cpu >= nr_cpu_ids) {
-		cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
-		dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed);
-
-		/*
-		 * Don't tell them about moving exiting tasks or
-		 * kernel threads (both mm NULL), since they never
-		 * leave kernel.
-		 */
-		if (p->mm && printk_ratelimit()) {
-			pr_info("process %d (%s) no longer affine to cpu%d\n",
-				task_pid_nr(p), p->comm, dead_cpu);
-		}
-	}
+	dest_cpu = select_fallback_rq(dead_cpu, p);
 
-move:
 	/* It can have affinity changed while we were choosing. */
 	if (unlikely(!__migrate_task_irq(p, dead_cpu, dest_cpu)))
 		goto again;