/*
   * stop-task scheduling class.
   *
   * The stop task is the highest priority task in the system, it preempts
   * everything and will be preempted by nothing.
   *
   * See kernel/stop_machine.c
   */
  
  #ifdef CONFIG_SMP
  static int
  select_task_rq_stop(struct rq *rq, struct task_struct *p,
  		    int sd_flag, int flags)
  {
  	return task_cpu(p); /* stop tasks as never migrate */
  }
  #endif /* CONFIG_SMP */
  
  static void
  check_preempt_curr_stop(struct rq *rq, struct task_struct *p, int flags)
  {
  	resched_task(rq->curr); /* we preempt everything */
  }
  
  static struct task_struct *pick_next_task_stop(struct rq *rq)
  {
  	struct task_struct *stop = rq->stop;
  
  	if (stop && stop->state == TASK_RUNNING)
  		return stop;
  
  	return NULL;
  }
  
  static void
  enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
  {
  }
  
  static void
  dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags)
  {
  }
  
  static void yield_task_stop(struct rq *rq)
  {
  	BUG(); /* the stop task should never yield, its pointless. */
  }
  
  static void put_prev_task_stop(struct rq *rq, struct task_struct *prev)
  {
  }
  
  static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued)
  {
  }
  
  static void set_curr_task_stop(struct rq *rq)
  {
  }
  
  static void switched_to_stop(struct rq *rq, struct task_struct *p,
  			     int running)
  {
  	BUG(); /* its impossible to change to this class */
  }
  
  static void prio_changed_stop(struct rq *rq, struct task_struct *p,
  			      int oldprio, int running)
  {
  	BUG(); /* how!?, what priority? */
  }
  
  static unsigned int
  get_rr_interval_stop(struct rq *rq, struct task_struct *task)
  {
  	return 0;
  }
  
  /*
   * Simple, special scheduling class for the per-CPU stop tasks:
   */
  static const struct sched_class stop_sched_class = {
  	.next			= &rt_sched_class,
  
  	.enqueue_task		= enqueue_task_stop,
  	.dequeue_task		= dequeue_task_stop,
  	.yield_task		= yield_task_stop,
  
  	.check_preempt_curr	= check_preempt_curr_stop,
  
  	.pick_next_task		= pick_next_task_stop,
  	.put_prev_task		= put_prev_task_stop,
  
  #ifdef CONFIG_SMP
  	.select_task_rq		= select_task_rq_stop,
  #endif
  
  	.set_curr_task          = set_curr_task_stop,
  	.task_tick		= task_tick_stop,
  
  	.get_rr_interval	= get_rr_interval_stop,
  
  	.prio_changed		= prio_changed_stop,
  	.switched_to		= switched_to_stop,
  
  	/* no .task_new for stop tasks */
  };