1// SPDX-License-Identifier: GPL-2.0
  2/*
  3 * stop-task scheduling class.
  4 *
  5 * The stop task is the highest priority task in the system, it preempts
  6 * everything and will be preempted by nothing.
  7 *
  8 * See kernel/stop_machine.c
  9 */
 10
 11#ifdef CONFIG_SMP
 12static int
 13select_task_rq_stop(struct task_struct *p, int cpu, int flags)
 14{
 15	return task_cpu(p); /* stop tasks as never migrate */
 16}
 17
 18static int
 19balance_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 20{
 21	return sched_stop_runnable(rq);
 22}
 23#endif /* CONFIG_SMP */
 24
 25static void
 26wakeup_preempt_stop(struct rq *rq, struct task_struct *p, int flags)
 27{
 28	/* we're never preempted */
 29}
 30
 31static void set_next_task_stop(struct rq *rq, struct task_struct *stop, bool first)
 32{
 33	stop->se.exec_start = rq_clock_task(rq);
 34}
 35
 36static struct task_struct *pick_task_stop(struct rq *rq)
 37{
 38	if (!sched_stop_runnable(rq))
 39		return NULL;
 40
 41	return rq->stop;
 42}
 43
 44static void
 45enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
 46{
 47	add_nr_running(rq, 1);
 48}
 49
 50static bool
 51dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags)
 52{
 53	sub_nr_running(rq, 1);
 54	return true;
 55}
 56
 57static void yield_task_stop(struct rq *rq)
 58{
 59	BUG(); /* the stop task should never yield, its pointless. */
 60}
 61
 62static void put_prev_task_stop(struct rq *rq, struct task_struct *prev, struct task_struct *next)
 63{
 64	update_curr_common(rq);
 65}
 66
 67/*
 68 * scheduler tick hitting a task of our scheduling class.
 69 *
 70 * NOTE: This function can be called remotely by the tick offload that
 71 * goes along full dynticks. Therefore no local assumption can be made
 72 * and everything must be accessed through the @rq and @curr passed in
 73 * parameters.
 74 */
 75static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued)
 76{
 77}
 78
 79static void switched_to_stop(struct rq *rq, struct task_struct *p)
 80{
 81	BUG(); /* its impossible to change to this class */
 82}
 83
 84static void
 85prio_changed_stop(struct rq *rq, struct task_struct *p, int oldprio)
 86{
 87	BUG(); /* how!?, what priority? */
 88}
 89
 90static void update_curr_stop(struct rq *rq)
 91{
 92}
 93
 94/*
 95 * Simple, special scheduling class for the per-CPU stop tasks:
 96 */
 97DEFINE_SCHED_CLASS(stop) = {
 98
 99	.enqueue_task		= enqueue_task_stop,
100	.dequeue_task		= dequeue_task_stop,
101	.yield_task		= yield_task_stop,
102
103	.wakeup_preempt		= wakeup_preempt_stop,
104
105	.pick_task		= pick_task_stop,
106	.put_prev_task		= put_prev_task_stop,
107	.set_next_task          = set_next_task_stop,
108
109#ifdef CONFIG_SMP
110	.balance		= balance_stop,
111	.select_task_rq		= select_task_rq_stop,
112	.set_cpus_allowed	= set_cpus_allowed_common,
113#endif
114
115	.task_tick		= task_tick_stop,
116
117	.prio_changed		= prio_changed_stop,
118	.switched_to		= switched_to_stop,
119	.update_curr		= update_curr_stop,
120};