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 struct task_struct *pick_next_task_stop(struct rq *rq)
45{
46	struct task_struct *p = pick_task_stop(rq);
47
48	if (p)
49		set_next_task_stop(rq, p, true);
50
51	return p;
52}
53
54static void
55enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
56{
57	add_nr_running(rq, 1);
58}
59
60static void
61dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags)
62{
63	sub_nr_running(rq, 1);
64}
65
66static void yield_task_stop(struct rq *rq)
67{
68	BUG(); /* the stop task should never yield, its pointless. */
69}
70
71static void put_prev_task_stop(struct rq *rq, struct task_struct *prev)
72{
73	update_curr_common(rq);
74}
75
76/*
77 * scheduler tick hitting a task of our scheduling class.
78 *
79 * NOTE: This function can be called remotely by the tick offload that
80 * goes along full dynticks. Therefore no local assumption can be made
81 * and everything must be accessed through the @rq and @curr passed in
82 * parameters.
83 */
84static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued)
85{
86}
87
88static void switched_to_stop(struct rq *rq, struct task_struct *p)
89{
90	BUG(); /* its impossible to change to this class */
91}
92
93static void
94prio_changed_stop(struct rq *rq, struct task_struct *p, int oldprio)
95{
96	BUG(); /* how!?, what priority? */
97}
98
99static void update_curr_stop(struct rq *rq)
100{
101}
102
103/*
104 * Simple, special scheduling class for the per-CPU stop tasks:
105 */
106DEFINE_SCHED_CLASS(stop) = {
107
108	.enqueue_task		= enqueue_task_stop,
109	.dequeue_task		= dequeue_task_stop,
110	.yield_task		= yield_task_stop,
111
112	.wakeup_preempt		= wakeup_preempt_stop,
113
114	.pick_next_task		= pick_next_task_stop,
115	.put_prev_task		= put_prev_task_stop,
116	.set_next_task          = set_next_task_stop,
117
118#ifdef CONFIG_SMP
119	.balance		= balance_stop,
120	.pick_task		= pick_task_stop,
121	.select_task_rq		= select_task_rq_stop,
122	.set_cpus_allowed	= set_cpus_allowed_common,
123#endif
124
125	.task_tick		= task_tick_stop,
126
127	.prio_changed		= prio_changed_stop,
128	.switched_to		= switched_to_stop,
129	.update_curr		= update_curr_stop,
130};
131