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
12 static int
13 select_task_rq_stop(struct task_struct *p, int cpu, int flags)
14 {
15 return task_cpu(p); /* stop tasks as never migrate */
16 }
17
18 static int
19 balance_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
25 static void
26 wakeup_preempt_stop(struct rq *rq, struct task_struct *p, int flags)
27 {
28 /* we're never preempted */
29 }
30
31 static 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
36 static 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
44 static 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
54 static void
55 enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
56 {
57 add_nr_running(rq, 1);
58 }
59
60 static void
61 dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags)
62 {
63 sub_nr_running(rq, 1);
64 }
65
66 static void yield_task_stop(struct rq *rq)
67 {
68 BUG(); /* the stop task should never yield, its pointless. */
69 }
70
71 static 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 */
84 static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued)
85 {
86 }
87
88 static void switched_to_stop(struct rq *rq, struct task_struct *p)
89 {
90 BUG(); /* its impossible to change to this class */
91 }
92
93 static void
94 prio_changed_stop(struct rq *rq, struct task_struct *p, int oldprio)
95 {
96 BUG(); /* how!?, what priority? */
97 }
98
99 static void update_curr_stop(struct rq *rq)
100 {
101 }
102
103 /*
104 * Simple, special scheduling class for the per-CPU stop tasks:
105 */
106 DEFINE_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
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