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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#include "sched.h"
11
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 rq_flags *rf)
20{
21 return sched_stop_runnable(rq);
22}
23
24static void
25wakeup_preempt_stop(struct rq *rq, struct task_struct *p, int flags)
26{
27 /* we're never preempted */
28}
29
30static void set_next_task_stop(struct rq *rq, struct task_struct *stop, enum snt_e type)
31{
32 if (type == SNT_REPICK)
33 return;
34
35 stop->se.exec_start = rq_clock_task(rq);
36}
37
38static struct task_struct *pick_task_stop(struct rq *rq, struct rq_flags *rf)
39{
40 if (!sched_stop_runnable(rq))
41 return NULL;
42
43 return rq->stop;
44}
45
46static void
47enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
48{
49 add_nr_running(rq, 1);
50}
51
52static bool
53dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags)
54{
55 sub_nr_running(rq, 1);
56 return true;
57}
58
59static void yield_task_stop(struct rq *rq)
60{
61 BUG(); /* the stop task should never yield, its pointless. */
62}
63
64static void put_prev_task_stop(struct rq *rq, struct task_struct *prev, struct task_struct *next)
65{
66 update_curr_common(rq);
67}
68
69/*
70 * scheduler tick hitting a task of our scheduling class.
71 *
72 * NOTE: This function can be called remotely by the tick offload that
73 * goes along full dynticks. Therefore no local assumption can be made
74 * and everything must be accessed through the @rq and @curr passed in
75 * parameters.
76 */
77static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued)
78{
79}
80
81static void switching_to_stop(struct rq *rq, struct task_struct *p)
82{
83 BUG(); /* its impossible to change to this class */
84}
85
86static void
87prio_changed_stop(struct rq *rq, struct task_struct *p, u64 oldprio)
88{
89 if (p->prio == oldprio)
90 return;
91
92 BUG(); /* how!?, what priority? */
93}
94
95static void update_curr_stop(struct rq *rq)
96{
97}
98
99/*
100 * Simple, special scheduling class for the per-CPU stop tasks:
101 */
102DEFINE_SCHED_CLASS(stop) = {
103 .enqueue_task = enqueue_task_stop,
104 .dequeue_task = dequeue_task_stop,
105 .yield_task = yield_task_stop,
106
107 .wakeup_preempt = wakeup_preempt_stop,
108
109 .pick_task = pick_task_stop,
110 .put_prev_task = put_prev_task_stop,
111 .set_next_task = set_next_task_stop,
112
113 .balance = balance_stop,
114 .select_task_rq = select_task_rq_stop,
115 .set_cpus_allowed = set_cpus_allowed_common,
116
117 .task_tick = task_tick_stop,
118
119 .prio_changed = prio_changed_stop,
120 .switching_to = switching_to_stop,
121 .update_curr = update_curr_stop,
122};