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
12 static int
select_task_rq_stop(struct task_struct * p,int cpu,int flags)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
balance_stop(struct rq * rq,struct task_struct * prev,struct rq_flags * rf)19 balance_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
20 {
21 return sched_stop_runnable(rq);
22 }
23
24 static void
wakeup_preempt_stop(struct rq * rq,struct task_struct * p,int flags)25 wakeup_preempt_stop(struct rq *rq, struct task_struct *p, int flags)
26 {
27 /* we're never preempted */
28 }
29
set_next_task_stop(struct rq * rq,struct task_struct * stop,bool first)30 static void set_next_task_stop(struct rq *rq, struct task_struct *stop, bool first)
31 {
32 stop->se.exec_start = rq_clock_task(rq);
33 }
34
pick_task_stop(struct rq * rq)35 static struct task_struct *pick_task_stop(struct rq *rq)
36 {
37 if (!sched_stop_runnable(rq))
38 return NULL;
39
40 return rq->stop;
41 }
42
43 static void
enqueue_task_stop(struct rq * rq,struct task_struct * p,int flags)44 enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
45 {
46 add_nr_running(rq, 1);
47 }
48
49 static bool
dequeue_task_stop(struct rq * rq,struct task_struct * p,int flags)50 dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags)
51 {
52 sub_nr_running(rq, 1);
53 return true;
54 }
55
yield_task_stop(struct rq * rq)56 static void yield_task_stop(struct rq *rq)
57 {
58 BUG(); /* the stop task should never yield, its pointless. */
59 }
60
put_prev_task_stop(struct rq * rq,struct task_struct * prev,struct task_struct * next)61 static void put_prev_task_stop(struct rq *rq, struct task_struct *prev, struct task_struct *next)
62 {
63 update_curr_common(rq);
64 }
65
66 /*
67 * scheduler tick hitting a task of our scheduling class.
68 *
69 * NOTE: This function can be called remotely by the tick offload that
70 * goes along full dynticks. Therefore no local assumption can be made
71 * and everything must be accessed through the @rq and @curr passed in
72 * parameters.
73 */
task_tick_stop(struct rq * rq,struct task_struct * curr,int queued)74 static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued)
75 {
76 }
77
switched_to_stop(struct rq * rq,struct task_struct * p)78 static void switched_to_stop(struct rq *rq, struct task_struct *p)
79 {
80 BUG(); /* its impossible to change to this class */
81 }
82
83 static void
prio_changed_stop(struct rq * rq,struct task_struct * p,int oldprio)84 prio_changed_stop(struct rq *rq, struct task_struct *p, int oldprio)
85 {
86 BUG(); /* how!?, what priority? */
87 }
88
update_curr_stop(struct rq * rq)89 static void update_curr_stop(struct rq *rq)
90 {
91 }
92
93 /*
94 * Simple, special scheduling class for the per-CPU stop tasks:
95 */
96 DEFINE_SCHED_CLASS(stop) = {
97
98 .enqueue_task = enqueue_task_stop,
99 .dequeue_task = dequeue_task_stop,
100 .yield_task = yield_task_stop,
101
102 .wakeup_preempt = wakeup_preempt_stop,
103
104 .pick_task = pick_task_stop,
105 .put_prev_task = put_prev_task_stop,
106 .set_next_task = set_next_task_stop,
107
108 .balance = balance_stop,
109 .select_task_rq = select_task_rq_stop,
110 .set_cpus_allowed = set_cpus_allowed_common,
111
112 .task_tick = task_tick_stop,
113
114 .prio_changed = prio_changed_stop,
115 .switched_to = switched_to_stop,
116 .update_curr = update_curr_stop,
117 };
118