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
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 #endif /* CONFIG_SMP */
24 
25 static void
check_preempt_curr_stop(struct rq * rq,struct task_struct * p,int flags)26 check_preempt_curr_stop(struct rq *rq, struct task_struct *p, int flags)
27 {
28 	/* we're never preempted */
29 }
30 
set_next_task_stop(struct rq * rq,struct task_struct * stop,bool first)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 
pick_task_stop(struct rq * rq)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 
pick_next_task_stop(struct rq * rq)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
enqueue_task_stop(struct rq * rq,struct task_struct * p,int flags)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
dequeue_task_stop(struct rq * rq,struct task_struct * p,int flags)61 dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags)
62 {
63 	sub_nr_running(rq, 1);
64 }
65 
yield_task_stop(struct rq * rq)66 static void yield_task_stop(struct rq *rq)
67 {
68 	BUG(); /* the stop task should never yield, its pointless. */
69 }
70 
put_prev_task_stop(struct rq * rq,struct task_struct * prev)71 static void put_prev_task_stop(struct rq *rq, struct task_struct *prev)
72 {
73 	struct task_struct *curr = rq->curr;
74 	u64 now, delta_exec;
75 
76 	now = rq_clock_task(rq);
77 	delta_exec = now - curr->se.exec_start;
78 	if (unlikely((s64)delta_exec < 0))
79 		delta_exec = 0;
80 
81 	schedstat_set(curr->stats.exec_max,
82 		      max(curr->stats.exec_max, delta_exec));
83 
84 	update_current_exec_runtime(curr, now, delta_exec);
85 }
86 
87 /*
88  * scheduler tick hitting a task of our scheduling class.
89  *
90  * NOTE: This function can be called remotely by the tick offload that
91  * goes along full dynticks. Therefore no local assumption can be made
92  * and everything must be accessed through the @rq and @curr passed in
93  * parameters.
94  */
task_tick_stop(struct rq * rq,struct task_struct * curr,int queued)95 static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued)
96 {
97 }
98 
switched_to_stop(struct rq * rq,struct task_struct * p)99 static void switched_to_stop(struct rq *rq, struct task_struct *p)
100 {
101 	BUG(); /* its impossible to change to this class */
102 }
103 
104 static void
prio_changed_stop(struct rq * rq,struct task_struct * p,int oldprio)105 prio_changed_stop(struct rq *rq, struct task_struct *p, int oldprio)
106 {
107 	BUG(); /* how!?, what priority? */
108 }
109 
update_curr_stop(struct rq * rq)110 static void update_curr_stop(struct rq *rq)
111 {
112 }
113 
114 /*
115  * Simple, special scheduling class for the per-CPU stop tasks:
116  */
117 DEFINE_SCHED_CLASS(stop) = {
118 
119 	.enqueue_task		= enqueue_task_stop,
120 	.dequeue_task		= dequeue_task_stop,
121 	.yield_task		= yield_task_stop,
122 
123 	.check_preempt_curr	= check_preempt_curr_stop,
124 
125 	.pick_next_task		= pick_next_task_stop,
126 	.put_prev_task		= put_prev_task_stop,
127 	.set_next_task          = set_next_task_stop,
128 
129 #ifdef CONFIG_SMP
130 	.balance		= balance_stop,
131 	.pick_task		= pick_task_stop,
132 	.select_task_rq		= select_task_rq_stop,
133 	.set_cpus_allowed	= set_cpus_allowed_common,
134 #endif
135 
136 	.task_tick		= task_tick_stop,
137 
138 	.prio_changed		= prio_changed_stop,
139 	.switched_to		= switched_to_stop,
140 	.update_curr		= update_curr_stop,
141 };
142