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#ifdef CONFIG_SMP
 13static int
 14select_task_rq_stop(struct task_struct *p, int cpu, int sd_flag, int flags)
 15{
 16	return task_cpu(p); /* stop tasks as never migrate */
 17}
 18
 19static int
 20balance_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 21{
 22	return sched_stop_runnable(rq);
 23}
 24#endif /* CONFIG_SMP */
 25
 26static void
 27check_preempt_curr_stop(struct rq *rq, struct task_struct *p, int flags)
 28{
 29	/* we're never preempted */
 30}
 31
 32static void set_next_task_stop(struct rq *rq, struct task_struct *stop)
 33{
 34	stop->se.exec_start = rq_clock_task(rq);
 35}
 36
 37static struct task_struct *
 38pick_next_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 39{
 40	WARN_ON_ONCE(prev || rf);
 41
 42	if (!sched_stop_runnable(rq))
 43		return NULL;
 44
 45	set_next_task_stop(rq, rq->stop);
 46	return rq->stop;
 47}
 48
 49static void
 50enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
 51{
 52	add_nr_running(rq, 1);
 53}
 54
 55static void
 56dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags)
 57{
 58	sub_nr_running(rq, 1);
 59}
 60
 61static void yield_task_stop(struct rq *rq)
 62{
 63	BUG(); /* the stop task should never yield, its pointless. */
 64}
 65
 66static void put_prev_task_stop(struct rq *rq, struct task_struct *prev)
 67{
 68	struct task_struct *curr = rq->curr;
 69	u64 delta_exec;
 70
 71	delta_exec = rq_clock_task(rq) - curr->se.exec_start;
 72	if (unlikely((s64)delta_exec < 0))
 73		delta_exec = 0;
 74
 75	schedstat_set(curr->se.statistics.exec_max,
 76			max(curr->se.statistics.exec_max, delta_exec));
 77
 78	curr->se.sum_exec_runtime += delta_exec;
 79	account_group_exec_runtime(curr, delta_exec);
 80
 81	curr->se.exec_start = rq_clock_task(rq);
 82	cgroup_account_cputime(curr, delta_exec);
 83}
 84
 85/*
 86 * scheduler tick hitting a task of our scheduling class.
 87 *
 88 * NOTE: This function can be called remotely by the tick offload that
 89 * goes along full dynticks. Therefore no local assumption can be made
 90 * and everything must be accessed through the @rq and @curr passed in
 91 * parameters.
 92 */
 93static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued)
 94{
 95}
 96
 97static void switched_to_stop(struct rq *rq, struct task_struct *p)
 98{
 99	BUG(); /* its impossible to change to this class */
100}
101
102static void
103prio_changed_stop(struct rq *rq, struct task_struct *p, int oldprio)
104{
105	BUG(); /* how!?, what priority? */
106}
107
108static unsigned int
109get_rr_interval_stop(struct rq *rq, struct task_struct *task)
110{
111	return 0;
112}
113
114static void update_curr_stop(struct rq *rq)
115{
116}
117
118/*
119 * Simple, special scheduling class for the per-CPU stop tasks:
120 */
121const struct sched_class stop_sched_class = {
122	.next			= &dl_sched_class,
123
124	.enqueue_task		= enqueue_task_stop,
125	.dequeue_task		= dequeue_task_stop,
126	.yield_task		= yield_task_stop,
127
128	.check_preempt_curr	= check_preempt_curr_stop,
129
130	.pick_next_task		= pick_next_task_stop,
131	.put_prev_task		= put_prev_task_stop,
132	.set_next_task          = set_next_task_stop,
133
134#ifdef CONFIG_SMP
135	.balance		= balance_stop,
136	.select_task_rq		= select_task_rq_stop,
137	.set_cpus_allowed	= set_cpus_allowed_common,
138#endif
139
140	.task_tick		= task_tick_stop,
141
142	.get_rr_interval	= get_rr_interval_stop,
143
144	.prio_changed		= prio_changed_stop,
145	.switched_to		= switched_to_stop,
146	.update_curr		= update_curr_stop,
147};