stop_task.c

   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 };