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