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 void 45 enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags) 46 { 47 add_nr_running(rq, 1); 48 } 49 50 static bool 51 dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags) 52 { 53 sub_nr_running(rq, 1); 54 return true; 55 } 56 57 static void yield_task_stop(struct rq *rq) 58 { 59 BUG(); /* the stop task should never yield, its pointless. */ 60 } 61 62 static void put_prev_task_stop(struct rq *rq, struct task_struct *prev, struct task_struct *next) 63 { 64 update_curr_common(rq); 65 } 66 67 /* 68 * scheduler tick hitting a task of our scheduling class. 69 * 70 * NOTE: This function can be called remotely by the tick offload that 71 * goes along full dynticks. Therefore no local assumption can be made 72 * and everything must be accessed through the @rq and @curr passed in 73 * parameters. 74 */ 75 static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued) 76 { 77 } 78 79 static void switched_to_stop(struct rq *rq, struct task_struct *p) 80 { 81 BUG(); /* its impossible to change to this class */ 82 } 83 84 static void 85 prio_changed_stop(struct rq *rq, struct task_struct *p, int oldprio) 86 { 87 BUG(); /* how!?, what priority? */ 88 } 89 90 static void update_curr_stop(struct rq *rq) 91 { 92 } 93 94 /* 95 * Simple, special scheduling class for the per-CPU stop tasks: 96 */ 97 DEFINE_SCHED_CLASS(stop) = { 98 99 .enqueue_task = enqueue_task_stop, 100 .dequeue_task = dequeue_task_stop, 101 .yield_task = yield_task_stop, 102 103 .wakeup_preempt = wakeup_preempt_stop, 104 105 .pick_task = pick_task_stop, 106 .put_prev_task = put_prev_task_stop, 107 .set_next_task = set_next_task_stop, 108 109 #ifdef CONFIG_SMP 110 .balance = balance_stop, 111 .select_task_rq = select_task_rq_stop, 112 .set_cpus_allowed = set_cpus_allowed_common, 113 #endif 114 115 .task_tick = task_tick_stop, 116 117 .prio_changed = prio_changed_stop, 118 .switched_to = switched_to_stop, 119 .update_curr = update_curr_stop, 120 }; 121