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 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 24 static void 25 wakeup_preempt_stop(struct rq *rq, struct task_struct *p, int flags) 26 { 27 /* we're never preempted */ 28 } 29 30 static void set_next_task_stop(struct rq *rq, struct task_struct *stop, bool first) 31 { 32 stop->se.exec_start = rq_clock_task(rq); 33 } 34 35 static struct task_struct *pick_task_stop(struct rq *rq, struct rq_flags *rf) 36 { 37 if (!sched_stop_runnable(rq)) 38 return NULL; 39 40 return rq->stop; 41 } 42 43 static void 44 enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags) 45 { 46 add_nr_running(rq, 1); 47 } 48 49 static bool 50 dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags) 51 { 52 sub_nr_running(rq, 1); 53 return true; 54 } 55 56 static void yield_task_stop(struct rq *rq) 57 { 58 BUG(); /* the stop task should never yield, its pointless. */ 59 } 60 61 static void put_prev_task_stop(struct rq *rq, struct task_struct *prev, struct task_struct *next) 62 { 63 update_curr_common(rq); 64 } 65 66 /* 67 * scheduler tick hitting a task of our scheduling class. 68 * 69 * NOTE: This function can be called remotely by the tick offload that 70 * goes along full dynticks. Therefore no local assumption can be made 71 * and everything must be accessed through the @rq and @curr passed in 72 * parameters. 73 */ 74 static void task_tick_stop(struct rq *rq, struct task_struct *curr, int queued) 75 { 76 } 77 78 static void switching_to_stop(struct rq *rq, struct task_struct *p) 79 { 80 BUG(); /* its impossible to change to this class */ 81 } 82 83 static void 84 prio_changed_stop(struct rq *rq, struct task_struct *p, u64 oldprio) 85 { 86 if (p->prio == oldprio) 87 return; 88 89 BUG(); /* how!?, what priority? */ 90 } 91 92 static void update_curr_stop(struct rq *rq) 93 { 94 } 95 96 /* 97 * Simple, special scheduling class for the per-CPU stop tasks: 98 */ 99 DEFINE_SCHED_CLASS(stop) = { 100 101 .queue_mask = 16, 102 103 .enqueue_task = enqueue_task_stop, 104 .dequeue_task = dequeue_task_stop, 105 .yield_task = yield_task_stop, 106 107 .wakeup_preempt = wakeup_preempt_stop, 108 109 .pick_task = pick_task_stop, 110 .put_prev_task = put_prev_task_stop, 111 .set_next_task = set_next_task_stop, 112 113 .balance = balance_stop, 114 .select_task_rq = select_task_rq_stop, 115 .set_cpus_allowed = set_cpus_allowed_common, 116 117 .task_tick = task_tick_stop, 118 119 .prio_changed = prio_changed_stop, 120 .switching_to = switching_to_stop, 121 .update_curr = update_curr_stop, 122 }; 123