kernel-hacking-2024-linux-steffo/kernel/sched_rt.c

/*
 * Real-Time Scheduling Class (mapped to the SCHED_FIFO and SCHED_RR
 * policies)
 */

/*
 * Update the current task's runtime statistics. Skip current tasks that
 * are not in our scheduling class.
 */
static void update_curr_rt(struct rq *rq)
{
	struct task_struct *curr = rq->curr;
	u64 delta_exec;

	if (!task_has_rt_policy(curr))
		return;

	delta_exec = rq->clock - curr->se.exec_start;
	if (unlikely((s64)delta_exec < 0))
		delta_exec = 0;

	schedstat_set(curr->se.exec_max, max(curr->se.exec_max, delta_exec));

	curr->se.sum_exec_runtime += delta_exec;
	curr->se.exec_start = rq->clock;
	cpuacct_charge(curr, delta_exec);
}

static inline void inc_rt_tasks(struct task_struct *p, struct rq *rq)
{
	WARN_ON(!rt_task(p));
	rq->rt.rt_nr_running++;
#ifdef CONFIG_SMP
	if (p->prio < rq->rt.highest_prio)
		rq->rt.highest_prio = p->prio;
#endif /* CONFIG_SMP */
}

static inline void dec_rt_tasks(struct task_struct *p, struct rq *rq)
{
	WARN_ON(!rt_task(p));
	WARN_ON(!rq->rt.rt_nr_running);
	rq->rt.rt_nr_running--;
#ifdef CONFIG_SMP
	if (rq->rt.rt_nr_running) {
		struct rt_prio_array *array;

		WARN_ON(p->prio < rq->rt.highest_prio);
		if (p->prio == rq->rt.highest_prio) {
			/* recalculate */
			array = &rq->rt.active;
			rq->rt.highest_prio =
				sched_find_first_bit(array->bitmap);
		} /* otherwise leave rq->highest prio alone */
	} else
		rq->rt.highest_prio = MAX_RT_PRIO;
#endif /* CONFIG_SMP */
}

static void enqueue_task_rt(struct rq *rq, struct task_struct *p, int wakeup)
{
	struct rt_prio_array *array = &rq->rt.active;

	list_add_tail(&p->run_list, array->queue + p->prio);
	__set_bit(p->prio, array->bitmap);
	inc_cpu_load(rq, p->se.load.weight);

	inc_rt_tasks(p, rq);
}

/*
 * Adding/removing a task to/from a priority array:
 */
static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int sleep)
{
	struct rt_prio_array *array = &rq->rt.active;

	update_curr_rt(rq);

	list_del(&p->run_list);
	if (list_empty(array->queue + p->prio))
		__clear_bit(p->prio, array->bitmap);
	dec_cpu_load(rq, p->se.load.weight);

	dec_rt_tasks(p, rq);
}

/*
 * Put task to the end of the run list without the overhead of dequeue
 * followed by enqueue.
 */
static void requeue_task_rt(struct rq *rq, struct task_struct *p)
{
	struct rt_prio_array *array = &rq->rt.active;

	list_move_tail(&p->run_list, array->queue + p->prio);
}

static void
yield_task_rt(struct rq *rq)
{
	requeue_task_rt(rq, rq->curr);
}

/*
 * Preempt the current task with a newly woken task if needed:
 */
static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p)
{
	if (p->prio < rq->curr->prio)
		resched_task(rq->curr);
}

static struct task_struct *pick_next_task_rt(struct rq *rq)
{
	struct rt_prio_array *array = &rq->rt.active;
	struct task_struct *next;
	struct list_head *queue;
	int idx;

	idx = sched_find_first_bit(array->bitmap);
	if (idx >= MAX_RT_PRIO)
		return NULL;

	queue = array->queue + idx;
	next = list_entry(queue->next, struct task_struct, run_list);

	next->se.exec_start = rq->clock;

	return next;
}

static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
{
	update_curr_rt(rq);
	p->se.exec_start = 0;
}

#ifdef CONFIG_SMP
/*
 * Load-balancing iterator. Note: while the runqueue stays locked
 * during the whole iteration, the current task might be
 * dequeued so the iterator has to be dequeue-safe. Here we
 * achieve that by always pre-iterating before returning
 * the current task:
 */
static struct task_struct *load_balance_start_rt(void *arg)
{
	struct rq *rq = arg;
	struct rt_prio_array *array = &rq->rt.active;
	struct list_head *head, *curr;
	struct task_struct *p;
	int idx;

	idx = sched_find_first_bit(array->bitmap);
	if (idx >= MAX_RT_PRIO)
		return NULL;

	head = array->queue + idx;
	curr = head->prev;

	p = list_entry(curr, struct task_struct, run_list);

	curr = curr->prev;

	rq->rt.rt_load_balance_idx = idx;
	rq->rt.rt_load_balance_head = head;
	rq->rt.rt_load_balance_curr = curr;

	return p;
}

static struct task_struct *load_balance_next_rt(void *arg)
{
	struct rq *rq = arg;
	struct rt_prio_array *array = &rq->rt.active;
	struct list_head *head, *curr;
	struct task_struct *p;
	int idx;

	idx = rq->rt.rt_load_balance_idx;
	head = rq->rt.rt_load_balance_head;
	curr = rq->rt.rt_load_balance_curr;

	/*
	 * If we arrived back to the head again then
	 * iterate to the next queue (if any):
	 */
	if (unlikely(head == curr)) {
		int next_idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);

		if (next_idx >= MAX_RT_PRIO)
			return NULL;

		idx = next_idx;
		head = array->queue + idx;
		curr = head->prev;

		rq->rt.rt_load_balance_idx = idx;
		rq->rt.rt_load_balance_head = head;
	}

	p = list_entry(curr, struct task_struct, run_list);

	curr = curr->prev;

	rq->rt.rt_load_balance_curr = curr;

	return p;
}

static unsigned long
load_balance_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
		unsigned long max_load_move,
		struct sched_domain *sd, enum cpu_idle_type idle,
		int *all_pinned, int *this_best_prio)
{
	struct rq_iterator rt_rq_iterator;

	rt_rq_iterator.start = load_balance_start_rt;
	rt_rq_iterator.next = load_balance_next_rt;
	/* pass 'busiest' rq argument into
	 * load_balance_[start|next]_rt iterators
	 */
	rt_rq_iterator.arg = busiest;

	return balance_tasks(this_rq, this_cpu, busiest, max_load_move, sd,
			     idle, all_pinned, this_best_prio, &rt_rq_iterator);
}

static int
move_one_task_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
		 struct sched_domain *sd, enum cpu_idle_type idle)
{
	struct rq_iterator rt_rq_iterator;

	rt_rq_iterator.start = load_balance_start_rt;
	rt_rq_iterator.next = load_balance_next_rt;
	rt_rq_iterator.arg = busiest;

	return iter_move_one_task(this_rq, this_cpu, busiest, sd, idle,
				  &rt_rq_iterator);
}
#endif

static void task_tick_rt(struct rq *rq, struct task_struct *p)
{
	update_curr_rt(rq);

	/*
	 * RR tasks need a special form of timeslice management.
	 * FIFO tasks have no timeslices.
	 */
	if (p->policy != SCHED_RR)
		return;

	if (--p->time_slice)
		return;

	p->time_slice = DEF_TIMESLICE;

	/*
	 * Requeue to the end of queue if we are not the only element
	 * on the queue:
	 */
	if (p->run_list.prev != p->run_list.next) {
		requeue_task_rt(rq, p);
		set_tsk_need_resched(p);
	}
}

static void set_curr_task_rt(struct rq *rq)
{
	struct task_struct *p = rq->curr;

	p->se.exec_start = rq->clock;
}

const struct sched_class rt_sched_class = {
	.next			= &fair_sched_class,
	.enqueue_task		= enqueue_task_rt,
	.dequeue_task		= dequeue_task_rt,
	.yield_task		= yield_task_rt,

	.check_preempt_curr	= check_preempt_curr_rt,

	.pick_next_task		= pick_next_task_rt,
	.put_prev_task		= put_prev_task_rt,

#ifdef CONFIG_SMP
	.load_balance		= load_balance_rt,
	.move_one_task		= move_one_task_rt,
#endif

	.set_curr_task          = set_curr_task_rt,
	.task_tick		= task_tick_rt,
};