linux内核中提供了两种实时调度策略：SCHED_FIFO和SCHED_RR，其中RR是带有时间片的FIFO。这两种调度算法实现的都是静态优先级。内核不为实时进程计算动态优先级。这能保证给定优先级别的实时进程总能抢占优先级比他低得进程。linux的实时调度算法提供了一种软实时工作方式。实时优先级范围从0到MAX_RT_PRIO减一。默认情况下，MAX_RT_PRIO为100，所以默认的实时优先级范围是从0到99.SCHED_NORMAL级进程的nice值共享了这个取值空间；他的取值范围是从MAX_RT_PRIO到MAX_RT_PRIO+40.也就是说，在默认情况下，nice值从-20到19直接对应的是从100到139的实时优先级范围。
数据结构
实时调度的优先级队列
实时调度的优先级队列是一组链表，每个优先级对应一个链表。
/*实时调度的优先级队列，实时调度中，运行进程运行队列
根据优先级放到对应的队列里面，对于相同的优先级
的进程后面来的进程放到同一优先级队列的队尾
对于FIFO/RR调度，各自的进程需要设置相关的属性
进程运行时，要根据task中的这些属性判断和设置
放弃cpu的时机(运行完或是时间片用完)*/
struct rt_prio_array {
DECLARE_BITMAP(bitmap, MAX_RT_PRIO+1); /* include 1 bit for delimiter */
struct list_head queue[MAX_RT_PRIO];
};
运行队列组织实时调度的相关信息
/* Real-Time classes' related field in a runqueue: */进程调度实体
struct rt_rq {
struct rt_prio_array active;
unsigned long rt_nr_running;
#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
struct {
int curr; /* highest queued rt task prio */
#ifdef CONFIG_SMP
int next; /* next highest */
#endif
} highest_prio;
#endif
#ifdef CONFIG_SMP
unsigned long rt_nr_migratory;
unsigned long rt_nr_total;
int overloaded;
struct plist_head pushable_tasks;
#endif
int rt_throttled;
u64 rt_time;
u64 rt_runtime;
/* Nests inside the rq lock: */
spinlock_t rt_runtime_lock;
#ifdef CONFIG_RT_GROUP_SCHED
unsigned long rt_nr_boosted;
struct rq *rq;
struct list_head leaf_rt_rq_list;
struct task_group *tg;
struct sched_rt_entity *rt_se;
#endif
};
struct sched_rt_entity {
struct list_head run_list;
unsigned long timeout;
unsigned int time_slice;
int nr_cpus_allowed;
struct sched_rt_entity *back;
#ifdef CONFIG_RT_GROUP_SCHED
struct sched_rt_entity*parent;
/* rq on which this entity is (to be) queued: */
struct rt_rq*rt_rq;
/* rq "owned" by this entity/group: */
struct rt_rq*my_q;
#endif
};
调度类
static 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
.select_task_rq= select_task_rq_rt,
.load_balance= load_balance_rt,
.move_one_task= move_one_task_rt,
.set_cpus_allowed = set_cpus_allowed_rt,
.rq_online = rq_online_rt,
.rq_offline = rq_offline_rt,
.pre_schedule= pre_schedule_rt,
.post_schedule= post_schedule_rt,
.task_wake_up= task_wake_up_rt,
.switched_from= switched_from_rt,
#endif
.set_curr_task = set_curr_task_rt,
.task_tick= task_tick_rt,
.get_rr_interval= get_rr_interval_rt,
.prio_changed= prio_changed_rt,
.switched_to= switched_to_rt,
};
从运行队列中移除函数dequeue_task_rt
static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int sleep)
{
struct sched_rt_entity *rt_se = &p->rt;
/*更新调度信息*/
update_curr_rt(rq);
/*实际工作，将rt_se从运行队列中删除然后
添加到队列尾部*/
dequeue_rt_entity(rt_se);
/*从hash表中删除*/
dequeue_pushable_task(rq, p);
}
/*
* 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;
struct sched_rt_entity *rt_se = &curr->rt;
struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
u64 delta_exec;
if (!task_has_rt_policy(curr))/*判断是否问实时调度进程*/
return;
/*执行时间*/
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));
/*更新当前进程的总的执行时间*/
curr->se.sum_exec_runtime += delta_exec;
account_group_exec_runtime(curr, delta_exec);
/*更新执行的开始时间*/
curr->se.exec_start = rq->clock;
cpuacct_charge(curr, delta_exec);/*主调度相关*/
/*更新调度信息*/
sched_rt_avg_update(rq, delta_exec);
if (!rt_bandwidth_enabled())
return;
for_each_sched_rt_entity(rt_se) {
rt_rq = rt_rq_of_se(rt_se);
if (sched_rt_runtime(rt_rq) != RUNTIME_INF) {
spin_lock(&rt_rq->rt_runtime_lock);
rt_rq->rt_time += delta_exec;
if (sched_rt_runtime_exceeded(rt_rq))
resched_task(curr);
spin_unlock(&rt_rq->rt_runtime_lock);
}
}
}
static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
{
/*从运行队列中删除*/
dequeue_rt_stack(rt_se);
for_each_sched_rt_entity(rt_se) {
struct rt_rq *rt_rq = group_rt_rq(rt_se);
if (rt_rq && rt_rq->rt_nr_running)
__enqueue_rt_entity(rt_se);/*添加到队列尾*/
}
}
/*删除的实际工作为：
* Because the prio of an upper entry depends on the lower
* entries, we must remove entries top - down.
*/
static void dequeue_rt_stack(struct sched_rt_entity *rt_se)
{
struct sched_rt_entity *back = NULL;
for_each_sched_rt_entity(rt_se) {/*遍历整个组调度实体*/
rt_se->back = back;/*可见rt_se的back实体为组调度中前一个调度实体*/
back = rt_se;
}
/*将组中的所有进程从运行队列中移除*/
for (rt_se = back; rt_se; rt_se = rt_se->back) {
if (on_rt_rq(rt_se))
__dequeue_rt_entity(rt_se);
}
}
static void __dequeue_rt_entity(struct sched_rt_entity *rt_se)添加进程到运行队列
{
struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
struct rt_prio_array *array = &rt_rq->active;
list_del_init(&rt_se->run_list);
if (list_empty(array->queue + rt_se_prio(rt_se)))
__clear_bit(rt_se_prio(rt_se), array->bitmap);
dec_rt_tasks(rt_se, rt_rq);
}
/*
* Adding/removing a task to/from a priority array:
*/
static void enqueue_task_rt(struct rq *rq, struct task_struct *p, int wakeup)
{
struct sched_rt_entity *rt_se = &p->rt;
if (wakeup)
rt_se->timeout = 0;
/*实际工作*/
enqueue_rt_entity(rt_se);
if (!task_current(rq, p) && p->rt.nr_cpus_allowed > 1)
enqueue_pushable_task(rq, p);/*添加到对应的hash表中*/
}
static void enqueue_rt_entity(struct sched_rt_entity *rt_se){dequeue_rt_stack(rt_se);/*从运行队列中删除*/for_each_sched_rt_entity(rt_se)__enqueue_rt_entity(rt_se);/*添加到运行队列尾部*/}实际的添加工作：
static void __enqueue_rt_entity(struct sched_rt_entity *rt_se)选择下一个进程运行
{
struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
struct rt_prio_array *array = &rt_rq->active;
struct rt_rq *group_rq = group_rt_rq(rt_se);
struct list_head *queue = array->queue + rt_se_prio(rt_se);
/*
* Don't enqueue the group if its throttled, or when empty.
* The latter is a consequence of the former when a child group
* get throttled and the current group doesn't have any other
* active members.
*/
if (group_rq && (rt_rq_throttled(group_rq) || !group_rq->rt_nr_running))
return;
list_add_tail(&rt_se->run_list, queue);
__set_bit(rt_se_prio(rt_se), array->bitmap);
inc_rt_tasks(rt_se, rt_rq);
}
static struct task_struct *pick_next_task_rt(struct rq *rq)
{
struct task_struct *p = _pick_next_task_rt(rq);/*实际工作*/
/* The running task is never eligible for pushing */
if (p)
dequeue_pushable_task(rq, p);
#ifdef CONFIG_SMP
/*
* We detect this state here so that we can avoid taking the RQ
* lock again later if there is no need to push
*/
rq->post_schedule = has_pushable_tasks(rq);
#endif
return p;
}
static struct task_struct *_pick_next_task_rt(struct rq *rq)
{
struct sched_rt_entity *rt_se;
struct task_struct *p;
struct rt_rq *rt_rq;
rt_rq = &rq->rt;
if (unlikely(!rt_rq->rt_nr_running))
return NULL;
if (rt_rq_throttled(rt_rq))
return NULL;
do {/*遍历组调度中的每个进程*/
rt_se = pick_next_rt_entity(rq, rt_rq);
BUG_ON(!rt_se);
rt_rq = group_rt_rq(rt_se);
} while (rt_rq);
p = rt_task_of(rt_se);
/*更新执行域*/
p->se.exec_start = rq->clock;
return p;
}
static struct sched_rt_entity *pick_next_rt_entity(struct rq *rq,总结：对于实时调度，基于linux内核调度框架下作的工作比较简单，把所有的运行进程根据优先级放到不用的队列里面，采用位图方式进行使用记录。进队列仅仅是删除原来队列里面的本进程，然后将他挂到队列尾部；而对于“移除”操作，也仅仅是从队列里面移除后添加到运行队列尾部。
struct rt_rq *rt_rq)
{
struct rt_prio_array *array = &rt_rq->active;
struct sched_rt_entity *next = NULL;
struct list_head *queue;
int idx;
/*找到第一个可用的*/
idx = sched_find_first_bit(array->bitmap);
BUG_ON(idx >= MAX_RT_PRIO);
/*从链表组中找到对应的链表*/
queue = array->queue + idx;
next = list_entry(queue->next, struct sched_rt_entity, run_list);
/*返回找到的运行实体*/
return next;
}