sched: RT-balance, add new methods to sched_class

[mirror_ubuntu-zesty-kernel.git] / kernel / sched_rt.c

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

#ifdef CONFIG_SMP

static inline int rt_overloaded(struct rq *rq)
{
        return atomic_read(&rq->rd->rto_count);
}

static inline void rt_set_overload(struct rq *rq)
{
        cpu_set(rq->cpu, rq->rd->rto_mask);
        /*
         * Make sure the mask is visible before we set
         * the overload count. That is checked to determine
         * if we should look at the mask. It would be a shame
         * if we looked at the mask, but the mask was not
         * updated yet.
         */
        wmb();
        atomic_inc(&rq->rd->rto_count);
}

static inline void rt_clear_overload(struct rq *rq)
{
        /* the order here really doesn't matter */
        atomic_dec(&rq->rd->rto_count);
        cpu_clear(rq->cpu, rq->rd->rto_mask);
}

static void update_rt_migration(struct rq *rq)
{
        if (rq->rt.rt_nr_migratory && (rq->rt.rt_nr_running > 1)) {
                rt_set_overload(rq);
                rq->rt.overloaded = 1;
        } else {
                rt_clear_overload(rq);
                rq->rt.overloaded = 0;
        }
}
#endif /* CONFIG_SMP */

/*
 * 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;
        if (p->nr_cpus_allowed > 1)
                rq->rt.rt_nr_migratory++;

        update_rt_migration(rq);
#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;
        if (p->nr_cpus_allowed > 1)
                rq->rt.rt_nr_migratory--;

        update_rt_migration(rq);
#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);
}

#ifdef CONFIG_SMP
static int find_lowest_rq(struct task_struct *task);

static int select_task_rq_rt(struct task_struct *p, int sync)
{
        struct rq *rq = task_rq(p);

        /*
         * If the current task is an RT task, then
         * try to see if we can wake this RT task up on another
         * runqueue. Otherwise simply start this RT task
         * on its current runqueue.
         *
         * We want to avoid overloading runqueues. Even if
         * the RT task is of higher priority than the current RT task.
         * RT tasks behave differently than other tasks. If
         * one gets preempted, we try to push it off to another queue.
         * So trying to keep a preempting RT task on the same
         * cache hot CPU will force the running RT task to
         * a cold CPU. So we waste all the cache for the lower
         * RT task in hopes of saving some of a RT task
         * that is just being woken and probably will have
         * cold cache anyway.
         */
        if (unlikely(rt_task(rq->curr)) &&
            (p->nr_cpus_allowed > 1)) {
                int cpu = find_lowest_rq(p);

                return (cpu == -1) ? task_cpu(p) : cpu;
        }

        /*
         * Otherwise, just let it ride on the affined RQ and the
         * post-schedule router will push the preempted task away
         */
        return task_cpu(p);
}
#endif /* CONFIG_SMP */

/*
 * 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
/* Only try algorithms three times */
#define RT_MAX_TRIES 3

static int double_lock_balance(struct rq *this_rq, struct rq *busiest);
static void deactivate_task(struct rq *rq, struct task_struct *p, int sleep);

static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
{
        if (!task_running(rq, p) &&
            (cpu < 0 || cpu_isset(cpu, p->cpus_allowed)) &&
            (p->nr_cpus_allowed > 1))
                return 1;
        return 0;
}

/* Return the second highest RT task, NULL otherwise */
static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu)
{
        struct rt_prio_array *array = &rq->rt.active;
        struct task_struct *next;
        struct list_head *queue;
        int idx;

        if (likely(rq->rt.rt_nr_running < 2))
                return NULL;

        idx = sched_find_first_bit(array->bitmap);
        if (unlikely(idx >= MAX_RT_PRIO)) {
                WARN_ON(1); /* rt_nr_running is bad */
                return NULL;
        }

        queue = array->queue + idx;
        BUG_ON(list_empty(queue));

        next = list_entry(queue->next, struct task_struct, run_list);
        if (unlikely(pick_rt_task(rq, next, cpu)))
                goto out;

        if (queue->next->next != queue) {
                /* same prio task */
                next = list_entry(queue->next->next, struct task_struct,
                                  run_list);
                if (pick_rt_task(rq, next, cpu))
                        goto out;
        }

 retry:
        /* slower, but more flexible */
        idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
        if (unlikely(idx >= MAX_RT_PRIO))
                return NULL;

        queue = array->queue + idx;
        BUG_ON(list_empty(queue));

        list_for_each_entry(next, queue, run_list) {
                if (pick_rt_task(rq, next, cpu))
                        goto out;
        }

        goto retry;

 out:
        return next;
}

static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);

static int find_lowest_cpus(struct task_struct *task, cpumask_t *lowest_mask)
{
        int       lowest_prio = -1;
        int       lowest_cpu  = -1;
        int       count       = 0;
        int       cpu;

        cpus_and(*lowest_mask, task_rq(task)->rd->online, task->cpus_allowed);

        /*
         * Scan each rq for the lowest prio.
         */
        for_each_cpu_mask(cpu, *lowest_mask) {
                struct rq *rq = cpu_rq(cpu);

                /* We look for lowest RT prio or non-rt CPU */
                if (rq->rt.highest_prio >= MAX_RT_PRIO) {
                        /*
                         * if we already found a low RT queue
                         * and now we found this non-rt queue
                         * clear the mask and set our bit.
                         * Otherwise just return the queue as is
                         * and the count==1 will cause the algorithm
                         * to use the first bit found.
                         */
                        if (lowest_cpu != -1) {
                                cpus_clear(*lowest_mask);
                                cpu_set(rq->cpu, *lowest_mask);
                        }
                        return 1;
                }

                /* no locking for now */
                if ((rq->rt.highest_prio > task->prio)
                    && (rq->rt.highest_prio >= lowest_prio)) {
                        if (rq->rt.highest_prio > lowest_prio) {
                                /* new low - clear old data */
                                lowest_prio = rq->rt.highest_prio;
                                lowest_cpu = cpu;
                                count = 0;
                        }
                        count++;
                } else
                        cpu_clear(cpu, *lowest_mask);
        }

        /*
         * Clear out all the set bits that represent
         * runqueues that were of higher prio than
         * the lowest_prio.
         */
        if (lowest_cpu > 0) {
                /*
                 * Perhaps we could add another cpumask op to
                 * zero out bits. Like cpu_zero_bits(cpumask, nrbits);
                 * Then that could be optimized to use memset and such.
                 */
                for_each_cpu_mask(cpu, *lowest_mask) {
                        if (cpu >= lowest_cpu)
                                break;
                        cpu_clear(cpu, *lowest_mask);
                }
        }

        return count;
}

static inline int pick_optimal_cpu(int this_cpu, cpumask_t *mask)
{
        int first;

        /* "this_cpu" is cheaper to preempt than a remote processor */
        if ((this_cpu != -1) && cpu_isset(this_cpu, *mask))
                return this_cpu;

        first = first_cpu(*mask);
        if (first != NR_CPUS)
                return first;

        return -1;
}

static int find_lowest_rq(struct task_struct *task)
{
        struct sched_domain *sd;
        cpumask_t *lowest_mask = &__get_cpu_var(local_cpu_mask);
        int this_cpu = smp_processor_id();
        int cpu      = task_cpu(task);
        int count    = find_lowest_cpus(task, lowest_mask);

        if (!count)
                return -1; /* No targets found */

        /*
         * There is no sense in performing an optimal search if only one
         * target is found.
         */
        if (count == 1)
                return first_cpu(*lowest_mask);

        /*
         * At this point we have built a mask of cpus representing the
         * lowest priority tasks in the system.  Now we want to elect
         * the best one based on our affinity and topology.
         *
         * We prioritize the last cpu that the task executed on since
         * it is most likely cache-hot in that location.
         */
        if (cpu_isset(cpu, *lowest_mask))
                return cpu;

        /*
         * Otherwise, we consult the sched_domains span maps to figure
         * out which cpu is logically closest to our hot cache data.
         */
        if (this_cpu == cpu)
                this_cpu = -1; /* Skip this_cpu opt if the same */

        for_each_domain(cpu, sd) {
                if (sd->flags & SD_WAKE_AFFINE) {
                        cpumask_t domain_mask;
                        int       best_cpu;

                        cpus_and(domain_mask, sd->span, *lowest_mask);

                        best_cpu = pick_optimal_cpu(this_cpu,
                                                    &domain_mask);
                        if (best_cpu != -1)
                                return best_cpu;
                }
        }

        /*
         * And finally, if there were no matches within the domains
         * just give the caller *something* to work with from the compatible
         * locations.
         */
        return pick_optimal_cpu(this_cpu, lowest_mask);
}

/* Will lock the rq it finds */
static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
{
        struct rq *lowest_rq = NULL;
        int tries;
        int cpu;

        for (tries = 0; tries < RT_MAX_TRIES; tries++) {
                cpu = find_lowest_rq(task);

                if ((cpu == -1) || (cpu == rq->cpu))
                        break;

                lowest_rq = cpu_rq(cpu);

                /* if the prio of this runqueue changed, try again */
                if (double_lock_balance(rq, lowest_rq)) {
                        /*
                         * We had to unlock the run queue. In
                         * the mean time, task could have
                         * migrated already or had its affinity changed.
                         * Also make sure that it wasn't scheduled on its rq.
                         */
                        if (unlikely(task_rq(task) != rq ||
                                     !cpu_isset(lowest_rq->cpu,
                                                task->cpus_allowed) ||
                                     task_running(rq, task) ||
                                     !task->se.on_rq)) {

                                spin_unlock(&lowest_rq->lock);
                                lowest_rq = NULL;
                                break;
                        }
                }

                /* If this rq is still suitable use it. */
                if (lowest_rq->rt.highest_prio > task->prio)
                        break;

                /* try again */
                spin_unlock(&lowest_rq->lock);
                lowest_rq = NULL;
        }

        return lowest_rq;
}

/*
 * If the current CPU has more than one RT task, see if the non
 * running task can migrate over to a CPU that is running a task
 * of lesser priority.
 */
static int push_rt_task(struct rq *rq)
{
        struct task_struct *next_task;
        struct rq *lowest_rq;
        int ret = 0;
        int paranoid = RT_MAX_TRIES;

        if (!rq->rt.overloaded)
                return 0;

        next_task = pick_next_highest_task_rt(rq, -1);
        if (!next_task)
                return 0;

 retry:
        if (unlikely(next_task == rq->curr)) {
                WARN_ON(1);
                return 0;
        }

        /*
         * It's possible that the next_task slipped in of
         * higher priority than current. If that's the case
         * just reschedule current.
         */
        if (unlikely(next_task->prio < rq->curr->prio)) {
                resched_task(rq->curr);
                return 0;
        }

        /* We might release rq lock */
        get_task_struct(next_task);

        /* find_lock_lowest_rq locks the rq if found */
        lowest_rq = find_lock_lowest_rq(next_task, rq);
        if (!lowest_rq) {
                struct task_struct *task;
                /*
                 * find lock_lowest_rq releases rq->lock
                 * so it is possible that next_task has changed.
                 * If it has, then try again.
                 */
                task = pick_next_highest_task_rt(rq, -1);
                if (unlikely(task != next_task) && task && paranoid--) {
                        put_task_struct(next_task);
                        next_task = task;
                        goto retry;
                }
                goto out;
        }

        deactivate_task(rq, next_task, 0);
        set_task_cpu(next_task, lowest_rq->cpu);
        activate_task(lowest_rq, next_task, 0);

        resched_task(lowest_rq->curr);

        spin_unlock(&lowest_rq->lock);

        ret = 1;
out:
        put_task_struct(next_task);

        return ret;
}

/*
 * TODO: Currently we just use the second highest prio task on
 *       the queue, and stop when it can't migrate (or there's
 *       no more RT tasks).  There may be a case where a lower
 *       priority RT task has a different affinity than the
 *       higher RT task. In this case the lower RT task could
 *       possibly be able to migrate where as the higher priority
 *       RT task could not.  We currently ignore this issue.
 *       Enhancements are welcome!
 */
static void push_rt_tasks(struct rq *rq)
{
        /* push_rt_task will return true if it moved an RT */
        while (push_rt_task(rq))
                ;
}

static int pull_rt_task(struct rq *this_rq)
{
        int this_cpu = this_rq->cpu, ret = 0, cpu;
        struct task_struct *p, *next;
        struct rq *src_rq;

        if (likely(!rt_overloaded(this_rq)))
                return 0;

        next = pick_next_task_rt(this_rq);

        for_each_cpu_mask(cpu, this_rq->rd->rto_mask) {
                if (this_cpu == cpu)
                        continue;

                src_rq = cpu_rq(cpu);
                if (unlikely(src_rq->rt.rt_nr_running <= 1)) {
                        /*
                         * It is possible that overlapping cpusets
                         * will miss clearing a non overloaded runqueue.
                         * Clear it now.
                         */
                        if (double_lock_balance(this_rq, src_rq)) {
                                /* unlocked our runqueue lock */
                                struct task_struct *old_next = next;

                                next = pick_next_task_rt(this_rq);
                                if (next != old_next)
                                        ret = 1;
                        }
                        if (likely(src_rq->rt.rt_nr_running <= 1)) {
                                /*
                                 * Small chance that this_rq->curr changed
                                 * but it's really harmless here.
                                 */
                                rt_clear_overload(this_rq);
                        } else {
                                /*
                                 * Heh, the src_rq is now overloaded, since
                                 * we already have the src_rq lock, go straight
                                 * to pulling tasks from it.
                                 */
                                goto try_pulling;
                        }
                        spin_unlock(&src_rq->lock);
                        continue;
                }

                /*
                 * We can potentially drop this_rq's lock in
                 * double_lock_balance, and another CPU could
                 * steal our next task - hence we must cause
                 * the caller to recalculate the next task
                 * in that case:
                 */
                if (double_lock_balance(this_rq, src_rq)) {
                        struct task_struct *old_next = next;

                        next = pick_next_task_rt(this_rq);
                        if (next != old_next)
                                ret = 1;
                }

                /*
                 * Are there still pullable RT tasks?
                 */
                if (src_rq->rt.rt_nr_running <= 1) {
                        spin_unlock(&src_rq->lock);
                        continue;
                }

 try_pulling:
                p = pick_next_highest_task_rt(src_rq, this_cpu);

                /*
                 * Do we have an RT task that preempts
                 * the to-be-scheduled task?
                 */
                if (p && (!next || (p->prio < next->prio))) {
                        WARN_ON(p == src_rq->curr);
                        WARN_ON(!p->se.on_rq);

                        /*
                         * There's a chance that p is higher in priority
                         * than what's currently running on its cpu.
                         * This is just that p is wakeing up and hasn't
                         * had a chance to schedule. We only pull
                         * p if it is lower in priority than the
                         * current task on the run queue or
                         * this_rq next task is lower in prio than
                         * the current task on that rq.
                         */
                        if (p->prio < src_rq->curr->prio ||
                            (next && next->prio < src_rq->curr->prio))
                                goto out;

                        ret = 1;

                        deactivate_task(src_rq, p, 0);
                        set_task_cpu(p, this_cpu);
                        activate_task(this_rq, p, 0);
                        /*
                         * We continue with the search, just in
                         * case there's an even higher prio task
                         * in another runqueue. (low likelyhood
                         * but possible)
                         *
                         * Update next so that we won't pick a task
                         * on another cpu with a priority lower (or equal)
                         * than the one we just picked.
                         */
                        next = p;

                }
 out:
                spin_unlock(&src_rq->lock);
        }

        return ret;
}

static void pre_schedule_rt(struct rq *rq, struct task_struct *prev)
{
        /* Try to pull RT tasks here if we lower this rq's prio */
        if (unlikely(rt_task(prev)) && rq->rt.highest_prio > prev->prio)
                pull_rt_task(rq);
}

static void post_schedule_rt(struct rq *rq)
{
        /*
         * If we have more than one rt_task queued, then
         * see if we can push the other rt_tasks off to other CPUS.
         * Note we may release the rq lock, and since
         * the lock was owned by prev, we need to release it
         * first via finish_lock_switch and then reaquire it here.
         */
        if (unlikely(rq->rt.overloaded)) {
                spin_lock_irq(&rq->lock);
                push_rt_tasks(rq);
                spin_unlock_irq(&rq->lock);
        }
}


static void task_wake_up_rt(struct rq *rq, struct task_struct *p)
{
        if (!task_running(rq, p) &&
            (p->prio >= rq->rt.highest_prio) &&
            rq->rt.overloaded)
                push_rt_tasks(rq);
}

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)
{
        /* don't touch RT tasks */
        return 0;
}

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)
{
        /* don't touch RT tasks */
        return 0;
}

static void set_cpus_allowed_rt(struct task_struct *p, cpumask_t *new_mask)
{
        int weight = cpus_weight(*new_mask);

        BUG_ON(!rt_task(p));

        /*
         * Update the migration status of the RQ if we have an RT task
         * which is running AND changing its weight value.
         */
        if (p->se.on_rq && (weight != p->nr_cpus_allowed)) {
                struct rq *rq = task_rq(p);

                if ((p->nr_cpus_allowed <= 1) && (weight > 1)) {
                        rq->rt.rt_nr_migratory++;
                } else if ((p->nr_cpus_allowed > 1) && (weight <= 1)) {
                        BUG_ON(!rq->rt.rt_nr_migratory);
                        rq->rt.rt_nr_migratory--;
                }

                update_rt_migration(rq);
        }

        p->cpus_allowed    = *new_mask;
        p->nr_cpus_allowed = weight;
}

/* Assumes rq->lock is held */
static void join_domain_rt(struct rq *rq)
{
        if (rq->rt.overloaded)
                rt_set_overload(rq);
}

/* Assumes rq->lock is held */
static void leave_domain_rt(struct rq *rq)
{
        if (rq->rt.overloaded)
                rt_clear_overload(rq);
}

/*
 * When switch from the rt queue, we bring ourselves to a position
 * that we might want to pull RT tasks from other runqueues.
 */
static void switched_from_rt(struct rq *rq, struct task_struct *p,
                           int running)
{
        /*
         * If there are other RT tasks then we will reschedule
         * and the scheduling of the other RT tasks will handle
         * the balancing. But if we are the last RT task
         * we may need to handle the pulling of RT tasks
         * now.
         */
        if (!rq->rt.rt_nr_running)
                pull_rt_task(rq);
}
#endif /* CONFIG_SMP */

/*
 * When switching a task to RT, we may overload the runqueue
 * with RT tasks. In this case we try to push them off to
 * other runqueues.
 */
static void switched_to_rt(struct rq *rq, struct task_struct *p,
                           int running)
{
        int check_resched = 1;

        /*
         * If we are already running, then there's nothing
         * that needs to be done. But if we are not running
         * we may need to preempt the current running task.
         * If that current running task is also an RT task
         * then see if we can move to another run queue.
         */
        if (!running) {
#ifdef CONFIG_SMP
                if (rq->rt.overloaded && push_rt_task(rq) &&
                    /* Don't resched if we changed runqueues */
                    rq != task_rq(p))
                        check_resched = 0;
#endif /* CONFIG_SMP */
                if (check_resched && p->prio < rq->curr->prio)
                        resched_task(rq->curr);
        }
}

/*
 * Priority of the task has changed. This may cause
 * us to initiate a push or pull.
 */
static void prio_changed_rt(struct rq *rq, struct task_struct *p,
                            int oldprio, int running)
{
        if (running) {
#ifdef CONFIG_SMP
                /*
                 * If our priority decreases while running, we
                 * may need to pull tasks to this runqueue.
                 */
                if (oldprio < p->prio)
                        pull_rt_task(rq);
                /*
                 * If there's a higher priority task waiting to run
                 * then reschedule.
                 */
                if (p->prio > rq->rt.highest_prio)
                        resched_task(p);
#else
                /* For UP simply resched on drop of prio */
                if (oldprio < p->prio)
                        resched_task(p);
#endif /* CONFIG_SMP */
        } else {
                /*
                 * This task is not running, but if it is
                 * greater than the current running task
                 * then reschedule.
                 */
                if (p->prio < rq->curr->prio)
                        resched_task(rq->curr);
        }
}


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,
#ifdef CONFIG_SMP
        .select_task_rq         = select_task_rq_rt,
#endif /* CONFIG_SMP */

        .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,
        .set_cpus_allowed       = set_cpus_allowed_rt,
        .join_domain            = join_domain_rt,
        .leave_domain           = leave_domain_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,

        .prio_changed           = prio_changed_rt,
        .switched_to            = switched_to_rt,
};