进程的调度方式之一就是周期性的Tick 调度, 该调度由Timer周期性的softirq触发.

/*
 * This function gets called by the timer code, with HZ frequency.
 * We call it with interrupts disabled.
 *
 * It also gets called by the fork code, when changing the parent's
 * timeslices.
 */
void scheduler_tick(void)
{
    int cpu = smp_processor_id();
    struct rq *rq = cpu_rq(cpu);
    struct task_struct *curr = rq->curr;

    sched_clock_tick();

    raw_spin_lock(&rq->lock);
    update_rq_clock(rq);
    update_cpu_load_active(rq);
    **curr->sched_class->task_tick(rq, curr, 0);** // 运行各调度class的tick 
    raw_spin_unlock(&rq->lock);

    perf_event_task_tick();

#ifdef CONFIG_SMP
    rq->idle_at_tick = idle_cpu(cpu);
    trigger_load_balance(rq, cpu);
#endif
}

各个调度算法中如果有实现Tick调度的代码 一般如下面:

/*
 * All the scheduling class methods:
 */
static const struct sched_class fair_sched_class = {
    .next           = &idle_sched_class,
    .enqueue_task       = enqueue_task_fair,
    .dequeue_task       = dequeue_task_fair,
    .yield_task     = yield_task_fair,
    .yield_to_task      = yield_to_task_fair,

    .check_preempt_curr = check_preempt_wakeup,

    .pick_next_task     = pick_next_task_fair,
    .put_prev_task      = put_prev_task_fair,

#ifdef CONFIG_SMP
    .select_task_rq     = select_task_rq_fair,

    .rq_online      = rq_online_fair,
    .rq_offline     = rq_offline_fair,

    .task_waking        = task_waking_fair,
#endif

    .set_curr_task          = set_curr_task_fair,
    **.task_tick        = task_tick_fair,** //CFS 的Tick 调度函数入口
    .task_fork      = task_fork_fair,

    .prio_changed       = prio_changed_fair,
    .switched_from      = switched_from_fair,
    .switched_to        = switched_to_fair,

    .get_rr_interval    = get_rr_interval_fair,

#ifdef CONFIG_FAIR_GROUP_SCHED
    .task_move_group    = task_move_group_fair,
#endif
};

CFS调度算法的task_tick_fair 实现如下:

/*
 * scheduler tick hitting a task of our scheduling class:
 */
static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
{
    struct cfs_rq *cfs_rq;
    struct sched_entity *se = &curr->se;

    for_each_sched_entity(se) {//当前进程的se 和parent的se 遍历
        cfs_rq = cfs_rq_of(se);//通过se 得到rq
        entity_tick(cfs_rq, se, queued);
    }
}

entity_tick 主要做了什么?看下面的代码.

static void
entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
{
    /*
     * Update run-time statistics of the 'current'.
     */
    update_curr(cfs_rq);//更想Current的运行统计数据

    /*
     * Update share accounting for long-running entities.
     */
    update_entity_shares_tick(cfs_rq);

#ifdef CONFIG_SCHED_HRTICK
    /*
     * queued ticks are scheduled to match the slice, so don't bother
     * validating it and just reschedule.
     */
    if (queued) {// curr->sched_class->task_tick(rq, curr, 0); 由上级的调用函数可知该参数为0
        resched_task(rq_of(cfs_rq)->curr);
        return;
    }
    /*
     * don't let the period tick interfere with the hrtick preemption
     */
    if (!sched_feat(DOUBLE_TICK) &&
            hrtimer_active(&rq_of(cfs_rq)->hrtick_timer))
        return;
#endif

    if (cfs_rq->nr_running > 1 || !sched_feat(WAKEUP_PREEMPT))
        check_preempt_tick(cfs_rq, curr);
}