/*

               * 遍历运行队列,查找优先级最高的进程, 优先级最高的进程将获得CPU

               */

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

              if (can_schedule(p, this_cpu)) {

                     /*

                      * goodness()中，如果是实时进程，则weight = 1000 + p->rt_priority,

                      * 使实时进程的优先级永远比非实时进程高

                      */

                     int weight = goodness(p, this_cpu, prev->active_mm);

                     if (weight > c)              //注意这里是”>”而不是”>=”，如果权值相同，则先来的先上

                            c = weight, next = p;

              }

       }

       /* Do we need to re-calculate counters? */

       if (unlikely(!c)) {    

              /* 

               * 如果当前优先级为0,那么整个运行队列中的进程将重新计算优先权

               */

              struct task_struct *p;

              spin_unlock_irq(&runqueue_lock);

              read_lock(&tasklist_lock);

              for_each_task(p)

                     p->counter = (p->counter >> 1) + NICE_TO_TICKS(p->nice);

              read_unlock(&tasklist_lock);

              spin_lock_irq(&runqueue_lock);

              goto repeat_schedule;

       }

       /*

        * from this point on nothing can prevent us from

        * switching to the next task, save this fact in sched_data.

        */

       sched_data->curr = next;

       task_set_cpu(next, this_cpu);

       spin_unlock_irq(&runqueue_lock);

       if (unlikely(prev == next)) {

              /* We won''''t go through the normal tail, so do this by hand */

              prev->policy &= ~SCHED_YIELD;

              goto same_process;

       }

       kstat.context_swtch++;

       /*

        * there are 3 processes which are affected by a context switch:

        *

        * prev == .... ==> (last => next)

        *

        * It''''s the ''''much more previous'''' ''''prev'''' that is on next''''s stack,

        * but prev is set to (the just run) ''''last'''' process by switch_to().

        * This might sound slightly confusing but makes tons of sense.

        */

       prepare_to_switch();   {

              struct mm_struct *mm = next->mm;

              struct mm_struct *oldmm = prev->active_mm;

              if (!mm) {                     //如果是内核线程的切换，则不做页表处理

                     if (next->active_mm) BUG();

                     next->active_mm = oldmm;

                     atomic_inc(&oldmm->mm_count);

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