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_head) { /*
* 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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