Linux 上下文切换的实现

上下文切换的具体步骤

场景：进程A下CPU，进程B上CPU
1. 保存进程A的上下文环境（程序计数器，程序状态字，其他寄存器…）
2. 用新状态和其他相关信息更新进程A的PCB
3. 把进程A移至合适的队列（就绪，阻塞…）
4. 将进程B的状态设置为运行态
5. 从进程B的PCB中恢复上下文（程序计数器，程序状态字，其他寄存器…）

Ref: 陈奶奶2018.3.29的课件

上下文切换的代码实现

schedule 调度函数

// Ref: linux-2.6.0\kernel\sched.c
/*
 * schedule() is the main scheduler function.
 */
asmlinkage void schedule(void)
{
    task_t *prev, *next;
    runqueue_t *rq;
    prio_array_t *array;
    struct list_head *queue;
    unsigned long long now;
    unsigned long run_time;
    int idx;

    /*
     * Test if we are atomic.  Since do_exit() needs to call into
     * schedule() atomically, we ignore that path for now.
     * Otherwise, whine if we are scheduling when we should not be.
     */
    if (likely(!(current->state & (TASK_DEAD | TASK_ZOMBIE)))) {
        if (unlikely(in_atomic())) {
            printk(KERN_ERR "bad: scheduling while atomic!\n");
            dump_stack();
        }
    }

need_resched:
    preempt_disable();
    prev = current;
    rq = this_rq();

    release_kernel_lock(prev);
    now = sched_clock();
    if (likely(now - prev->timestamp < NS_MAX_SLEEP_AVG))
        run_time = now - prev->timestamp;
    else
        run_time = NS_MAX_SLEEP_AVG;

    /*
     * Tasks with interactive credits get charged less run_time
     * at high sleep_avg to delay them losing their interactive
     * status
     */
    if (HIGH_CREDIT(prev))
        run_time /= (CURRENT_BONUS(prev) ? : 1);

    spin_lock_irq(&rq->lock);

    /*
     * if entering off of a kernel preemption go straight
     * to picking the next task.
     */
    if (unlikely(preempt_count() & PREEMPT_ACTIVE))
        goto pick_next_task;

    switch (prev->state) {
    case TASK_INTERRUPTIBLE:
        if (unlikely(signal_pending(prev))) {
            prev->