7-进程切换

进程切换

• P1 进入内核，切换到调度器进程，调度器进程切换到 P2

• 核心函数为 swtch()函数；该函数保存并加载部分寄存器的值（RISC-V 中存在调用者保存并恢复的寄存器（caller-saved registers），不需要保存全部寄存器）

切换过程

• yield 调用了 sched 函数

• sched 函数进行合理性检查，最后调用 swtch 函数交换当前进程的上下文和 CPU 调度线程的上下文，返回地址为切换后上下文的 ra 寄存器存的值；实际上是 scheduler 函数

  void
  sched(void)
  {
    int intena;
    struct proc *p = myproc();
   
    if(!holding(&p->lock))
      panic("sched p->lock");
    if(mycpu()->noff != 1)
      panic("sched locks");
    if(p->state == RUNNING)
      panic("sched running");
    if(intr_get())
      panic("sched interruptible");
   
    intena = mycpu()->intena;
    swtch(&p->context, &mycpu()->context);
    mycpu()->intena = intena;
  }

• 调度器找到一个可运行的进程，再次运行 swtch 函数切换上下文， 此时的 ra 寄存器是之前是被定时器中断通过 sched 函数挂起的

  void
  scheduler(void)
  {
    struct proc *p;
    struct cpu *c = mycpu();
   
    c->proc = 0;
    for(;;){
      // The most recent process to run may have had interrupts
      // turned off; enable them to avoid a deadlock if all
      // processes are waiting.
      intr_on();
   
      for(p = proc; p < &proc[NPROC]; p++) {
        acquire(&p->lock);
        if(p->state == RUNNABLE) {
          // Switch to chosen process.  It is the process's job
          // to release its lock and then reacquire it
          // before jumping back to us.
          p->state = RUNNING;
          c->proc = p;
          swtch(&c->context, &p->context);
   
          // Process is done running for now.
          // It should have changed its p->state before coming back.
          c->proc = 0;
        }
        release(&p->lock);
      }
    }
  }

• 第一次切换进程时，构造一个 forkret，allocproc 设置了 ra 和 sp 寄存器，forkret 本身只释放锁，调用 usertrapret