Context switching

Required reading: proc.c (focus on scheduler() and sched()), swtch.S. Also process creation: sys_fork() and copyproc().

Overview

Big picture: more programs than CPUs. How to share the limited number of CPUs among the programs?

Idea: have the kernel transparently switch the CPU(s) among programs, giving them a "virtual CPU" abstraction.

Terminology: I'm going to call a switch from one program to another a context switch.

The program's view of context switching

For this discussion we're going to assume that the O/S runs each program in a "process", which couples an address space with a single thread of control. For the most part each process appears to have a dedicated CPU, but if processes compare notes they can see that the CPU is actually multiplexed, with this process-visible behavior:

When a process makes a system call that blocks, the kernel arranges to take the CPU away and run a different process. For example, if read() needs to wait for the hard disk to finish reading a block, the kernel will cause a different process to run until the disk has finished.
The kernel time-slices the CPU among processes that compute a lot.
Both of the above are largely transparent to the process, in the sense that the kernel resumes a process with the same register and memory state it had when the kernel suspended it. (Except that a system call may explicitly modify state required to express its return value or data read by e.g. read().)

The kernel also exposes system calls for handling processes; here are the main XV6 process system calls:

fork: create a new process, which is a copy of the parent
exec: execute a program file
exit: terminate the calling process
wait: wait for a child process to call exit
kill: kill process
sbrk: grow the address space of a process.

The kernel's view of context switching

As you know, the xv6 kernel maintains a kernel stack for each process, on which to run system calls for the process. At any given time, multiple processes may be executing system calls inside the kernel. This is a common kernel arrangement -- multiple threads of control in the kernel address space, one per user-level process. A kernel thread must be capable of suspending and giving the CPU away, because the implementation of many system calls involves blocking to wait for I/O. That's the reason why each xv6 process has its own kernel stack.

Terminology: if a process has entered the kernel, the kernel stack and CPU registers of the resulting thread of control is called the processes kernel thread.

In this arrangment, the kernel doesn't directly switch CPUs among processes. Instead, the kernel has a number of mechanisms which combine to switch among processes:

User->kernel transitions, which we've seen when we looked at system calls and interrrupts. If a process is running and makes a user->kernel transition, we're then running in the process's kernel thread. This transition saves the state of the user process -- saving its register in a struct trapframe on the process's kernel stack. A user->kernel transition initializes fresh register state for the process's kernel thread.
Switching between kernel threads. If a kernel thread calls sched() or yield() or sleep(), the kernel saves that kernel thread's registers, picks another runnable kernel thread, and restores its registers -- restoring ESP and EIP cause a stack switch and control switch. scheduler() also switches user segments; this has no immediate effect, it only affects what happens on a future return from kernel to user.
Kernel->user transitions. A return to user space restores the user registers from the trapframe, and implicitly loses the register state of the process's kernel thread.
Timer interrupts. The timer interrupt handler implements time-slicing by calling yield(). At a low level yield() only switches between kernel threads, but the interrupt may have caused a user->kernel transition, so the net effect is often to switch between user processes.

xv6 code examples

Let's watch what happens when a timer interrupt causes an involuntary context switch between two compute-bound user processes. The user process:

main(){
  fork();
  while(1)
    ;
}

Break in yield(). On kernel stack of one of the processes. The return EIP should be trap() (but might be alltraps; why?).

Why does yield() need to acquire proc_table_lock? When will the lock be released?

Why state = RUNNABLE? What was state's previous value?

yield() calls sched() which calls swtch(). Look at definition of struct context: it's a set of saved registers. swtch() will save into one context, restore from another. What are the two contexts?

Where do we think swtch() will return to?

Why doesn't swtch() save EAX in struct context?

Why doesn't swtch() just leave the return EIP on the stack? Why does it copy the return EIP from the stack to the struct context, then back from the struct context to the stack?

At end of swtch(), what stack are we running on?

Where does print-stack show swtch() will return to? Why did this happen?

scheduler() is in the middle of a loop that checks to see whether each process would like to run (is RUNNABLE).

If scheduler() finds no RUNNABLE process, it finishes the loop, releases the lock, but then immediately re-acquires the lock at the start of the outer loop. Why release just to re-acquire?

That is, suppose for a while there are no RUNNABLE processes. What will the CPUs spend their time doing?

scheduler() has its own stack (in cpus[x]). Why? Why not just call it directly from sched()? I.e. why not have scheduler() run on the stack of the last process that gave up the CPU?

What policy does scheduler() use to give CPU time to processes?

We know scheduler() is going to run the other cpu-bound process, by calling setupsegs(p) and swtch() to restore other process's registers and stack. Set state to RUNNING to prevent another CPU's scheduler() from also running this process.

Break in swtch(), and check that we are indeed switching to the other process's stack, and that we are returning from its call to yield() from trap().

Wrap-up

More sophisticated operating systems usually allow multiple threads of control in a process. How could we do this for xv6? Each thread would have its own user-space stack in the process address space. There would be a few more system calls: perhaps thread_fork(), thread_exit(), and thread_wait(). The kernel would have to split struct proc into a structure describing overall process resources (address space and open files), and a kernel stack and set of saved registers per thread. That way multiple threads from a process could be executing system calls inside the kernel.

It's worth comparing xv6's fairly conventional context switches with those of JOS. JOS has only one thread active in the kernel at a time, and thus only a single kernel stack (not one per process) and no thread switching inside the kernel. It only has an analogue to xv6's kernel entry/exit to save/restore user thread state. But as a direct consequence JOS 1) can't run the kernel concurrently on multiple CPUs and 2) cannot have system calls that block in the middle (e.g. to look up a multi-component path name).

We've seen the complete story of how xv6 switches between user processes. This is the concrete realization of the virtual CPU abstraction. The core mechanism is swtch(), which saves and restores thread register state and thus switches stacks (ESP) and changes the control flow (EIP). We also saw a bit of what it takes to integrate thread switching with the concurrency brought by multiple CPUs and by interrupts.