Class 17
CS 202
3 April 2023

On the board
------------
1. Last time
2. Context switches (WeensyOS)
3. User-level threading, intro
4. Context switches (user-level threading)
    -swtch()
    -yield()
    -I/O
5. Cooperative multithreading
6. Preemptive user-level multithreading

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1. Last time

    I/O (CPU/device)

    mmap

    today, a mixture of topics


2. Context switches in WeensyOS

    - on interrupt, hardware saves "trapframe":
        %rip/%rsp/%eflags and lots of other things
        saves all of that on the *kernel's stack* at a well-known place in kernel memory
    - %rdi set equal to stack pointer
    - %rdi shows up, in C code, as the argument to exception()
    - exception does a brute force struct copy into process-specific memory in kernel space
    - now all of the process's registers just live in the PCB / struct proc.
    - kernel does its thing.
    - kernel gets ready to choose another process
    - remember, that process had the same thing happen
    - so all of *the new process's* registers are sitting in the same kind of memory mentioned above.
    - now, exception_return (&p->p_registers)
    - note: %rdi holds address of saved registers
    - set the stack pointer equal to that address
    - that means that popq will do the "right" thing
    - pop the saved registers into the CPU's
    - add 16 to skip past saved codes (error code if pg fault handler and int code in all cases).
    - now stack pointer is pointing to the trapframe at the time of the trap
    - that trapframe includes %rip and trap-time %rsp
    - iretq brings us back into user space with the %rip,%rsp at the time of the trap

3. User-level threading

    Setting: there's a _threading package_

    --Review: what *is* a kernel-managed thread? (We refer to that
    as "kernel-level threading.")

        --basically same as a process, except two threads in the same
        process have the same value for %cr3 

        --recall: kernel threads are always preemptive

    --We can also have *user*-level threading, in which the kernel is
    completely ignorant of the existence of threading.

               [draw picture]

            T1     T2     T3
                thr package
                   OS
                   H/W


    --in this case, the threading package is the layer of software that
    maintains the array of TCBs (thread control blocks)

    --threading package has other responsibilities as well:

        --make a new stack for each new thread.

        --scheduling!

     --user-level threading can be non-preemptive (cooperative) or
     preemptive. we'll look at both.

     --but first, revisit context switches, this time for user-space.
     
4. Context switches in user-space

    Note confusion: a "context switch" really is two things:
        - switching the view of memory (%cr3)
        - switching the registers

    The first one isn't relevant for user-level threading.

    Workhorse: swtch() switches registers

        [draw pictures; see handout]

    swtch() is called by yield()
        
        [see handout]

    yield() is called by any thread that couldn't make further progress.

    Good example of simultaneous use of synchronous and asynchronous
    interface. 

        [see handout]

    (Kernels also need swtch(), to switch the *kernel* stack. 

        [for those who want to learn more:
        
        here is an example in an instructional OS, see MIT's sv6

        x86 version:
            https://pdos.csail.mit.edu/6.828/2018/xv6.html
            https://pdos.csail.mit.edu/6.828/2018/xv6/xv6-rev7.pdf

        RISC version:
            https://pdos.csail.mit.edu/6.828/2019/xv6.html

        search for uses of swtch().
        ]

        WeensyOS doesn't use this mechanism because there is only a
        single kernel stack.
    )


5. Cooperative multithreading

    --This is also called *non-preemptive multithreading*.
    
    --It means that a context switch takes place only at well-defined
    points: when the thread calls yield() and when the thread would
    block on I/O.

6. Preemptive multithreading in user-level

    How can we build a user-level threading package that does
    context switches at any time?

    Need to arrange for the package to get interrupted.

    How?

    Signals!

    Deliver a periodic timer interrupt or signal to a thread
    scheduler [setitimer() ]. When it gets its interrupt, swap out
    the thread, run another one

    Makes programming with user-level threads more complex -- all the
    complexity of programming with kernel-level threads, but few of the
    advantages (except perhaps performance from fewer system calls).

    in practice, systems aren't usually built this way, but sometimes it
    is what you want (for example, if you're simulating some OS-like
    thing inside a process, and you want to simulate the non-determinism
    that arises from hardware timer interrupts).

    A larger point: signals are instructive, and are used for many
    things. What a signal is really doing is abstracting a key hardware
    feature: interrupts.

    So this is another example of the fact that the OS's job is to give
    a user-space process the illusion that it's running on something
    like a machine, by creating abstractions. In this example, the
    abstraction is the signal, and the thing that it's abstracting is an
    interrupt.