Homework: User-level threads

In this assignment you will complete a simple user-level thread package by implementing the code to perform context switching between threads. Submit your solutions before the beginning of the next lecture to the submission web site.

Please feel free to collaborate with others on these exercises.

Switching threads

Download uthread.c and uthread_switch.S into your xv6 directory. Make sure uthread_switch.S ends with .S, not .s. Add the following rule to the xv6 Makefile after the _forktest rule:

_uthread: uthread.o uthread_switch.o
	$(LD) $(LDFLAGS) -N -e main -Ttext 0 -o _uthread uthread.o uthread_switch.o $(ULIB)
	$(OBJDUMP) -S _uthread > uthread.asm
Make sure that the blank space at the start of each line is a tab, not spaces.

Add _uthread in the Makefile to the list of user programs defined by UPROGS.

Run xv6, and run uthread from the xv6 shell, and you will observe a panic.

Your job is to complete thread_switch.S, so that you see output similar to this (make sure to run with CPUS=1):

~/classes/6828/xv6$ make CPUS=1 qemu-nox
dd if=/dev/zero of=xv6.img count=10000
10000+0 records in
10000+0 records out
5120000 bytes transferred in 0.037167 secs (137756344 bytes/sec)
dd if=bootblock of=xv6.img conv=notrunc
1+0 records in
1+0 records out
512 bytes transferred in 0.000026 secs (19701685 bytes/sec)
dd if=kernel of=xv6.img seek=1 conv=notrunc
307+1 records in
307+1 records out
157319 bytes transferred in 0.003590 secs (43820143 bytes/sec)
qemu -nographic -hdb fs.img xv6.img -smp 1 -m 512 
Could not open option rom 'sgabios.bin': No such file or directory
cpu0: starting
init: starting sh
$ uthread
my thread running
my thread 0x2A30
my thread running
my thread 0x4A40
my thread 0x2A30
my thread 0x4A40
my thread 0x2A30
my thread 0x4A40

uthread creates two threads and switches back and forth between them. Each thread prints "my thread ..." and then yields to give the other thread a chance to run.

To observe the above output, you need to complete thread_switch.S, but before jumping into uthread_switch.S, first understand how uthread.c uses thread_switch. uthread.c has two global variables current_thread and next_thread. Each is a pointer to a thread structure. The thread structure has a stack for a thread and a saved stack pointer (sp, which points into the thread's stack). The job of uthread_switch is to save the current thread state into the structure pointed to by current_thread, restore next_thread's state, and make current_thread point to where next_thread was pointing to, so that when uthread_switch returns next_thread is running and is the current_thread.

You should study thread_create, which sets up the initial stack for a new thread. It provides you good hint what thread_switch should do. In particular, note that thread_create allocates 32 bytes of space for saving registers. That is all x86 registers, and the intent is that thread_switch use the assembly instructions popal and pushal to restore and save registers.

To write the assembly in thread_switch, you need to know how the C compiler lays out struct thread in memory, which is as follows:

    | 4 bytes for state|
    | stack size bytes |
    | for stack        |
    | 4 bytes for sp   |
    --------------------  <--- current_thread

    | 4 bytes for state|
    | stack size bytes |
    | for stack        |
    | 4 bytes for sp   |
    --------------------  <--- next_thread
The variables next_thread and current_thread each contain the address of a struct thread.

To write the sp field of the struct that current_thread points to, you should write assembly like this:

   movl current_thread, %eax
   movl %esp, (%eax)
This saves %esp in current_thread->sp. This works because sp is at offset 0 in the struct. You can study the assembly the compiler generates for uthread.c by looking at uthread.asm.

To test your code it might be helpful to single step through your thread_switch using gdb. You can do this as follows:

(gdb) symbol-file _uthread
Load new symbol table from "/Users/kaashoek/classes/6828/xv6/_uthread"? (y or n) y
Reading symbols from /Users/kaashoek/classes/6828/xv6/_uthread...done.
(gdb) b thread_switch
Breakpoint 1 at 0x204: file uthread_switch.S, line 9.
(gdb) c

Note the continue to skip the first time that gdb breaks. This first break is not in uthread. uthread is not even running yet. (Why is gdb breaking then?)

Once your xv6 shell runs, type "uthread", and gdb will break at thread_switch. Now you can type commands like the following to inspect the state of uthread:

(gdb) print /x *next_thread
$1 = {sp = 0x4d48, stack = {0x0 , 0x61, 0x1, 0x0, 0x0}, state = 0x1}
What address is 0x161, which sits on the top of the stack of next_thread?

Submit: your modified uthread_switch.S

Challenge exercises

The user-level thread package interacts badly with the operating system in several ways. For example, if one user-level thread blocks in a system call, another user-level thread won't run, because the user-level threads scheduler doesn't know that one of its threads has been descheduled by the xv6 scheduler. As another example, two user-level threads will not run concurrently on different cores, because the xv6 scheduler isn't aware that there are multiple threads that could run in parallel. Note that if two user-level threads were to run truly in parallel, this implementation won't work because of several races (e.g., two threads on different processors could call thread_schedule concurrently, select the same runnable thread, and both run it on different processors.)

There are several ways of addressing these problems. One is using scheduler activations and another is to use one kernel thread per user-level thread (as Linux kernels do). Implement one of these ways in xv6.

Add locks, condition variables, barriers, etc. to your thread package.