Added a description of fork, 2012-36.

Last time we got our kernel switch back and forth between a user mode task. The focus in this post is going to be adding the ability to run multiple tasks.

Hack Two Tasks in There

Our kernel currently has no way for tasks to exit, so we need to make sure they never try to return, since that will probably cause an error and hang the system (in the best case). So change the syscall invocation at the end of first to be in an infinite loop, such that if the task gets re-activated after it is done it just immediately calls back into the kernel.

Then, add a second function so that we have something else to run:

void task(void) {
	bwputs("In other task\n");
	while(1) syscall();
}

We’re going to run two tasks, so change the first_stack and first_stack_start to something like:

unsigned int stacks[2][256];
unsigned int *tasks[2];

And setup both tasks, in basically the same way as we did before:

tasks[0] = stacks[0] + 256 - 16;
tasks[0][0] = 0x10;
tasks[0][1] = (unsigned int)&first;

tasks[1] = stacks[1] + 256 - 16;
tasks[1][0] = 0x10;
tasks[1][1] = (unsigned int)&task;

Then, call activate with tasks[0] and tasks[1] instead of first_stack_start. Recompile and run, you should see both tasks running in the order that you activate them.

Some abstractions

We’ve hardcoded a bunch of stuff again. We should clean it up so that we don’t have to keep copying these chunks every time we want a new task. Let’s move those magic numbers into constants and make a function to set up a new task:

#define STACK_SIZE 256 /* Size of task stacks in words */
#define TASK_LIMIT 2   /* Max number of tasks we can handle */

unsigned int *init_task(unsigned int *stack, void (*start)(void)) {
	stack += STACK_SIZE - 16; /* End of stack, minus what we're about to push */
	stack[0] = 0x10; /* User mode, interrupts on */
	stack[1] = (unsigned int)start;
	return stack;
}

Then, back in main clean up the task initialisation to use these abstractions:

unsigned int stacks[TASK_LIMIT][STACK_SIZE];
unsigned int *tasks[TASK_LIMIT];

tasks[0] = init_task(stacks[0], &first);
tasks[1] = init_task(stacks[1], &task);

Scheduling

Scheduling (deciding what tasks to run, when, and for how long, in a multitasking system) is a big topic. We are going to implement the simplest possible (in our situation) schedule: round-robin. This means that we’ll just activate each task in the order that they were created, and then go back to the beginning and activate each in sequence again.

Since we have no way for tasks to exit, make sure the following is at the end of each task:

while(1) syscall();

This will just cause the task to repeatedly call back into the kernel when it is done.

We’ll also need to keep track in main of how many tasks there are and which one we’re currently running:

size_t task_count = 0;
size_t current_task = 0;

In order to use size_t you’ll need to include stddef.h. Now just make sure that task_count gets set correctly, and replace the code that activates user tasks with this:

while(1) {
	tasks[current_task] = activate(tasks[current_task]);
	current_task++;
	if(current_task >= task_count) current_task = 0;
}

This just activates each task in order, repeatedly, forever. Fairly simple.

You may be wondering why I didn’t use the classic current_task = (current_task + 1) % task_count trick. The reason is that, at least on ARMv6, gcc actually generates a library call for the % operator. You could try linking in libgcc, but there are some problems. For this case, it just wasn’t worth it, and so I’m using an if statement instead.

Code so far on GitHub

Setup for new Syscall

So, our syscall syscall is cute, but it’s not very useful. For good multitasking, we want a way for a user mode task to create new tasks. We’ll do this the way Unix does: fork. This syscall copies the current process, and then returns the ID of the new process to the parent, and 0 to the child.

We need a way for the kernel to know what the user mode task wants it to do. As I mentioned in a previous post, this is what the “argument” to svc is for. We could add code to our context switch to mask the bits off of the end of the svc instruction, but let’s not bother our context switch now. We’re going to store the id of our syscall in a register. syscalls.s is now:

.global syscall
syscall:
	push {r7}
	mov r7, #0x0
	svc 0
	pop {r7}
	bx lr

.global fork
fork:
	push {r7}
	mov r7, #0x1
	svc 0
	pop {r7}
	bx lr

That’s two syscalls, and the context switch will save the new value of r7 on the top of the stack, which we can read in the kernel. The actual value of r7 gets saved and restored by the syscall wrapper.

In order to call our new syscall, we’ll need to add the following to asm.h:

int fork(void);

memcpy

As you may have guessed, we’re going to implement the fork syscall, in order to make our multitasking more actually useful. To do that, we’re going to need to copy stuff. Because we’re not linking in libc at this point, we need our own memcpy, like so:

void *memcpy(void *dest, const void *src, size_t n) {
	char *d = dest;
	const char *s = src;
	size_t i;
	for(i = 0; i < n; i++) {
		d[i] = s[i];
	}
	return d;
}

Forking

We’re going to use fork to spawn task from first. So, move task up above first, and replace the first call to syscall with:

if(!fork()) task();

Then, modify main so that only first gets set up to run.

The Actual Syscall

So, we’re jumping into the kernel now, but so far we don’t actually do anything with this new syscall. How can we even tell which syscall is being called? Well, remember that the syscall id is in r7 when the context switch happens, so by the time we get to the kernel, we can access it like so:

switch(tasks[current_task][2+7]) {
	case 0x1:
		bwputs("fork!\n");
	break;
}

What’s that 2+7 for? Well, remember, we push SPSR and the supervisor mode lr on the top of the stack, so we have to move past those to get to the registers.

Our kernel has a limitation. Specifically, TASK_LIMIT. If we call fork when there is no space for a new task, that would be bad, so we should return an error:

if(task_count == TASK_LIMIT) {
	tasks[current_task][2+0] = -1;
} else {
}

What are we doing here? We’re changing the saved value of r0, which, you will recall, is the return value of a function. So, we are setting the return value that the user mode task will see to -1.

Now, what does fork actually need to do? It copies all the state from one task into a new one. The new task will need a stack pointer, and said stack pointer will need to be exactly as far from the end of the stack as the current task’s stack pointer:

size_t used = stacks[current_task] + STACK_SIZE - tasks[current_task];
tasks[task_count] = stacks[task_count] + STACK_SIZE - used;

Now we actually have to copy the stack over. Luckily, we know exactly how much to copy, since it’s the same as the distance from the stack pointer to the end:

memcpy(tasks[task_count], tasks[current_task], used*sizeof(*tasks[current_task]));

fork is specified to return the new PID in the parent process, and 0 in the child process:

tasks[current_task][2+0] = task_count;
tasks[task_count][2+0] = 0;

And, finally, we should probably record the fact that there’s a new task:

task_count++;

That’s it!

We now have a multitasking kernel (remember to increase TASK_LIMIT if you want to run more tasks!), and user mode tasks can start new tasks. Next time: hardware interrupts!

Code for this post is on GitHub.