pluto/src/kernel/scheduler.zig

const std = @import("std");
const expectEqual = std.testing.expectEqual;
const expectError = std.testing.expectError;
const assert = std.debug.assert;
const builtin = @import("builtin");
const is_test = builtin.is_test;
const build_options = @import("build_options");
const mock_path = build_options.mock_path;
const arch = @import("arch.zig").internals;
const panic = if (is_test) @import(mock_path ++ "panic_mock.zig").panic else @import("panic.zig").panic;
const task = if (is_test) @import(mock_path ++ "task_mock.zig") else @import("task.zig");
const Task = task.Task;
const Allocator = std.mem.Allocator;
const TailQueue = std.TailQueue;

/// The function type for the entry point.
const EntryPointFn = fn () void;

/// The default stack size of a task. Currently this is set to a page size.
const STACK_SIZE: u32 = arch.MEMORY_BLOCK_SIZE / @sizeOf(usize);

/// Pointer to the start of the main kernel stack
extern var KERNEL_STACK_START: []u32;

/// The current task running
var current_task: *Task = undefined;

/// Array list of all runnable tasks
var tasks: TailQueue(*Task) = undefined;

/// Whether the scheduler is allowed to switch tasks.
var can_switch: bool = true;

///
/// The idle task that just halts the CPU but the CPU can still handle interrupts.
///
fn idle() noreturn {
    arch.spinWait();
}

pub fn taskSwitching(enabled: bool) void {
    can_switch = enabled;
}

///
/// Round robin. This will first save the the current tasks stack pointer, then will pick the next
/// task to be run from the queue. It will add the current task to the end of the queue and pop the
/// next task from the front as set this as the current task. Then will return the stack pointer
/// of the next task to be loaded into the stack register to load the next task stack to pop off
/// its state. Interrupts are assumed disabled.
///
/// Argument:
///     IN ctx: *arch.CpuState - Pointer to the exception context containing the contents
///                              of the registers at the time of a exception.
///
/// Return: usize
///     The new stack pointer to the next stack of the next task.
///
pub fn pickNextTask(ctx: *arch.CpuState) usize {
    // Save the stack pointer from old task
    current_task.stack_pointer = @ptrToInt(ctx);

    // If we can't switch, then continue with the current task
    if (!can_switch) {
        return current_task.stack_pointer;
    }

    // Pick the next task
    // If there isn't one, then just return the same task
    if (tasks.pop()) |new_task_node| {
        // Get the next task
        const next_task = new_task_node.data;

        // Move some pointers to don't need to allocate memory, speeds things up
        new_task_node.data = current_task;
        new_task_node.prev = null;
        new_task_node.next = null;

        // Add the 'current_task' node to the end of the queue
        tasks.prepend(new_task_node);

        current_task = next_task;
    }

    // Context switch in the interrupt stub handler which will pop the next task state off the
    // stack
    return current_task.stack_pointer;
}

///
/// Create a new task and add it to the scheduling queue. No locking.
///
/// Arguments:
///     IN entry_point: EntryPointFn - The entry point into the task. This must be a function.
///
/// Error: Allocator.Error
///     OutOfMemory - If there isn't enough memory for the a task/stack. Any memory allocated will
///                   be freed on return.
///
pub fn scheduleTask(new_task: *Task, allocator: *Allocator) Allocator.Error!void {
    var task_node = try tasks.createNode(new_task, allocator);
    tasks.prepend(task_node);
}

///
/// Initialise the scheduler. This will set up the current task to the code that is currently
/// running. So if there is a task switch before kmain can finish, can continue when switched back.
/// This will set the stack to KERNEL_STACK_START from the linker stript. This will also create the
/// idle task for when there is no more tasks to run.
///
/// Arguments:
///     IN allocator: *Allocator - The allocator to use when needing to allocate memory.
///
/// Error: Allocator.Error
///     OutOfMemory - There is no more memory. Any memory allocated will be freed on return.
///
pub fn init(allocator: *Allocator) Allocator.Error!void {
    // TODO: Maybe move the task init here?
    std.log.info(.scheduler, "Init\n", .{});
    defer std.log.info(.scheduler, "Done\n", .{});

    // Init the task list for round robin
    tasks = TailQueue(*Task).init();

    // Set up the init task to continue execution
    current_task = try allocator.create(Task);
    errdefer allocator.destroy(current_task);
    // PID 0
    current_task.pid = 0;
    current_task.stack = @intToPtr([*]u32, @ptrToInt(&KERNEL_STACK_START))[0..4096];
    // ESP will be saved on next schedule

    // Run the runtime tests here
    switch (build_options.test_mode) {
        .Scheduler => runtimeTests(allocator),
        else => {},
    }

    // Create the idle task when there are no more tasks left
    var idle_task = try Task.create(idle, allocator);
    errdefer idle_task.destroy(allocator);

    try scheduleTask(idle_task, allocator);
}

// For testing the errdefer
const FailingAllocator = std.testing.FailingAllocator;
const testing_allocator = &std.testing.base_allocator_instance.allocator;

fn test_fn1() void {}
fn test_fn2() void {}

var test_pid_counter: u7 = 1;

fn task_create(entry_point: EntryPointFn, allocator: *Allocator) Allocator.Error!*Task {
    var t = try allocator.create(Task);
    errdefer allocator.destroy(t);
    t.pid = test_pid_counter;
    // Just alloc something
    t.stack = try allocator.alloc(u32, 1);
    t.stack_pointer = 0;
    test_pid_counter += 1;
    return t;
}

fn task_destroy(self: *Task, allocator: *Allocator) void {
    if (@ptrToInt(self.stack.ptr) != @ptrToInt(&KERNEL_STACK_START)) {
        allocator.free(self.stack);
    }
    allocator.destroy(self);
}

test "pickNextTask" {
    task.initTest();
    defer task.freeTest();

    task.addConsumeFunction("Task.create", task_create);
    task.addConsumeFunction("Task.create", task_create);
    task.addRepeatFunction("Task.destroy", task_destroy);

    var ctx: arch.CpuState = std.mem.zeroes(arch.CpuState);

    var allocator = std.testing.allocator;
    tasks = TailQueue(*Task).init();

    // Set up a current task
    current_task = try allocator.create(Task);
    defer allocator.destroy(current_task);
    current_task.pid = 0;
    current_task.stack = @intToPtr([*]u32, @ptrToInt(&KERNEL_STACK_START))[0..4096];
    current_task.stack_pointer = @ptrToInt(&KERNEL_STACK_START);

    // Create two tasks and schedule them
    var test_fn1_task = try Task.create(test_fn1, allocator);
    defer test_fn1_task.destroy(allocator);
    try scheduleTask(test_fn1_task, allocator);

    var test_fn2_task = try Task.create(test_fn2, allocator);
    defer test_fn2_task.destroy(allocator);
    try scheduleTask(test_fn2_task, allocator);

    // Get the stack pointers of the created tasks
    const fn1_stack_pointer = tasks.first.?.data.stack_pointer;
    const fn2_stack_pointer = tasks.first.?.next.?.data.stack_pointer;

    expectEqual(pickNextTask(&ctx), fn1_stack_pointer);
    // The stack pointer of the re-added task should point to the context
    expectEqual(tasks.first.?.data.stack_pointer, @ptrToInt(&ctx));

    // Should be the PID of the next task
    expectEqual(current_task.pid, 1);

    expectEqual(pickNextTask(&ctx), fn2_stack_pointer);
    // The stack pointer of the re-added task should point to the context
    expectEqual(tasks.first.?.data.stack_pointer, @ptrToInt(&ctx));

    // Should be the PID of the next task
    expectEqual(current_task.pid, 2);

    expectEqual(pickNextTask(&ctx), @ptrToInt(&ctx));
    // The stack pointer of the re-added task should point to the context
    expectEqual(tasks.first.?.data.stack_pointer, @ptrToInt(&ctx));

    // Should be back tot he beginning
    expectEqual(current_task.pid, 0);

    // Reset the test pid
    test_pid_counter = 1;

    // Free the queue
    while (tasks.pop()) |elem| {
        tasks.destroyNode(elem, allocator);
    }
}

test "createNewTask add new task" {
    task.initTest();
    defer task.freeTest();

    task.addConsumeFunction("Task.create", task_create);
    task.addConsumeFunction("Task.destroy", task_destroy);

    // Set the global allocator
    var allocator = std.testing.allocator;

    // Init the task list
    tasks = TailQueue(*Task).init();

    var test_fn1_task = try Task.create(test_fn1, allocator);
    defer test_fn1_task.destroy(allocator);
    try scheduleTask(test_fn1_task, allocator);

    expectEqual(tasks.len, 1);

    // Free the memory
    tasks.destroyNode(tasks.first.?, allocator);
}

test "init" {
    task.initTest();
    defer task.freeTest();

    task.addConsumeFunction("Task.create", task_create);
    task.addRepeatFunction("Task.destroy", task_destroy);

    var allocator = std.testing.allocator;

    try init(allocator);

    expectEqual(current_task.pid, 0);
    expectEqual(current_task.stack, @intToPtr([*]u32, @ptrToInt(&KERNEL_STACK_START))[0..4096]);

    expectEqual(tasks.len, 1);

    // Free the tasks created
    current_task.destroy(allocator);
    while (tasks.pop()) |elem| {
        elem.data.destroy(allocator);
        tasks.destroyNode(elem, allocator);
    }
}

/// A volatile pointer used to control a loop outside the task. This is so to ensure a task switch
/// ocurred.
var is_set: *volatile bool = undefined;

///
/// The test task function.
///
fn task_function() noreturn {
    std.log.info(.scheduler, "Switched\n", .{});
    is_set.* = false;
    while (true) {}
}

///
/// This tests that variables in registers and on the stack are preserved when a task switch
/// occurs. Also tests that a global volatile can be test in one task and be reacted to in another.
///
/// Arguments:
///     IN allocator: *Allocator - The allocator to use when needing to allocate memory.
///
fn rt_variable_preserved(allocator: *Allocator) void {
    // Create the memory for the boolean
    is_set = allocator.create(bool) catch unreachable;
    defer allocator.destroy(is_set);
    is_set.* = true;

    var test_task = Task.create(task_function, allocator) catch unreachable;
    scheduleTask(test_task, allocator) catch unreachable;
    // TODO: Need to add the ability to remove tasks

    var w: u32 = 0;
    var x: u32 = 1;
    var y: u32 = 2;
    var z: u32 = 3;

    while (is_set.*) {
        if (w != 0) {
            panic(@errorReturnTrace(), "FAILED: w not 0, but: {}\n", .{w});
        }
        if (x != 1) {
            panic(@errorReturnTrace(), "FAILED: x not 1, but: {}\n", .{x});
        }
        if (y != 2) {
            panic(@errorReturnTrace(), "FAILED: y not 2, but: {}\n", .{y});
        }
        if (z != 3) {
            panic(@errorReturnTrace(), "FAILED: z not 3, but: {}\n", .{z});
        }
    }
    // Make sure these are the same values
    if (w != 0) {
        panic(@errorReturnTrace(), "FAILED: w not 0, but: {}\n", .{w});
    }
    if (x != 1) {
        panic(@errorReturnTrace(), "FAILED: x not 1, but: {}\n", .{x});
    }
    if (y != 2) {
        panic(@errorReturnTrace(), "FAILED: y not 2, but: {}\n", .{y});
    }
    if (z != 3) {
        panic(@errorReturnTrace(), "FAILED: z not 3, but: {}\n", .{z});
    }

    std.log.info(.scheduler, "SUCCESS: Scheduler variables preserved\n", .{});
}

///
/// The scheduler runtime tests that will test the scheduling functionality.
///
/// Arguments:
///     IN allocator: *Allocator - The allocator to use when needing to allocate memory.
///
fn runtimeTests(allocator: *Allocator) void {
    arch.enableInterrupts();
    rt_variable_preserved(allocator);
    while (true) {}
}