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add usr app: green_threads

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Yu Chen 3 years ago
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commit 7f5fa3355f
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user/src/bin/green_threads.rs
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#![no_std]
#![no_main]
#![feature(naked_functions)]
#[macro_use]
extern crate user_lib;
#[macro_use]
extern crate alloc;
use core::arch::global_asm;
use alloc::vec::Vec;
//use user_lib::{close, open, read, OpenFlags}; //for linux
use user_lib::{exit};
// In our simple example we set most constraints here.
//const DEFAULT_STACK_SIZE: usize = 1024 * 1024 * 2; //for linux
const DEFAULT_STACK_SIZE: usize = 1024;
const MAX_TASKS: usize = 4;
static mut RUNTIME: usize = 0;
pub struct Runtime {
tasks: Vec<Task>,
current: usize,
}
#[derive(PartialEq, Eq, Debug)]
enum State {
Available,
Running,
Ready,
}
struct Task {
id: usize,
stack: Vec<u8>,
ctx: TaskContext,
state: State,
}
#[derive(Debug, Default)]
#[repr(C)] // not strictly needed but Rust ABI is not guaranteed to be stable
pub struct TaskContext {
// 15 u64
x1: u64, //ra: return addres
x2: u64, //sp
x8: u64, //s0,fp
x9: u64, //s1
x18: u64, //x18-27: s2-11
x19: u64,
x20: u64,
x21: u64,
x22: u64,
x23: u64,
x24: u64,
x25: u64,
x26: u64,
x27: u64,
nx1: u64, //new return addres
}
impl Task {
fn new(id: usize) -> Self {
// We initialize each task here and allocate the stack. This is not neccesary,
// we can allocate memory for it later, but it keeps complexity down and lets us focus on more interesting parts
// to do it here. The important part is that once allocated it MUST NOT move in memory.
Task {
id,
stack: vec![0_u8; DEFAULT_STACK_SIZE],
ctx: TaskContext::default(),
state: State::Available,
}
}
}
impl Runtime {
pub fn new() -> Self {
// This will be our base task, which will be initialized in the `running` state
let base_task = Task {
id: 0,
stack: vec![0_u8; DEFAULT_STACK_SIZE],
ctx: TaskContext::default(),
state: State::Running,
};
// We initialize the rest of our tasks.
let mut tasks = vec![base_task];
let mut available_tasks: Vec<Task> = (1..MAX_TASKS).map(|i| Task::new(i)).collect();
tasks.append(&mut available_tasks);
Runtime {
tasks,
current: 0,
}
}
/// This is cheating a bit, but we need a pointer to our Runtime stored so we can call yield on it even if
/// we don't have a reference to it.
pub fn init(&self) {
unsafe {
let r_ptr: *const Runtime = self;
RUNTIME = r_ptr as usize;
}
}
/// This is where we start running our runtime. If it is our base task, we call yield until
/// it returns false (which means that there are no tasks scheduled) and we are done.
pub fn run(&mut self) -> ! {
while self.t_yield() {}
// std::process::exit(0); //for linux
exit(0);
}
/// This is our return function. The only place we use this is in our `guard` function.
/// If the current task is not our base task we set its state to Available. It means
/// we're finished with it. Then we yield which will schedule a new task to be run.
fn t_return(&mut self) {
if self.current != 0 {
self.tasks[self.current].state = State::Available;
self.t_yield();
}
}
/// This is the heart of our runtime. Here we go through all tasks and see if anyone is in the `Ready` state.
/// If no task is `Ready` we're all done. This is an extremely simple sceduler using only a round-robin algorithm.
///
/// If we find a task that's ready to be run we change the state of the current task from `Running` to `Ready`.
/// Then we call switch which will save the current context (the old context) and load the new context
/// into the CPU which then resumes based on the context it was just passed.
fn t_yield(&mut self) -> bool {
let mut pos = self.current;
while self.tasks[pos].state != State::Ready {
pos += 1;
if pos == self.tasks.len() {
pos = 0;
}
if pos == self.current {
return false;
}
}
if self.tasks[self.current].state != State::Available {
self.tasks[self.current].state = State::Ready;
}
self.tasks[pos].state = State::Running;
let old_pos = self.current;
self.current = pos;
unsafe {
__switch(&mut self.tasks[old_pos].ctx, &self.tasks[pos].ctx);
}
// NOTE: this might look strange and it is. Normally we would just mark this as `unreachable!()` but our compiler
// is too smart for it's own good so it optimized our code away on release builds. Curiously this happens on windows
// and not on linux. This is a common problem in tests so Rust has a `black_box` function in the `test` crate that
// will "pretend" to use a value we give it to prevent the compiler from eliminating code. I'll just do this instead,
// this code will never be run anyways and if it did it would always be `true`.
self.tasks.len() > 0
}
/// While `yield` is the logically interesting function I think this the technically most interesting.
///
/// When we spawn a new task we first check if there are any available tasks (tasks in `Parked` state).
/// If we run out of tasks we panic in this scenario but there are several (better) ways to handle that.
/// We keep things simple for now.
///
/// When we find an available task we get the stack length and a pointer to our u8 bytearray.
///
/// The next part we have to use some unsafe functions. First we write an address to our `guard` function
/// that will be called if the function we provide returns. Then we set the address to the function we
/// pass inn.
///
/// Third, we set the value of `sp` which is the stack pointer to the address of our provided function so we start
/// executing that first when we are scheuled to run.
///
/// Lastly we set the state as `Ready` which means we have work to do and is ready to do it.
pub fn spawn(&mut self, f: fn()) {
let available = self
.tasks
.iter_mut()
.find(|t| t.state == State::Available)
.expect("no available task.");
let size = available.stack.len();
unsafe {
let s_ptr = available.stack.as_mut_ptr().offset(size as isize);
// make sure our stack itself is 8 byte aligned - it will always
// offset to a lower memory address. Since we know we're at the "high"
// memory address of our allocated space, we know that offsetting to
// a lower one will be a valid address (given that we actually allocated)
// enough space to actually get an aligned pointer in the first place).
let s_ptr = (s_ptr as usize & !7) as *mut u8;
available.ctx.x1 = guard as u64; //ctx.x1 is old return address
available.ctx.nx1 = f as u64; //ctx.nx2 is new return address
available.ctx.x2 = s_ptr.offset(-32) as u64; //cxt.x2 is sp
}
available.state = State::Ready;
}
}
/// This is our guard function that we place on top of the stack. All this function does is set the
/// state of our current task and then `yield` which will then schedule a new task to be run.
fn guard() {
unsafe {
let rt_ptr = RUNTIME as *mut Runtime;
(*rt_ptr).t_return();
};
}
/// We know that Runtime is alive the length of the program and that we only access from one core
/// (so no datarace). We yield execution of the current task by dereferencing a pointer to our
/// Runtime and then calling `t_yield`
pub fn yield_task() {
unsafe {
let rt_ptr = RUNTIME as *mut Runtime;
(*rt_ptr).t_yield();
};
}
/// So here is our inline Assembly. As you remember from our first example this is just a bit more elaborate where we first
/// read out the values of all the registers we need and then sets all the register values to the register values we
/// saved when we suspended exceution on the "new" task.
///
/// This is essentially all we need to do to save and resume execution.
///
/// Some details about inline assembly.
///
/// The assembly commands in the string literal is called the assemblt template. It is preceeded by
/// zero or up to four segments indicated by ":":
///
/// - First ":" we have our output parameters, this parameters that this function will return.
/// - Second ":" we have the input parameters which is our contexts. We only read from the "new" context
/// but we modify the "old" context saving our registers there (see volatile option below)
/// - Third ":" This our clobber list, this is information to the compiler that these registers can't be used freely
/// - Fourth ":" This is options we can pass inn, Rust has 3: "alignstack", "volatile" and "intel"
///
/// For this to work on windows we need to use "alignstack" where the compiler adds the neccesary padding to
/// make sure our stack is aligned. Since we modify one of our inputs, our assembly has "side effects"
/// therefore we should use the `volatile` option. I **think** this is actually set for us by default
/// when there are no output parameters given (my own assumption after going through the source code)
/// for the `asm` macro, but we should make it explicit anyway.
///
/// One last important part (it will not work without this) is the #[naked] attribute. Basically this lets us have full
/// control over the stack layout since normal functions has a prologue-and epilogue added by the
/// compiler that will cause trouble for us. We avoid this by marking the funtion as "Naked".
/// For this to work on `release` builds we also need to use the `#[inline(never)] attribute or else
/// the compiler decides to inline this function (curiously this currently only happens on Windows).
/// If the function is inlined we get a curious runtime error where it fails when switching back
/// to as saved context and in general our assembly will not work as expected.
///
/// see: https://github.com/rust-lang/rfcs/blob/master/text/1201-naked-fns.md
global_asm!(r#"
.section .text
.globl __switch
__switch:
sd x1, 0x00(a0)
sd x2, 0x08(a0)
sd x8, 0x10(a0)
sd x9, 0x18(a0)
sd x18, 0x20(a0)
sd x19, 0x28(a0)
sd x20, 0x30(a0)
sd x21, 0x38(a0)
sd x22, 0x40(a0)
sd x23, 0x48(a0)
sd x24, 0x50(a0)
sd x25, 0x58(a0)
sd x26, 0x60(a0)
sd x27, 0x68(a0)
sd x1, 0x70(a0)
ld x1, 0x00(a1)
ld x2, 0x08(a1)
ld x8, 0x10(a1)
ld x9, 0x18(a1)
ld x18, 0x20(a1)
ld x19, 0x28(a1)
ld x20, 0x30(a1)
ld x21, 0x38(a1)
ld x22, 0x40(a1)
ld x23, 0x48(a1)
ld x24, 0x50(a1)
ld x25, 0x58(a1)
ld x26, 0x60(a1)
ld x27, 0x68(a1)
ld t0, 0x70(a1)
jr t0
"#);
extern "C" {
pub fn __switch(old: *mut TaskContext, new: *const TaskContext);
}
// #[naked]
// #[inline(never)]
// unsafe fn switch(old: *mut TaskContext, new: *const TaskContext) {
// // a0: _old, a1: _new
// asm!("
// sd x1, 0x00(a0)
// sd x2, 0x08(a0)
// sd x8, 0x10(a0)
// sd x9, 0x18(a0)
// sd x18, 0x20(a0)
// sd x19, 0x28(a0)
// sd x20, 0x30(a0)
// sd x21, 0x38(a0)
// sd x22, 0x40(a0)
// sd x23, 0x48(a0)
// sd x24, 0x50(a0)
// sd x25, 0x58(a0)
// sd x26, 0x60(a0)
// sd x27, 0x68(a0)
// sd x1, 0x70(a0)
// ld x1, 0x00(a1)
// ld x2, 0x08(a1)
// ld x8, 0x10(a1)
// ld x9, 0x18(a1)
// ld x18, 0x20(a1)
// ld x19, 0x28(a1)
// ld x20, 0x30(a1)
// ld x21, 0x38(a1)
// ld x22, 0x40(a1)
// ld x23, 0x48(a1)
// ld x24, 0x50(a1)
// ld x25, 0x58(a1)
// ld x26, 0x60(a1)
// ld x27, 0x68(a1)
// ld t0, 0x70(a1)
// jr t0
// ", options( noreturn)
// );
// }
#[no_mangle]
pub fn main() -> ! {
let mut runtime = Runtime::new();
runtime.init();
runtime.spawn(|| {
println!("TASK 1 STARTING");
let id = 1;
for i in 0..10 {
println!("task: {} counter: {}", id, i);
yield_task();
}
println!("TASK 1 FINISHED");
});
runtime.spawn(|| {
println!("TASK 2 STARTING");
let id = 2;
for i in 0..15 {
println!("task: {} counter: {}", id, i);
yield_task();
}
println!("TASK 2 FINISHED");
});
runtime.run();
}
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