1 files changed, 194 insertions, 0 deletions

diff --git a/embassy-executor/src/raw/run_queue.rs b/embassy-executor/src/raw/run_queue.rs
new file mode 100644
index 000000000..d98c26f73
--- /dev/null
+++ b/embassy-executor/src/raw/run_queue.rs
@@ -0,0 +1,194 @@
+use core::ptr::{addr_of_mut, NonNull};
+
+use cordyceps::sorted_list::Links;
+use cordyceps::Linked;
+#[cfg(feature = "scheduler-deadline")]
+use cordyceps::SortedList;
+
+#[cfg(target_has_atomic = "ptr")]
+type TransferStack<T> = cordyceps::TransferStack<T>;
+
+#[cfg(not(target_has_atomic = "ptr"))]
+type TransferStack<T> = MutexTransferStack<T>;
+
+use super::{TaskHeader, TaskRef};
+
+/// Use `cordyceps::sorted_list::Links` as the singly linked list
+/// for RunQueueItems.
+pub(crate) type RunQueueItem = Links<TaskHeader>;
+
+/// Implements the `Linked` trait, allowing for singly linked list usage
+/// of any of cordyceps' `TransferStack` (used for the atomic runqueue),
+/// `SortedList` (used with the DRS scheduler), or `Stack`, which is
+/// popped atomically from the `TransferStack`.
+unsafe impl Linked<Links<TaskHeader>> for TaskHeader {
+    type Handle = TaskRef;
+
+    // Convert a TaskRef into a TaskHeader ptr
+    fn into_ptr(r: TaskRef) -> NonNull<TaskHeader> {
+        r.ptr
+    }
+
+    // Convert a TaskHeader into a TaskRef
+    unsafe fn from_ptr(ptr: NonNull<TaskHeader>) -> TaskRef {
+        TaskRef { ptr }
+    }
+
+    // Given a pointer to a TaskHeader, obtain a pointer to the Links structure,
+    // which can be used to traverse to other TaskHeader nodes in the linked list
+    unsafe fn links(ptr: NonNull<TaskHeader>) -> NonNull<Links<TaskHeader>> {
+        let ptr: *mut TaskHeader = ptr.as_ptr();
+        NonNull::new_unchecked(addr_of_mut!((*ptr).run_queue_item))
+    }
+}
+
+/// Atomic task queue using a very, very simple lock-free linked-list queue:
+///
+/// To enqueue a task, task.next is set to the old head, and head is atomically set to task.
+///
+/// Dequeuing is done in batches: the queue is emptied by atomically replacing head with
+/// null. Then the batch is iterated following the next pointers until null is reached.
+///
+/// Note that batches will be iterated in the reverse order as they were enqueued. This is OK
+/// for our purposes: it can't create fairness problems since the next batch won't run until the
+/// current batch is completely processed, so even if a task enqueues itself instantly (for example
+/// by waking its own waker) can't prevent other tasks from running.
+pub(crate) struct RunQueue {
+    stack: TransferStack<TaskHeader>,
+}
+
+impl RunQueue {
+    pub const fn new() -> Self {
+        Self {
+            stack: TransferStack::new(),
+        }
+    }
+
+    /// Enqueues an item. Returns true if the queue was empty.
+    ///
+    /// # Safety
+    ///
+    /// `item` must NOT be already enqueued in any queue.
+    #[inline(always)]
+    pub(crate) unsafe fn enqueue(&self, task: TaskRef, _tok: super::state::Token) -> bool {
+        self.stack.push_was_empty(
+            task,
+            #[cfg(not(target_has_atomic = "ptr"))]
+            _tok,
+        )
+    }
+
+    /// # Standard atomic runqueue
+    ///
+    /// Empty the queue, then call `on_task` for each task that was in the queue.
+    /// NOTE: It is OK for `on_task` to enqueue more tasks. In this case they're left in the queue
+    /// and will be processed by the *next* call to `dequeue_all`, *not* the current one.
+    #[cfg(not(feature = "scheduler-deadline"))]
+    pub(crate) fn dequeue_all(&self, on_task: impl Fn(TaskRef)) {
+        let taken = self.stack.take_all();
+        for taskref in taken {
+            run_dequeue(&taskref);
+            on_task(taskref);
+        }
+    }
+
+    /// # Earliest Deadline First Scheduler
+    ///
+    /// This algorithm will loop until all enqueued tasks are processed.
+    ///
+    /// Before polling a task, all currently enqueued tasks will be popped from the
+    /// runqueue, and will be added to the working `sorted` list, a linked-list that
+    /// sorts tasks by their deadline, with nearest deadline items in the front, and
+    /// furthest deadline items in the back.
+    ///
+    /// After popping and sorting all pending tasks, the SOONEST task will be popped
+    /// from the front of the queue, and polled by calling `on_task` on it.
+    ///
+    /// This process will repeat until the local `sorted` queue AND the global
+    /// runqueue are both empty, at which point this function will return.
+    #[cfg(feature = "scheduler-deadline")]
+    pub(crate) fn dequeue_all(&self, on_task: impl Fn(TaskRef)) {
+        let mut sorted =
+            SortedList::<TaskHeader>::new_with_cmp(|lhs, rhs| lhs.metadata.deadline().cmp(&rhs.metadata.deadline()));
+
+        loop {
+            // For each loop, grab any newly pended items
+            let taken = self.stack.take_all();
+
+            // Sort these into the list - this is potentially expensive! We do an
+            // insertion sort of new items, which iterates the linked list.
+            //
+            // Something on the order of `O(n * m)`, where `n` is the number
+            // of new tasks, and `m` is the number of already pending tasks.
+            sorted.extend(taken);
+
+            // Pop the task with the SOONEST deadline. If there are no tasks
+            // pending, then we are done.
+            let Some(taskref) = sorted.pop_front() else {
+                return;
+            };
+
+            // We got one task, mark it as dequeued, and process the task.
+            run_dequeue(&taskref);
+            on_task(taskref);
+        }
+    }
+}
+
+/// atomic state does not require a cs...
+#[cfg(target_has_atomic = "ptr")]
+#[inline(always)]
+fn run_dequeue(taskref: &TaskRef) {
+    taskref.header().state.run_dequeue();
+}
+
+/// ...while non-atomic state does
+#[cfg(not(target_has_atomic = "ptr"))]
+#[inline(always)]
+fn run_dequeue(taskref: &TaskRef) {
+    critical_section::with(|cs| {
+        taskref.header().state.run_dequeue(cs);
+    })
+}
+
+/// A wrapper type that acts like TransferStack by wrapping a normal Stack in a CS mutex
+#[cfg(not(target_has_atomic = "ptr"))]
+struct MutexTransferStack<T: Linked<cordyceps::stack::Links<T>>> {
+    inner: critical_section::Mutex<core::cell::UnsafeCell<cordyceps::Stack<T>>>,
+}
+
+#[cfg(not(target_has_atomic = "ptr"))]
+impl<T: Linked<cordyceps::stack::Links<T>>> MutexTransferStack<T> {
+    const fn new() -> Self {
+        Self {
+            inner: critical_section::Mutex::new(core::cell::UnsafeCell::new(cordyceps::Stack::new())),
+        }
+    }
+
+    /// Push an item to the transfer stack, returning whether the stack was previously empty
+    fn push_was_empty(&self, item: T::Handle, token: super::state::Token) -> bool {
+        // SAFETY: The critical-section mutex guarantees that there is no *concurrent* access
+        // for the lifetime of the token, but does NOT protect against re-entrant access.
+        // However, we never *return* the reference, nor do we recurse (or call another method
+        // like `take_all`) that could ever allow for re-entrant aliasing. Therefore, the
+        // presence of the critical section is sufficient to guarantee exclusive access to
+        // the `inner` field for the purposes of this function.
+        let inner = unsafe { &mut *self.inner.borrow(token).get() };
+        let is_empty = inner.is_empty();
+        inner.push(item);
+        is_empty
+    }
+
+    fn take_all(&self) -> cordyceps::Stack<T> {
+        critical_section::with(|cs| {
+            // SAFETY: The critical-section mutex guarantees that there is no *concurrent* access
+            // for the lifetime of the token, but does NOT protect against re-entrant access.
+            // However, we never *return* the reference, nor do we recurse (or call another method
+            // like `push_was_empty`) that could ever allow for re-entrant aliasing. Therefore, the
+            // presence of the critical section is sufficient to guarantee exclusive access to
+            // the `inner` field for the purposes of this function.
+            let inner = unsafe { &mut *self.inner.borrow(cs).get() };
+            inner.take_all()
+        })
+    }
+}