Merge pull request #79 from embassy-rs/optimized-wakers

Optimized wakers

embassy/Cargo.toml

@@ -13,6 +13,8 @@ defmt-info = []
 defmt-warn = []
 defmt-error = []
 
+executor-agnostic = []
+
 [dependencies]
 defmt = { version = "0.2.0", optional = true }
 log = { version = "0.4.11", optional = true }

embassy/src/util/mod.rs

@@ -3,6 +3,8 @@ mod forever;
 mod mutex;
 mod portal;
 mod signal;
+
+#[cfg_attr(feature = "executor-agnostic", path = "waker_agnostic.rs")]
 mod waker;
 
 pub use drop_bomb::*;

embassy/src/util/waker.rs

@@ -1,11 +1,14 @@
-use core::mem;
-use core::task::Context;
+use core::ptr::{self, NonNull};
 use core::task::Waker;
 
+use atomic_polyfill::{AtomicPtr, Ordering};
+
+use crate::executor::raw::{task_from_waker, wake_task, Task};
+
 /// Utility struct to register and wake a waker.
 #[derive(Debug)]
 pub struct WakerRegistration {
-    waker: Option<Waker>,
+    waker: Option<NonNull<Task>>,
 }
 
 impl WakerRegistration {
@@ -15,37 +18,61 @@ impl WakerRegistration {
 
     /// Register a waker. Overwrites the previous waker, if any.
     pub fn register(&mut self, w: &Waker) {
+        let w = unsafe { task_from_waker(w) };
         match self.waker {
-            // Optimization: If both the old and new Wakers wake the same task, we can simply
-            // keep the old waker, skipping the clone. (In most executor implementations,
-            // cloning a waker is somewhat expensive, comparable to cloning an Arc).
-            Some(ref w2) if (w2.will_wake(w)) => {}
-            _ => {
-                // clone the new waker and store it
-                if let Some(old_waker) = mem::replace(&mut self.waker, Some(w.clone())) {
-                    // We had a waker registered for another task. Wake it, so the other task can
-                    // reregister itself if it's still interested.
-                    //
-                    // If two tasks are waiting on the same thing concurrently, this will cause them
-                    // to wake each other in a loop fighting over this WakerRegistration. This wastes
-                    // CPU but things will still work.
-                    //
-                    // If the user wants to have two tasks waiting on the same thing they should use
-                    // a more appropriate primitive that can store multiple wakers.
-                    old_waker.wake()
-                }
+            // Optimization: If both the old and new Wakers wake the same task, do nothing.
+            Some(w2) if w == w2 => {}
+            Some(w2) => {
+                // We had a waker registered for another task. Wake it, so the other task can
+                // reregister itself if it's still interested.
+                //
+                // If two tasks are waiting on the same thing concurrently, this will cause them
+                // to wake each other in a loop fighting over this WakerRegistration. This wastes
+                // CPU but things will still work.
+                //
+                // If the user wants to have two tasks waiting on the same thing they should use
+                // a more appropriate primitive that can store multiple wakers.
+
+                unsafe { wake_task(w2) }
+                self.waker = Some(w);
             }
+            None => self.waker = Some(w),
         }
     }
 
     /// Wake the registered waker, if any.
     pub fn wake(&mut self) {
         if let Some(w) = self.waker.take() {
-            w.wake()
+            unsafe { wake_task(w) }
+        }
+    }
+}
+
+pub struct AtomicWakerRegistration {
+    waker: AtomicPtr<Task>,
+}
+
+impl AtomicWakerRegistration {
+    pub const fn new() -> Self {
+        Self {
+            waker: AtomicPtr::new(ptr::null_mut()),
         }
     }
 
-    pub fn context(&self) -> Option<Context<'_>> {
-        self.waker.as_ref().map(|w| Context::from_waker(w))
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&self, w: &Waker) {
+        let w = unsafe { task_from_waker(w) };
+        let w2 = self.waker.swap(w.as_ptr(), Ordering::Relaxed);
+        if !w2.is_null() && w2 != w.as_ptr() {
+            unsafe { wake_task(NonNull::new_unchecked(w2)) };
+        }
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&self) {
+        let w2 = self.waker.swap(ptr::null_mut(), Ordering::Relaxed);
+        if !w2.is_null() {
+            unsafe { wake_task(NonNull::new_unchecked(w2)) };
+        }
     }
 }

embassy/src/util/waker_agnostic.rs

@@ -0,0 +1,87 @@
+use core::cell::Cell;
+use core::mem;
+use core::task::Waker;
+
+use cortex_m::interrupt::Mutex;
+
+/// Utility struct to register and wake a waker.
+#[derive(Debug)]
+pub struct WakerRegistration {
+    waker: Option<Waker>,
+}
+
+impl WakerRegistration {
+    pub const fn new() -> Self {
+        Self { waker: None }
+    }
+
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&mut self, w: &Waker) {
+        match self.waker {
+            // Optimization: If both the old and new Wakers wake the same task, we can simply
+            // keep the old waker, skipping the clone. (In most executor implementations,
+            // cloning a waker is somewhat expensive, comparable to cloning an Arc).
+            Some(ref w2) if (w2.will_wake(w)) => {}
+            _ => {
+                // clone the new waker and store it
+                if let Some(old_waker) = mem::replace(&mut self.waker, Some(w.clone())) {
+                    // We had a waker registered for another task. Wake it, so the other task can
+                    // reregister itself if it's still interested.
+                    //
+                    // If two tasks are waiting on the same thing concurrently, this will cause them
+                    // to wake each other in a loop fighting over this WakerRegistration. This wastes
+                    // CPU but things will still work.
+                    //
+                    // If the user wants to have two tasks waiting on the same thing they should use
+                    // a more appropriate primitive that can store multiple wakers.
+                    old_waker.wake()
+                }
+            }
+        }
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&mut self) {
+        if let Some(w) = self.waker.take() {
+            w.wake()
+        }
+    }
+}
+
+/// Utility struct to register and wake a waker.
+pub struct AtomicWakerRegistration {
+    waker: Mutex<Cell<Option<Waker>>>,
+}
+
+impl AtomicWakerRegistration {
+    pub const fn new() -> Self {
+        Self {
+            waker: Mutex::new(Cell::new(None)),
+        }
+    }
+
+    /// Register a waker. Overwrites the previous waker, if any.
+    pub fn register(&mut self, w: &Waker) {
+        cortex_m::interrupt::free(|cs| {
+            let cell = self.waker.borrow(cs);
+            cell.set(match cell.replace(None) {
+                Some(w2) if (w2.will_wake(w)) => Some(w2),
+                Some(w2) => {
+                    w2.wake();
+                    Some(w.clone())
+                }
+                None => Some(w.clone()),
+            })
+        })
+    }
+
+    /// Wake the registered waker, if any.
+    pub fn wake(&mut self) {
+        cortex_m::interrupt::free(|cs| {
+            let cell = self.waker.borrow(cs);
+            if let Some(w) = cell.replace(None) {
+                w.wake()
+            }
+        })
+    }
+}