time: eliminate timer wheel allocations (tokio-rs#6779)

Author: tglane

tokio/src/loom/mocked.rs

@@ -24,6 +24,11 @@ pub(crate) mod sync {
         pub(crate) fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
             self.0.try_lock().ok()
         }
+
+        #[inline]
+        pub(crate) fn get_mut(&mut self) -> &mut T {
+            self.0.get_mut().unwrap()
+        }
     }
     pub(crate) use loom::sync::*;
 

tokio/src/loom/std/mutex.rs

@@ -33,4 +33,12 @@ impl<T> Mutex<T> {
             Err(TryLockError::WouldBlock) => None,
         }
     }
+
+    #[inline]
+    pub(crate) fn get_mut(&mut self) -> &mut T {
+        match self.0.get_mut() {
+            Ok(val) => val,
+            Err(p_err) => p_err.into_inner(),
+        }
+    }
 }

tokio/src/runtime/time/mod.rs

@@ -28,6 +28,7 @@ use crate::util::WakeList;
 
 use crate::loom::sync::atomic::AtomicU64;
 use std::fmt;
+use std::sync::RwLock;
 use std::{num::NonZeroU64, ptr::NonNull};
 
 struct AtomicOptionNonZeroU64(AtomicU64);
@@ -115,7 +116,10 @@ struct Inner {
     next_wake: AtomicOptionNonZeroU64,
 
     /// Sharded Timer wheels.
-    wheels: Box<[Mutex<wheel::Wheel>]>,
+    wheels: RwLock<ShardedWheel>,
+
+    /// Number of entries in the sharded timer wheels.
+    wheels_len: u32,
 
     /// True if the driver is being shutdown.
     pub(super) is_shutdown: AtomicBool,
@@ -130,6 +134,9 @@ struct Inner {
     did_wake: AtomicBool,
 }
 
+/// Wrapper around the sharded timer wheels.
+struct ShardedWheel(Box<[Mutex<wheel::Wheel>]>);
+
 // ===== impl Driver =====
 
 impl Driver {
@@ -149,7 +156,8 @@ impl Driver {
             time_source,
             inner: Inner {
                 next_wake: AtomicOptionNonZeroU64::new(None),
-                wheels: wheels.into_boxed_slice(),
+                wheels: RwLock::new(ShardedWheel(wheels.into_boxed_slice())),
+                wheels_len: shards,
                 is_shutdown: AtomicBool::new(false),
                 #[cfg(feature = "test-util")]
                 did_wake: AtomicBool::new(false),
@@ -190,23 +198,27 @@ impl Driver {
         assert!(!handle.is_shutdown());
 
         // Finds out the min expiration time to park.
-        let locks = (0..rt_handle.time().inner.get_shard_size())
-            .map(|id| rt_handle.time().inner.lock_sharded_wheel(id))
-            .collect::<Vec<_>>();
-
-        let expiration_time = locks
-            .iter()
-            .filter_map(|lock| lock.next_expiration_time())
-            .min();
-
-        rt_handle
-            .time()
-            .inner
-            .next_wake
-            .store(next_wake_time(expiration_time));
-
-        // Safety: After updating the `next_wake`, we drop all the locks.
-        drop(locks);
+        let expiration_time = {
+            let mut wheels_lock = rt_handle
+                .time()
+                .inner
+                .wheels
+                .write()
+                .expect("Timer wheel shards poisoned");
+            let expiration_time = wheels_lock
+                .0
+                .iter_mut()
+                .filter_map(|wheel| wheel.get_mut().next_expiration_time())
+                .min();
+
+            rt_handle
+                .time()
+                .inner
+                .next_wake
+                .store(next_wake_time(expiration_time));
+
+            expiration_time
+        };
 
         match expiration_time {
             Some(when) => {
@@ -312,7 +324,12 @@ impl Handle {
     // Returns the next wakeup time of this shard.
     pub(self) fn process_at_sharded_time(&self, id: u32, mut now: u64) -> Option<u64> {
         let mut waker_list = WakeList::new();
-        let mut lock = self.inner.lock_sharded_wheel(id);
+        let mut wheels_lock = self
+            .inner
+            .wheels
+            .read()
+            .expect("Timer wheel shards poisoned");
+        let mut lock = wheels_lock.lock_sharded_wheel(id);
 
         if now < lock.elapsed() {
             // Time went backwards! This normally shouldn't happen as the Rust language
@@ -334,15 +351,22 @@ impl Handle {
                 if !waker_list.can_push() {
                     // Wake a batch of wakers. To avoid deadlock, we must do this with the lock temporarily dropped.
                     drop(lock);
+                    drop(wheels_lock);
 
                     waker_list.wake_all();
 
-                    lock = self.inner.lock_sharded_wheel(id);
+                    wheels_lock = self
+                        .inner
+                        .wheels
+                        .read()
+                        .expect("Timer wheel shards poisoned");
+                    lock = wheels_lock.lock_sharded_wheel(id);
                 }
             }
         }
         let next_wake_up = lock.poll_at();
         drop(lock);
+        drop(wheels_lock);
 
         waker_list.wake_all();
         next_wake_up
@@ -360,7 +384,12 @@ impl Handle {
     /// `add_entry` must not be called concurrently.
     pub(self) unsafe fn clear_entry(&self, entry: NonNull<TimerShared>) {
         unsafe {
-            let mut lock = self.inner.lock_sharded_wheel(entry.as_ref().shard_id());
+            let wheels_lock = self
+                .inner
+                .wheels
+                .read()
+                .expect("Timer wheel shards poisoned");
+            let mut lock = wheels_lock.lock_sharded_wheel(entry.as_ref().shard_id());
 
             if entry.as_ref().might_be_registered() {
                 lock.remove(entry);
@@ -383,7 +412,13 @@ impl Handle {
         entry: NonNull<TimerShared>,
     ) {
         let waker = unsafe {
-            let mut lock = self.inner.lock_sharded_wheel(entry.as_ref().shard_id());
+            let wheels_lock = self
+                .inner
+                .wheels
+                .read()
+                .expect("Timer wheel shards poisoned");
+
+            let mut lock = wheels_lock.lock_sharded_wheel(entry.as_ref().shard_id());
 
             // We may have raced with a firing/deregistration, so check before
             // deregistering.
@@ -443,24 +478,14 @@ impl Handle {
 // ===== impl Inner =====
 
 impl Inner {
-    /// Locks the driver's sharded wheel structure.
-    pub(super) fn lock_sharded_wheel(
-        &self,
-        shard_id: u32,
-    ) -> crate::loom::sync::MutexGuard<'_, Wheel> {
-        let index = shard_id % (self.wheels.len() as u32);
-        // Safety: This modulo operation ensures that the index is not out of bounds.
-        unsafe { self.wheels.get_unchecked(index as usize).lock() }
-    }
-
     // Check whether the driver has been shutdown
     pub(super) fn is_shutdown(&self) -> bool {
         self.is_shutdown.load(Ordering::SeqCst)
     }
 
     // Gets the number of shards.
     fn get_shard_size(&self) -> u32 {
-        self.wheels.len() as u32
+        self.wheels_len
     }
 }
 
@@ -470,5 +495,19 @@ impl fmt::Debug for Inner {
     }
 }
 
+// ===== impl ShardedWheel =====
+
+impl ShardedWheel {
+    /// Locks the driver's sharded wheel structure.
+    pub(super) fn lock_sharded_wheel(
+        &self,
+        shard_id: u32,
+    ) -> crate::loom::sync::MutexGuard<'_, Wheel> {
+        let index = shard_id % (self.0.len() as u32);
+        // Safety: This modulo operation ensures that the index is not out of bounds.
+        unsafe { self.0.get_unchecked(index as usize) }.lock()
+    }
+}
+
 #[cfg(test)]
 mod tests;