Implemented various backoffs and per-CPU rng

Cargo.toml

@@ -50,6 +50,7 @@ members = [
     "modules/ruxruntime",
     "modules/ruxtask",
     "modules/ruxfutex",
+    "modules/ruxrand",
 
     "api/ruxfeat",
     "api/arceos_api",

crates/spinlock/Cargo.toml

@@ -2,7 +2,10 @@
 name = "spinlock"
 version = "0.1.0"
 edition = "2021"
-authors = ["Yuekai Jia <[email protected]>"]
+authors = [
+    "Yuekai Jia <[email protected]>",
+    "Igna <[email protected]>",
+]
 description = "`no_std` spin lock implementation that can disable kernel local IRQs or preemption while locking"
 license = "GPL-3.0-or-later OR Apache-2.0"
 homepage = "https://github.com/rcore-os/arceos"

crates/spinlock/src/base.rs

@@ -24,17 +24,23 @@ use core::sync::atomic::{AtomicBool, Ordering};
 
 use kernel_guard::BaseGuard;
 
+use crate::{strategy, Strategy};
+
+/// The default strategy used in spinlocks.
+pub type DefaultStrategy = strategy::Once;
+
 /// A [spin lock](https://en.m.wikipedia.org/wiki/Spinlock) providing mutually
 /// exclusive access to data.
 ///
 /// This is a base struct, the specific behavior depends on the generic
 /// parameter `G` that implements [`BaseGuard`], such as whether to disable
-/// local IRQs or kernel preemption before acquiring the lock.
+/// local IRQs or kernel preemption before acquiring the lock. The parameter `S`
+/// that implements [`Strategy`] defines the behavior when encountering contention.
 ///
 /// For single-core environment (without the "smp" feature), we remove the lock
 /// state, CPU can always get the lock if we follow the proper guard in use.
-pub struct BaseSpinLock<G: BaseGuard, T: ?Sized> {
-    _phantom: PhantomData<G>,
+pub struct BaseSpinLock<DG: BaseGuard, T: ?Sized, S: Strategy = DefaultStrategy> {
+    _phantom: PhantomData<(DG, S)>,
     #[cfg(feature = "smp")]
     lock: AtomicBool,
     data: UnsafeCell<T>,
@@ -52,10 +58,10 @@ pub struct BaseSpinLockGuard<'a, G: BaseGuard, T: ?Sized + 'a> {
 }
 
 // Same unsafe impls as `std::sync::Mutex`
-unsafe impl<G: BaseGuard, T: ?Sized + Send> Sync for BaseSpinLock<G, T> {}
-unsafe impl<G: BaseGuard, T: ?Sized + Send> Send for BaseSpinLock<G, T> {}
+unsafe impl<G: BaseGuard, T: ?Sized + Send, B: Strategy> Sync for BaseSpinLock<G, T, B> {}
+unsafe impl<G: BaseGuard, T: ?Sized + Send, B: Strategy> Send for BaseSpinLock<G, T, B> {}
 
-impl<G: BaseGuard, T> BaseSpinLock<G, T> {
+impl<G: BaseGuard, T, S: Strategy> BaseSpinLock<G, T, S> {
     /// Creates a new [`BaseSpinLock`] wrapping the supplied data.
     #[inline(always)]
     pub const fn new(data: T) -> Self {
@@ -77,26 +83,33 @@ impl<G: BaseGuard, T> BaseSpinLock<G, T> {
     }
 }
 
-impl<G: BaseGuard, T: ?Sized> BaseSpinLock<G, T> {
-    /// Locks the [`BaseSpinLock`] and returns a guard that permits access to the inner data.
+impl<G: BaseGuard, T: ?Sized, S: Strategy> BaseSpinLock<G, T, S> {
+    /// Locks the [`BaseSpinLock`] using the given guard type and backoff strategy,
+    /// and returns a guard that permits access to the inner data.
     ///
     /// The returned value may be dereferenced for data access
     /// and the lock will be dropped when the guard falls out of scope.
-    #[inline(always)]
-    pub fn lock(&self) -> BaseSpinLockGuard<G, T> {
-        let irq_state = G::acquire();
+    pub fn lock_as<GT: BaseGuard, ST: Strategy>(&self) -> BaseSpinLockGuard<GT, T> {
+        let irq_state = GT::acquire();
+
         #[cfg(feature = "smp")]
         {
+            use crate::strategy::{Backoff, Relax};
+
+            let mut backoff = <ST as Strategy>::new_backoff();
             // Can fail to lock even if the spinlock is not locked. May be more efficient than `try_lock`
             // when called in a loop.
             while self
                 .lock
                 .compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed)
                 .is_err()
             {
+                backoff.backoff();
+                let mut relax = <ST as Strategy>::new_relax();
+
                 // Wait until the lock looks unlocked before retrying
                 while self.is_locked() {
-                    core::hint::spin_loop();
+                    relax.relax();
                 }
             }
         }
@@ -109,6 +122,16 @@ impl<G: BaseGuard, T: ?Sized> BaseSpinLock<G, T> {
         }
     }
 
+    /// Locks the [`BaseSpinLock`] using the "default" strategy specified by lock type,
+    /// and returns a guard that permits access to the inner data.
+    ///
+    /// The returned value may be dereferenced for data access
+    /// and the lock will be dropped when the guard falls out of scope.
+    #[inline(always)]
+    pub fn lock(&self) -> BaseSpinLockGuard<G, T> {
+        self.lock_as::<G, S>()
+    }
+
     /// Returns `true` if the lock is currently held.
     ///
     /// # Safety
@@ -183,14 +206,14 @@ impl<G: BaseGuard, T: ?Sized> BaseSpinLock<G, T> {
     }
 }
 
-impl<G: BaseGuard, T: ?Sized + Default> Default for BaseSpinLock<G, T> {
+impl<G: BaseGuard, T: ?Sized + Default, S: Strategy> Default for BaseSpinLock<G, T, S> {
     #[inline(always)]
     fn default() -> Self {
         Self::new(Default::default())
     }
 }
 
-impl<G: BaseGuard, T: ?Sized + fmt::Debug> fmt::Debug for BaseSpinLock<G, T> {
+impl<G: BaseGuard, T: ?Sized + fmt::Debug, S: Strategy> fmt::Debug for BaseSpinLock<G, T, S> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self.try_lock() {
             Some(guard) => write!(f, "SpinLock {{ data: ")

crates/spinlock/src/lib.rs

@@ -16,15 +16,23 @@
 //!   environment (without this feature), the lock state is unnecessary and
 //!   optimized out. CPU can always get the lock if we follow the proper guard
 //!   in use. By default, this feature is disabled.
+//! - `rand`: Provide extra contention-alleviating strategy using exponential
+//!   backoff algorithm. The user is responsible for providing the random number
+//!   generator implementation.
 
 #![cfg_attr(not(test), no_std)]
 
 mod base;
 
-use kernel_guard::{NoOp, NoPreempt, NoPreemptIrqSave};
+/// Defines the strategies used when encountering lock contention.
+pub mod strategy;
+
+use kernel_guard::{NoPreempt, NoPreemptIrqSave};
 
 pub use self::base::{BaseSpinLock, BaseSpinLockGuard};
 
+pub use self::strategy::*;
+
 /// A spin lock that disables kernel preemption while trying to lock, and
 /// re-enables it after unlocking.
 ///
@@ -48,7 +56,7 @@ pub type SpinNoIrqGuard<'a, T> = BaseSpinLockGuard<'a, NoPreemptIrqSave, T>;
 ///
 /// It must be used in the preemption-disabled and local IRQ-disabled context,
 /// or never be used in interrupt handlers.
-pub type SpinRaw<T> = BaseSpinLock<NoOp, T>;
+pub type SpinRaw<T> = BaseSpinLock<kernel_guard::NoOp, T>;
 
 /// A guard that provides mutable data access for [`SpinRaw`].
-pub type SpinRawGuard<'a, T> = BaseSpinLockGuard<'a, NoOp, T>;
+pub type SpinRawGuard<'a, T> = BaseSpinLockGuard<'a, kernel_guard::NoOp, T>;

crates/spinlock/src/strategy.rs

@@ -0,0 +1,161 @@
+/* Copyright (c) [2023] [Syswonder Community]
+ *   [Ruxos] is licensed under Mulan PSL v2.
+ *   You can use this software according to the terms and conditions of the Mulan PSL v2.
+ *   You may obtain a copy of Mulan PSL v2 at:
+ *               http://license.coscl.org.cn/MulanPSL2
+ *   THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO NON-INFRINGEMENT, MERCHANTABILITY OR FIT FOR A PARTICULAR PURPOSE.
+ *   See the Mulan PSL v2 for more details.
+ */
+
+use core::marker::PhantomData;
+
+// Re-exports extra strategies when feature enabled.
+#[cfg(feature = "rand")]
+pub use crate::rand_strategy::*;
+
+#[inline(always)]
+fn exp_backoff(current_limit: &mut u32, max: u32) {
+    let limit = *current_limit;
+    *current_limit = max.max(limit << 1);
+    for _ in 0..limit {
+        core::hint::spin_loop();
+    }
+}
+
+/// Defines the backoff behavior of a spinlock.
+pub trait Backoff {
+    /// Backoff behavior when failed to acquire the lock.
+    fn backoff(&mut self);
+}
+
+/// Defines the relax behavior of a spinlock.
+pub trait Relax {
+    /// Relax behavior when the lock seemed still held.
+    fn relax(&mut self);
+}
+
+/// Defines the lock behavior when encountering contention.
+/// [`Backoff::backoff`] is called when failed to acquire the lock, and
+/// [`Relax::relax`]` is called when the lock seemed still held.
+///
+/// One can easily define a new [`Strategy`] impl that
+/// combines existing backoff/relax behaviors.
+pub trait Strategy {
+    /// The type that defines the relax behavior.
+    type Relax: Relax;
+
+    /// The type that defines the backoff behavior.
+    type Backoff: Backoff;
+
+    /// Create a new relax state every time after failed to acquire the lock.
+    fn new_relax() -> Self::Relax;
+
+    /// Create a new backoff state every time after the locking procedure began.
+    fn new_backoff() -> Self::Backoff;
+}
+
+impl<T: Relax + Backoff + Default> Strategy for T {
+    type Relax = T;
+    type Backoff = T;
+
+    #[inline(always)]
+    fn new_relax() -> Self::Relax {
+        T::default()
+    }
+
+    #[inline(always)]
+    fn new_backoff() -> Self::Backoff {
+        T::default()
+    }
+}
+
+/// Do nothing when backoff/relax is required.
+/// It can be used as a baseline, or under rare circumstances be used as a
+/// performance improvement.
+///
+/// Note that under most modern CPU design, not using any backoff/relax strategy
+/// would normally make things slower.
+#[derive(Debug, Default)]
+pub struct NoOp;
+
+/// Call [`core::hint::spin_loop`] once when backoff/relax is required.
+///
+/// This may improve performance by said, reducing bus traffic. The exact
+/// behavior and benefits depend on the machine.
+#[derive(Debug, Default)]
+pub struct Once;
+
+/// Call [`core::hint::spin_loop`] with exponentially increased time when
+/// backoff/relax is required.
+///
+/// This would generally increase performance when the lock is highly contended.
+#[derive(Debug)]
+pub struct Exp<const MAX: u32>(u32);
+
+/// Combines a [`Relax`] and a [`Backoff`] into a strategy.
+#[derive(Debug, Default)]
+pub struct Combine<R: Relax, B: Backoff>(PhantomData<(R, B)>);
+
+impl Relax for NoOp {
+    #[inline(always)]
+    fn relax(&mut self) {}
+}
+
+impl Backoff for NoOp {
+    #[inline(always)]
+    fn backoff(&mut self) {}
+}
+
+impl Relax for Once {
+    #[inline(always)]
+    fn relax(&mut self) {
+        core::hint::spin_loop();
+    }
+}
+
+impl Backoff for Once {
+    #[inline(always)]
+    fn backoff(&mut self) {
+        core::hint::spin_loop();
+    }
+}
+
+impl<const N: u32> Relax for Exp<N> {
+    #[inline(always)]
+    fn relax(&mut self) {
+        exp_backoff(&mut self.0, N);
+    }
+}
+
+impl<const N: u32> Backoff for Exp<N> {
+    #[inline(always)]
+    fn backoff(&mut self) {
+        exp_backoff(&mut self.0, N);
+    }
+}
+
+impl<const N: u32> Default for Exp<N> {
+    #[inline(always)]
+    fn default() -> Self {
+        Self(1)
+    }
+}
+
+impl<R, B> Strategy for Combine<R, B>
+where
+    R: Relax + Default,
+    B: Backoff + Default,
+{
+    type Relax = R;
+    type Backoff = B;
+
+    #[inline(always)]
+    fn new_relax() -> Self::Relax {
+        R::default()
+    }
+
+    #[inline(always)]
+    fn new_backoff() -> Self::Backoff {
+        B::default()
+    }
+}