ci: speed up multi-threaded runtime loom tests. (#5723)

Increase max preemption back to 2 while running the tests in under 90 minutes.

.github/workflows/loom.yml

@@ -28,13 +28,13 @@ jobs:
           - scope: --skip loom_pool
             max_preemptions: 2
           - scope: loom_pool::group_a
-            max_preemptions: 1
+            max_preemptions: 2
           - scope: loom_pool::group_b
             max_preemptions: 2
           - scope: loom_pool::group_c
-            max_preemptions: 1
+            max_preemptions: 2
           - scope: loom_pool::group_d
-            max_preemptions: 1
+            max_preemptions: 2
           - scope: time::driver
             max_preemptions: 2
     steps:

tokio/src/runtime/scheduler/multi_thread/park.rs

@@ -4,7 +4,6 @@
 
 use crate::loom::sync::atomic::AtomicUsize;
 use crate::loom::sync::{Arc, Condvar, Mutex};
-use crate::loom::thread;
 use crate::runtime::driver::{self, Driver};
 use crate::util::TryLock;
 
@@ -104,18 +103,14 @@ impl Unparker {
 impl Inner {
     /// Parks the current thread for at most `dur`.
     fn park(&self, handle: &driver::Handle) {
-        for _ in 0..3 {
-            // If we were previously notified then we consume this notification and
-            // return quickly.
-            if self
-                .state
-                .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
-                .is_ok()
-            {
-                return;
-            }
-
-            thread::yield_now();
+        // If we were previously notified then we consume this notification and
+        // return quickly.
+        if self
+            .state
+            .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
+            .is_ok()
+        {
+            return;
         }
 
         if let Some(mut driver) = self.shared.driver.try_lock() {

tokio/src/runtime/tests/loom_pool.rs

@@ -323,8 +323,7 @@ mod group_d {
             // Spawn a task
             let c2 = c1.clone();
             pool.spawn(track(async move {
-                gated().await;
-                gated().await;
+                multi_gated().await;
 
                 if 1 == c1.fetch_add(1, Relaxed) {
                     done_tx1.assert_send(());
@@ -333,8 +332,7 @@ mod group_d {
 
             // Spawn a second task
             pool.spawn(track(async move {
-                gated().await;
-                gated().await;
+                multi_gated().await;
 
                 if 1 == c2.fetch_add(1, Relaxed) {
                     done_tx2.assert_send(());
@@ -349,14 +347,11 @@ mod group_d {
 fn mk_pool(num_threads: usize) -> Runtime {
     runtime::Builder::new_multi_thread()
         .worker_threads(num_threads)
+        .event_interval(2)
         .build()
         .unwrap()
 }
 
-fn gated() -> impl Future<Output = &'static str> {
-    gated2(false)
-}
-
 fn gated2(thread: bool) -> impl Future<Output = &'static str> {
     use loom::thread;
     use std::sync::Arc;
@@ -394,6 +389,38 @@ fn gated2(thread: bool) -> impl Future<Output = &'static str> {
     })
 }
 
+async fn multi_gated() {
+    struct Gate {
+        waker: loom::future::AtomicWaker,
+        count: AtomicUsize,
+    }
+
+    let gate = Arc::new(Gate {
+        waker: loom::future::AtomicWaker::new(),
+        count: AtomicUsize::new(0),
+    });
+
+    {
+        let gate = gate.clone();
+        spawn(track(async move {
+            for i in 1..3 {
+                gate.count.store(i, SeqCst);
+                gate.waker.wake();
+            }
+        }));
+    }
+
+    poll_fn(move |cx| {
+        if gate.count.load(SeqCst) < 2 {
+            gate.waker.register_by_ref(cx.waker());
+            Poll::Pending
+        } else {
+            Poll::Ready(())
+        }
+    })
+    .await;
+}
+
 fn track<T: Future>(f: T) -> Track<T> {
     Track {
         inner: f,