rt: coop should yield using waker defer strategy (#7185)

tokio/src/task/coop/mod.rs

@@ -305,7 +305,7 @@ cfg_coop! {
 
                 Poll::Ready(restore)
             } else {
-                cx.waker().wake_by_ref();
+                defer(cx);
                 Poll::Pending
             }
         }).unwrap_or(Poll::Ready(RestoreOnPending(Cell::new(Budget::unconstrained()))))
@@ -325,11 +325,19 @@ cfg_coop! {
             #[inline(always)]
             fn inc_budget_forced_yield_count() {}
         }
+
+        fn defer(cx: &mut Context<'_>) {
+            context::defer(cx.waker());
+        }
     }
 
     cfg_not_rt! {
         #[inline(always)]
         fn inc_budget_forced_yield_count() {}
+
+        fn defer(cx: &mut Context<'_>) {
+            cx.waker().wake_by_ref();
+        }
     }
 
     impl Budget {

tokio/tests/rt_common.rs

@@ -745,7 +745,25 @@ rt_test! {
     #[cfg_attr(miri, ignore)] // No `socket` in miri.
     fn yield_defers_until_park() {
         for _ in 0..10 {
-            if yield_defers_until_park_inner() {
+            if yield_defers_until_park_inner(false) {
+                // test passed
+                return;
+            }
+
+            // Wait a bit and run the test again.
+            std::thread::sleep(std::time::Duration::from_secs(2));
+        }
+
+        panic!("yield_defers_until_park is failing consistently");
+    }
+
+    /// Same as above, but with cooperative scheduling.
+    #[test]
+    #[cfg(not(target_os="wasi"))]
+    #[cfg_attr(miri, ignore)] // No `socket` in miri.
+    fn coop_yield_defers_until_park() {
+        for _ in 0..10 {
+            if yield_defers_until_park_inner(true) {
                 // test passed
                 return;
             }
@@ -760,10 +778,12 @@ rt_test! {
     /// Implementation of `yield_defers_until_park` test. Returns `true` if the
     /// test passed.
     #[cfg(not(target_os="wasi"))]
-    fn yield_defers_until_park_inner() -> bool {
+    fn yield_defers_until_park_inner(use_coop: bool) -> bool {
         use std::sync::atomic::{AtomicBool, Ordering::SeqCst};
         use std::sync::Barrier;
 
+        const BUDGET: usize = 128;
+
         let rt = rt();
 
         let flag = Arc::new(AtomicBool::new(false));
@@ -802,7 +822,15 @@ rt_test! {
                         // Yield until connected
                         let mut cnt = 0;
                         while !flag_clone.load(SeqCst){
+                            if use_coop {
+                                // Consume a good chunk of budget, which should
+                                // force at least one yield.
+                                for _ in 0..BUDGET {
+                                    tokio::task::consume_budget().await;
+                                }
+                            } else {
                                 tokio::task::yield_now().await;
+                            }
                             cnt += 1;
 
                             if cnt >= 10 {