runtime: improvements for global_queue_interval (#6445)

tokio/src/runtime/builder.rs

@@ -758,6 +758,10 @@ impl Builder {
     ///
     /// [the module documentation]: crate::runtime#multi-threaded-runtime-behavior-at-the-time-of-writing
     ///
+    /// # Panics
+    ///
+    /// This function will panic if 0 is passed as an argument.
+    ///
     /// # Examples
     ///
     /// ```
@@ -768,7 +772,9 @@ impl Builder {
     ///     .build();
     /// # }
     /// ```
+    #[track_caller]
     pub fn global_queue_interval(&mut self, val: u32) -> &mut Self {
+        assert!(val > 0, "global_queue_interval must be greater than 0");
         self.global_queue_interval = Some(val);
         self
     }

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

@@ -63,7 +63,7 @@ impl Stats {
         let tasks_per_interval = (TARGET_GLOBAL_QUEUE_INTERVAL / self.task_poll_time_ewma) as u32;
 
         cmp::max(
-            // We don't want to return less than 2 as that would result in the
+            // If we are using self-tuning, we don't want to return less than 2 as that would result in the
             // global queue always getting checked first.
             2,
             cmp::min(

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

@@ -985,8 +985,6 @@ impl Core {
             .stats
             .tuned_global_queue_interval(&worker.handle.shared.config);
 
-        debug_assert!(next > 1);
-
         // Smooth out jitter
         if abs_diff(self.global_queue_interval, next) > 2 {
             self.global_queue_interval = next;

tokio/src/runtime/scheduler/multi_thread_alt/stats.rs

@@ -82,7 +82,7 @@ impl Stats {
         let tasks_per_interval = (TARGET_GLOBAL_QUEUE_INTERVAL / self.task_poll_time_ewma) as u32;
 
         cmp::max(
-            // We don't want to return less than 2 as that would result in the
+            // If we are using self-tuning, we don't want to return less than 2 as that would result in the
             // global queue always getting checked first.
             2,
             cmp::min(

tokio/src/runtime/scheduler/multi_thread_alt/worker.rs

@@ -664,7 +664,6 @@ impl Worker {
     /// Ensure core's state is set correctly for the worker to start using.
     fn reset_acquired_core(&mut self, cx: &Context, synced: &mut Synced, core: &mut Core) {
         self.global_queue_interval = core.stats.tuned_global_queue_interval(&cx.shared().config);
-        debug_assert!(self.global_queue_interval > 1);
 
         // Reset `lifo_enabled` here in case the core was previously stolen from
         // a task that had the LIFO slot disabled.
@@ -1288,8 +1287,6 @@ impl Worker {
     fn tune_global_queue_interval(&mut self, cx: &Context, core: &mut Core) {
         let next = core.stats.tuned_global_queue_interval(&cx.shared().config);
 
-        debug_assert!(next > 1);
-
         // Smooth out jitter
         if abs_diff(self.global_queue_interval, next) > 2 {
             self.global_queue_interval = next;

tokio/tests/rt_panic.rs

@@ -70,6 +70,18 @@ fn builder_max_blocking_threads_panic_caller() -> Result<(), Box<dyn Error>> {
     Ok(())
 }
 
+#[test]
+fn builder_global_queue_interval_panic_caller() -> Result<(), Box<dyn Error>> {
+    let panic_location_file = test_panic(|| {
+        let _ = Builder::new_multi_thread().global_queue_interval(0).build();
+    });
+
+    // The panic location should be in this file
+    assert_eq!(&panic_location_file.unwrap(), file!());
+
+    Ok(())
+}
+
 fn current_thread() -> Runtime {
     tokio::runtime::Builder::new_current_thread()
         .enable_all()

tokio/tests/rt_threaded.rs

@@ -10,8 +10,8 @@ use tokio_test::{assert_err, assert_ok};
 use futures::future::poll_fn;
 use std::future::Future;
 use std::pin::Pin;
-use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering::Relaxed;
+use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{mpsc, Arc, Mutex};
 use std::task::{Context, Poll, Waker};
 
@@ -486,6 +486,34 @@ fn max_blocking_threads_set_to_zero() {
         .unwrap();
 }
 
+/// Regression test for #6445.
+///
+/// After #6445, setting `global_queue_interval` to 1 is now technically valid.
+/// This test confirms that there is no regression in `multi_thread_runtime`
+/// when global_queue_interval is set to 1.
+#[test]
+fn global_queue_interval_set_to_one() {
+    let rt = tokio::runtime::Builder::new_multi_thread()
+        .global_queue_interval(1)
+        .build()
+        .unwrap();
+
+    // Perform a simple work.
+    let cnt = Arc::new(AtomicUsize::new(0));
+    rt.block_on(async {
+        let mut set = tokio::task::JoinSet::new();
+        for _ in 0..10 {
+            let cnt = cnt.clone();
+            set.spawn(async move { cnt.fetch_add(1, Ordering::Relaxed) });
+        }
+
+        while let Some(res) = set.join_next().await {
+            res.unwrap();
+        }
+    });
+    assert_eq!(cnt.load(Relaxed), 10);
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn hang_on_shutdown() {
     let (sync_tx, sync_rx) = std::sync::mpsc::channel::<()>();

tokio/tests/rt_threaded_alt.rs

@@ -487,6 +487,33 @@ fn max_blocking_threads_set_to_zero() {
         .unwrap();
 }
 
+/// Regression test for #6445.
+///
+/// After #6445, setting `global_queue_interval` to 1 is now technically valid.
+/// This test confirms that there is no regression in `multi_thread_runtime`
+/// when global_queue_interval is set to 1.
+#[test]
+fn global_queue_interval_set_to_one() {
+    let rt = tokio::runtime::Builder::new_multi_thread_alt()
+        .global_queue_interval(1)
+        .build()
+        .unwrap();
+
+    // Perform a simple work.
+    let cnt = Arc::new(AtomicUsize::new(0));
+    rt.block_on(async {
+        let mut set = tokio::task::JoinSet::new();
+        for _ in 0..10 {
+            let cnt = cnt.clone();
+            set.spawn(async move { cnt.fetch_add(1, Relaxed) });
+        }
+        while let Some(res) = set.join_next().await {
+            res.unwrap();
+        }
+    });
+    assert_eq!(cnt.load(Relaxed), 10);
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn hang_on_shutdown() {
     let (sync_tx, sync_rx) = std::sync::mpsc::channel::<()>();