[WASI] improve single-threaded threadpool (dotnet#107395)

* fix dotnet#104803

* PollWasiEventLoopUntilResolvedVoid

* more

* wip

* CPU-bound work to do

* fix exit

* Update src/mono/sample/wasi/http-p2/Program.cs

Co-authored-by: Larry Ewing <lewing@microsoft.com>

* feedback

---------

Co-authored-by: Larry Ewing <lewing@microsoft.com>

src/libraries/Common/tests/WasmTestRunner/WasmTestRunner.cs

@@ -20,7 +20,7 @@ public static int Main(string[] args)
         return PollWasiEventLoopUntilResolved((Thread)null!, MainAsync(args));
 
         [UnsafeAccessor(UnsafeAccessorKind.StaticMethod, Name = "PollWasiEventLoopUntilResolved")]
-        static extern int PollWasiEventLoopUntilResolved(Thread t, Task<int> mainTask);
+        static extern T PollWasiEventLoopUntilResolved<T>(Thread t, Task<T> mainTask);
     }
 
 

src/libraries/System.Private.CoreLib/src/System/Threading/Thread.Unix.cs

@@ -18,22 +18,16 @@ internal static System.Threading.Tasks.Task RegisterWasiPollableHandle(int handl
             return WasiEventLoop.RegisterWasiPollableHandle(handle, cancellationToken);
         }
 
-        internal static int PollWasiEventLoopUntilResolved(Task<int> mainTask)
+        internal static T PollWasiEventLoopUntilResolved<T>(Task<T> mainTask)
         {
-            while (!mainTask.IsCompleted)
-            {
-                WasiEventLoop.DispatchWasiEventLoop();
-            }
-            var exception = mainTask.Exception;
-            if (exception is not null)
-            {
-                throw exception;
-            }
-
-            return mainTask.Result;
+            return WasiEventLoop.PollWasiEventLoopUntilResolved<T>(mainTask);
         }
 
-#endif
+        internal static void PollWasiEventLoopUntilResolvedVoid(Task mainTask)
+        {
+            WasiEventLoop.PollWasiEventLoopUntilResolvedVoid(mainTask);
+        }
+#endif // TARGET_WASI
 
         // the closest analog to Sleep(0) on Unix is sched_yield
         internal static void UninterruptibleSleep0() => Thread.Yield();

src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolWorkQueue.cs

@@ -906,10 +906,7 @@ internal static bool Dispatch()
             // thread because it sees a Determining or Scheduled stage, and the current thread is the last thread processing
             // work items, the current thread must either see the work item queued by the enqueuer, or it must see a stage of
             // Scheduled, and try to dequeue again or request another thread.
-#if !TARGET_WASI
-            // TODO https://github.com/dotnet/runtime/issues/104803
             Debug.Assert(workQueue._separated.queueProcessingStage == QueueProcessingStage.Scheduled);
-#endif
             workQueue._separated.queueProcessingStage = QueueProcessingStage.Determining;
             Interlocked.MemoryBarrier();
 

src/libraries/System.Private.CoreLib/src/System/Threading/Wasi/WasiEventLoop.cs

@@ -5,6 +5,7 @@
 using System.Threading.Tasks;
 using WasiPollWorld.wit.imports.wasi.io.v0_2_1;
 using Pollable = WasiPollWorld.wit.imports.wasi.io.v0_2_1.IPoll.Pollable;
+using MonotonicClockInterop = WasiPollWorld.wit.imports.wasi.clocks.v0_2_1.MonotonicClockInterop;
 
 namespace System.Threading
 {
@@ -14,7 +15,9 @@ internal static class WasiEventLoop
         // it will be leaked and stay in this list forever.
         // it will also keep the Pollable handle alive and prevent it from being disposed
         private static readonly List<PollableHolder> s_pollables = new();
-        private static bool s_tasksCanceled;
+        private static bool s_checkScheduled;
+        private static Pollable? s_resolvedPollable;
+        private static Task? s_mainTask;
 
         internal static Task RegisterWasiPollableHandle(int handle, CancellationToken cancellationToken)
         {
@@ -29,18 +32,70 @@ internal static Task RegisterWasiPollable(Pollable pollable, CancellationToken c
             // this will register the pollable holder into s_pollables
             var holder = new PollableHolder(pollable, cancellationToken);
             s_pollables.Add(holder);
+
+            ScheduleCheck();
+
             return holder.taskCompletionSource.Task;
         }
 
-        // this is not thread safe
-        internal static void DispatchWasiEventLoop()
+
+        internal static T PollWasiEventLoopUntilResolved<T>(Task<T> mainTask)
+        {
+            try
+            {
+                s_mainTask = mainTask;
+                while (!mainTask.IsCompleted)
+                {
+                    ThreadPoolWorkQueue.Dispatch();
+                }
+            }
+            finally
+            {
+                s_mainTask = null;
+            }
+            var exception = mainTask.Exception;
+            if (exception is not null)
+            {
+                throw exception;
+            }
+
+            return mainTask.Result;
+        }
+
+        internal static void PollWasiEventLoopUntilResolvedVoid(Task mainTask)
+        {
+            try
+            {
+                s_mainTask = mainTask;
+                while (!mainTask.IsCompleted)
+                {
+                    ThreadPoolWorkQueue.Dispatch();
+                }
+            }
+            finally
+            {
+                s_mainTask = null;
+            }
+
+            var exception = mainTask.Exception;
+            if (exception is not null)
+            {
+                throw exception;
+            }
+        }
+
+        internal static void ScheduleCheck()
         {
-            ThreadPoolWorkQueue.Dispatch();
-            if (s_tasksCanceled)
+            if (!s_checkScheduled && s_pollables.Count > 0)
             {
-                s_tasksCanceled = false;
-                return;
+                s_checkScheduled = true;
+                ThreadPool.UnsafeQueueUserWorkItem(CheckPollables, null);
             }
+        }
+
+        internal static void CheckPollables(object? _)
+        {
+            s_checkScheduled = false;
 
             var holders = new List<PollableHolder>(s_pollables.Count);
             var pending = new List<Pollable>(s_pollables.Count);
@@ -58,13 +113,28 @@ internal static void DispatchWasiEventLoop()
 
             if (pending.Count > 0)
             {
+                var resolvedPollableIndex = -1;
+                // if there is CPU-bound work to do, we should not block on PollInterop.Poll below
+                // so we will append pollable resolved in 0ms
+                // in effect, the PollInterop.Poll would not block us
+                if (ThreadPool.PendingWorkItemCount > 0 || (s_mainTask != null && s_mainTask.IsCompleted))
+                {
+                    s_resolvedPollable ??= MonotonicClockInterop.SubscribeDuration(0);
+                    resolvedPollableIndex = pending.Count;
+                    pending.Add(s_resolvedPollable);
+                }
+
                 var readyIndexes = PollInterop.Poll(pending);
                 for (int i = 0; i < readyIndexes.Length; i++)
                 {
                     uint readyIndex = readyIndexes[i];
-                    var holder = holders[(int)readyIndex];
-                    holder.ResolveAndDispose();
+                    if (resolvedPollableIndex != readyIndex)
+                    {
+                        var holder = holders[(int)readyIndex];
+                        holder.ResolveAndDispose();
+                    }
                 }
+
                 for (int i = 0; i < holders.Count; i++)
                 {
                     PollableHolder holder = holders[i];
@@ -73,6 +143,8 @@ internal static void DispatchWasiEventLoop()
                         s_pollables.Add(holder);
                     }
                 }
+
+                ScheduleCheck();
             }
         }
 
@@ -112,19 +184,14 @@ public void ResolveAndDispose()
             }
 
             // for GC of abandoned Tasks or for cancellation
-            private static void CancelAndDispose(object? s)
+            public static void CancelAndDispose(object? s)
             {
                 PollableHolder self = (PollableHolder)s!;
                 if (self.isDisposed)
                 {
                     return;
                 }
 
-                // Tell event loop to exit early, giving the application a
-                // chance to quit if the task(s) it is interested in have
-                // completed.
-                s_tasksCanceled = true;
-
                 // it will be removed from s_pollables on the next run
                 self.isDisposed = true;
                 self.pollable.Dispose();

src/mono/System.Private.CoreLib/src/System/Threading/ThreadPool.Wasi.Mono.cs

@@ -35,7 +35,7 @@ public static partial class ThreadPool
     {
         // Indicates whether the thread pool should yield the thread from the dispatch loop to the runtime periodically so that
         // the runtime may use the thread for processing other work
-        internal static bool YieldFromDispatchLoop => false;
+        internal static bool YieldFromDispatchLoop => true;
 
         private const bool IsWorkerTrackingEnabledInConfig = false;
 

src/mono/sample/wasi/http-p2/Program.cs

@@ -13,6 +13,7 @@ public static class WasiMainWrapper
 {
     public static async Task<int> MainAsync(string[] args)
     {
+        _ = Task.Delay(100_000_000);  // create a task that will not complete before main
         await Task.Delay(100);
         GC.Collect(); // test that Pollable->Task is not collected until resolved
 
@@ -78,7 +79,7 @@ public static int Main(string[] args)
         return PollWasiEventLoopUntilResolved((Thread)null!, MainAsync(args));
 
         [UnsafeAccessor(UnsafeAccessorKind.StaticMethod, Name = "PollWasiEventLoopUntilResolved")]
-        static extern int PollWasiEventLoopUntilResolved(Thread t, Task<int> mainTask);
+        static extern T PollWasiEventLoopUntilResolved<T>(Thread t, Task<T> mainTask);
     }
 
 }

src/mono/wasi/testassets/Http.cs

@@ -83,6 +83,6 @@ public static int Main(string[] args)
         return PollWasiEventLoopUntilResolved((Thread)null!, MainAsync(args));
 
         [UnsafeAccessor(UnsafeAccessorKind.StaticMethod, Name = "PollWasiEventLoopUntilResolved")]
-        static extern int PollWasiEventLoopUntilResolved(Thread t, Task<int> mainTask);
+        static extern T PollWasiEventLoopUntilResolved<T>(Thread t, Task<T> mainTask);
     }
 }