1 files changed, 51 insertions, 0 deletions
diff --git a/testsuite/tests/lib-systhreads/testyield.ml b/testsuite/tests/lib-systhreads/testyield.ml
new file mode 100644
index 0000000000..30e70ce946
--- /dev/null
+++ b/testsuite/tests/lib-systhreads/testyield.ml
@@ -0,0 +1,51 @@
+(* TEST
+   (* Test that yielding between busy threads reliably triggers a thread
+      switch. *)
+   include systhreads
+   * not-windows
+   ** bytecode
+   ** native
+*)
+
+let threads = 4
+
+let are_ready = ref 0
+
+let yields = ref 0
+
+let iters = 50000
+
+let last = ref (-1)
+
+let report thread run_length =
+  (* The below loop tests how many times in a row a loop that calls yield runs
+     without changing threads. Ideally the answer would *always* be one, but
+     it's not clear we can reliably guarantee that unless nothing else ever
+     drops the Ocaml lock, so instead just rely on it being small. *)
+  if run_length > 3
+  then Printf.printf "Thread %d ran %d consecutive iters\n" thread run_length
+
+
+let threads =
+  List.init threads (Thread.create (fun i ->
+    incr are_ready;
+    (* Don't make any progress until all threads are spawned and properly
+       contending for the Ocaml lock. *)
+    while !are_ready < threads do
+      Thread.yield ()
+    done;
+    let consecutive = ref 0 in
+    while !yields < iters do
+      incr yields;
+      last := i;
+      Thread.yield ();
+      incr consecutive;
+      if not (!last = i)
+      then (
+        report i !consecutive;
+        consecutive := 0)
+    done;
+    if !consecutive > 0 then report i !consecutive;
+  ));;
+
+List.iter Thread.join threads