diff --git a/src/allmydata/test/blocking.py b/src/allmydata/test/blocking.py
new file mode 100644
index 000000000..ce7274106
--- /dev/null
+++ b/src/allmydata/test/blocking.py
@@ -0,0 +1,40 @@
+import sys
+import traceback
+import signal
+import threading
+
+from twisted.internet import reactor
+
+
+def print_stacks():
+    print("Uh oh, something is blocking the event loop!")
+    current_thread = threading.get_ident()
+    for thread_id, frame in sys._current_frames().items():
+        if thread_id == current_thread:
+            traceback.print_stack(frame)
+            break
+
+
+def catch_blocking_in_event_loop(test=None):
+    """
+    Print tracebacks if the event loop is blocked for more than a short amount
+    of time.
+    """
+    signal.signal(signal.SIGALRM, lambda *args: print_stacks())
+
+    current_scheduled = [None]
+
+    def cancel_and_rerun():
+        signal.setitimer(signal.ITIMER_REAL, 0)
+        signal.setitimer(signal.ITIMER_REAL, 0.15)
+        current_scheduled[0] = reactor.callLater(0.1, cancel_and_rerun)
+
+    cancel_and_rerun()
+
+    def cleanup():
+        signal.signal(signal.SIGALRM, signal.SIG_DFL)
+        signal.setitimer(signal.ITIMER_REAL, 0)
+        current_scheduled[0].cancel()
+
+    if test is not None:
+        test.addCleanup(cleanup)