Looks like a memory-fence type issue on PyPy on Win.

Try killing the pool to see if that buys us what we need.

src/gevent/tests/test__threadpool.py

@@ -786,9 +786,11 @@ class TestWorkerProfileAndTrace(TestCase):
 
         self.ClassUnderTest = Pool
 
-        pool = self._makeOne(1)
+        pool = self._makeOne(1, create_all_worker_threads=True)
         assert isinstance(pool, Pool)
 
+        # Do this after creating the pool and its thread to verify we don't
+        # capture the function at thread creation time.
         setter(callback)
 
 
@@ -796,6 +798,16 @@ class TestWorkerProfileAndTrace(TestCase):
         self.assertEqual(res, 1701)
         self.assertGreaterEqual(called[0], 1)
 
+        # Shutdown the pool. PyPy2.7-7.3.1 on Windows/Appveyor was
+        # properly seeing the before_task value, but after_task was empty.
+        # That suggested a memory consistency type issue, where the updates
+        # written by the other thread weren't fully visible to this thread
+        # yet. Try to kill it to see if that helps. (Couldn't reproduce
+        # on macOS).
+        #
+        # https://ci.appveyor.com/project/jamadden/gevent/build/job/wo9likk85cduui7n#L867
+        pool.kill()
+
         # The function is active only for the scope of the function
         self.assertEqual(before_task, [task, None, callback])
         self.assertEqual(after_task, [task, callback, None])

src/gevent/threadpool.py

@@ -460,7 +460,7 @@ class ThreadPool(GroupMappingMixin):
             self.fork_watcher.stop()
 
     def _adjust_wait(self):
-        delay = 0.0001
+        delay = self.hub.loop.approx_timer_resolution
         while True:
             self._adjust_step()
             if len(self._worker_greenlets) <= self._maxsize: