diff --git a/doc/source/dev/architecture.rst b/doc/source/dev/architecture.rst
index 8a6d6f985e..46aebfa81d 100644
--- a/doc/source/dev/architecture.rst
+++ b/doc/source/dev/architecture.rst
@@ -93,20 +93,18 @@ driver itself or on any interface with driver_periodic_task_ decorator, e.g.
         def task2(self, manager, context):
             pass  # do something
 
-        @base.driver_periodic_task(parallel=False)
-        def blocking_task(self, manager, context):
-            pass  # do something fast, this blocks other tasks from starting!
-
 
 Here the ``spacing`` argument is a period in seconds for a given periodic task.
 For example 'spacing=5' means every 5 seconds.
 
-The ``parallel`` argument may be passed to driver_periodic_task_ and defaults
-to True. If False, this task will be run in the periodic task loop, rather
-than a separate greenthread. This should be used with caution, as it will
-cause all other periodic tasks to be blocked from starting while the
-non-parallel task is running. Long running tasks, especially any tasks that
-make a remote call (to a BMC, HTTP, etc.) **must** be parallelized.
+.. note::
+    The ``parallel`` argument may be passed to driver_periodic_task_.
+    If it's set to False, this task will be run in the periodic task loop,
+    rather than a separate greenthread.
+
+    This is deprecated as of Liberty release, and the parallel argument will be
+    ignored starting in the Mitaka cycle, as such task would prevent all other
+    periodic tasks from starting while it is is running.
 
 .. note::
     By default periodic task names are derived from method names,
diff --git a/ironic/drivers/base.py b/ironic/drivers/base.py
index 9b7f56b618..e767d3789b 100644
--- a/ironic/drivers/base.py
+++ b/ironic/drivers/base.py
@@ -32,7 +32,7 @@ from oslo_utils import excutils
 import six
 
 from ironic.common import exception
-from ironic.common.i18n import _LE
+from ironic.common.i18n import _LE, _LW
 from ironic.common import raid
 
 LOG = logging.getLogger(__name__)
@@ -1034,11 +1034,8 @@ def driver_periodic_task(parallel=True, **other):
 
     :param parallel: If True (default), this task is run in a separate thread.
             If False, this task will be run in the conductor's periodic task
-            loop, rather than a separate greenthread. False should be used with
-            caution, as it will cause all other periodic tasks to be blocked
-            from starting while the non-parallel task is running. Long running
-            tasks, especially any tasks that make a remote call (to a BMC,
-            HTTP, etc.) must be parallelized.
+            loop, rather than a separate greenthread. This parameter is
+            deprecated and will be ignored starting with Mitaka cycle.
     :param other: arguments to pass to @periodic_task.periodic_task
     """
     # TODO(dtantsur): drop all this magic once
@@ -1055,6 +1052,10 @@ def driver_periodic_task(parallel=True, **other):
 
                 eventlet.greenthread.spawn_n(_internal)
             else:
+                LOG.warn(_LW(
+                    'Using periodic tasks with parallel=False is deprecated, '
+                    '"parallel" argument will be ignored starting with '
+                    'the Mitaka release'))
                 func(*args, **kwargs)
 
         # NOTE(dtantsur): name should be unique