diff --git a/lisa/energy_model.py b/lisa/energy_model.py
index 870b3515ce4095f31fe21c1fe78b2a9f31a13cec..2b44258e0a7e394062fccd646103c1959a27e46b 100644
--- a/lisa/energy_model.py
+++ b/lisa/energy_model.py
@@ -78,9 +78,6 @@ class ActiveState(namedtuple('ActiveState', ['capacity', 'power'])):
     def __new__(cls, capacity=None, power=None):
         return super(ActiveState, cls).__new__(cls, capacity, power)
 
-    # helpers for yaml serialization
-    yaml_tag = 'em_active_state:capacity,power'
-
 class _CpuTree(Loggable):
     """Internal class. Abstract representation of a CPU topology.
 
diff --git a/lisa/utils.py b/lisa/utils.py
index 041fa4557e3dde814d97451a41c8e67e5b3a3c9b..1f989bbb54eb836923847d45395181483975d5d2 100644
--- a/lisa/utils.py
+++ b/lisa/utils.py
@@ -108,13 +108,13 @@ def import_all_submodules(pkg):
     ]
 
 class UnknownTagPlaceholder:
-    def __init__(self, yaml_tag, data, location=None):
-        self.yaml_tag = yaml_tag
+    def __init__(self, tag, data, location=None):
+        self.tag = tag
         self.data = data
         self.location = location
 
     def __str__(self):
-        return '<UnknownTagPlaceholder of {}>'.format(self.yaml_tag)
+        return '<UnknownTagPlaceholder of {}>'.format(self.tag)
 
 class Serializable(Loggable):
     """
@@ -245,13 +245,6 @@ class Serializable(Loggable):
 
     @classmethod
     def _to_path(cls, instance, filepath, fmt):
-        # On Python >= 3.6, __init_subclass__ will take care of that
-        if sys.version_info < (3, 6):
-            # Better late than never. Doing it here avoids using a metaclass
-            # just to register the class. If we don't do that, yaml_tag class
-            # attribute will be ignored.
-            cls._yaml.register_class(type(instance))
-
         if fmt is None:
             fmt = cls.DEFAULT_SERIALIZATION_FMT
 
@@ -353,12 +346,6 @@ class Serializable(Loggable):
     def __deepcopy__(self):
        return super().__deepcopy__()
 
-   # Will only be called with Python >= 3.6
-    def __init_subclass__(cls, **kwargs):
-        # Register the class to ensure yaml_tag will be used
-        cls._yaml.register_class(cls)
-        super().__init_subclass__(**kwargs)
-
 Serializable._init_yaml()
 
 class SerializableConfABC(Serializable, abc.ABC):