Fix Context Parallel validation checks (#12446)

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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

src/diffusers/models/_modeling_parallel.py

@@ -44,11 +44,16 @@ class ContextParallelConfig:
 
     Args:
         ring_degree (`int`, *optional*, defaults to `1`):
-            Number of devices to use for ring attention within a context parallel region. Must be a divisor of the
-            total number of devices in the context parallel mesh.
+            Number of devices to use for Ring Attention. Sequence is split across devices. Each device computes
+            attention between its local Q and KV chunks passed sequentially around ring. Lower memory (only holds 1/N
+            of KV at a time), overlaps compute with communication, but requires N iterations to see all tokens. Best
+            for long sequences with limited memory/bandwidth. Number of devices to use for ring attention within a
+            context parallel region. Must be a divisor of the total number of devices in the context parallel mesh.
         ulysses_degree (`int`, *optional*, defaults to `1`):
-            Number of devices to use for ulysses attention within a context parallel region. Must be a divisor of the
-            total number of devices in the context parallel mesh.
+            Number of devices to use for Ulysses Attention. Sequence split is across devices. Each device computes
+            local QKV, then all-gathers all KV chunks to compute full attention in one pass. Higher memory (stores all
+            KV), requires high-bandwidth all-to-all communication, but lower latency. Best for moderate sequences with
+            good interconnect bandwidth.
         convert_to_fp32 (`bool`, *optional*, defaults to `True`):
             Whether to convert output and LSE to float32 for ring attention numerical stability.
         rotate_method (`str`, *optional*, defaults to `"allgather"`):
@@ -79,28 +84,45 @@ class ContextParallelConfig:
         if self.ulysses_degree is None:
             self.ulysses_degree = 1
 
+        if self.ring_degree == 1 and self.ulysses_degree == 1:
+            raise ValueError(
+                "Either ring_degree or ulysses_degree must be greater than 1 in order to use context parallel inference"
+            )
+        if self.ring_degree < 1 or self.ulysses_degree < 1:
+            raise ValueError("`ring_degree` and `ulysses_degree` must be greater than or equal to 1.")
+        if self.ring_degree > 1 and self.ulysses_degree > 1:
+            raise ValueError(
+                "Unified Ulysses-Ring attention is not yet supported. Please set either `ring_degree` or `ulysses_degree` to 1."
+            )
+        if self.rotate_method != "allgather":
+            raise NotImplementedError(
+                f"Only rotate_method='allgather' is supported for now, but got {self.rotate_method}."
+            )
+
+    @property
+    def mesh_shape(self) -> Tuple[int, int]:
+        return (self.ring_degree, self.ulysses_degree)
+
+    @property
+    def mesh_dim_names(self) -> Tuple[str, str]:
+        """Dimension names for the device mesh."""
+        return ("ring", "ulysses")
+
     def setup(self, rank: int, world_size: int, device: torch.device, mesh: torch.distributed.device_mesh.DeviceMesh):
         self._rank = rank
         self._world_size = world_size
         self._device = device
         self._mesh = mesh
-        if self.ring_degree is None:
-            self.ring_degree = 1
-        if self.ulysses_degree is None:
-            self.ulysses_degree = 1
-        if self.rotate_method != "allgather":
-            raise NotImplementedError(
-                f"Only rotate_method='allgather' is supported for now, but got {self.rotate_method}."
+
+        if self.ulysses_degree * self.ring_degree > world_size:
+            raise ValueError(
+                f"The product of `ring_degree` ({self.ring_degree}) and `ulysses_degree` ({self.ulysses_degree}) must not exceed the world size ({world_size})."
             )
-        if self._flattened_mesh is None:
+
         self._flattened_mesh = self._mesh._flatten()
-        if self._ring_mesh is None:
         self._ring_mesh = self._mesh["ring"]
-        if self._ulysses_mesh is None:
         self._ulysses_mesh = self._mesh["ulysses"]
-        if self._ring_local_rank is None:
         self._ring_local_rank = self._ring_mesh.get_local_rank()
-        if self._ulysses_local_rank is None:
         self._ulysses_local_rank = self._ulysses_mesh.get_local_rank()
 
 
@@ -119,7 +141,7 @@ class ParallelConfig:
     _rank: int = None
     _world_size: int = None
     _device: torch.device = None
-    _cp_mesh: torch.distributed.device_mesh.DeviceMesh = None
+    _mesh: torch.distributed.device_mesh.DeviceMesh = None
 
     def setup(
         self,
@@ -127,14 +149,14 @@ class ParallelConfig:
         world_size: int,
         device: torch.device,
         *,
-        cp_mesh: Optional[torch.distributed.device_mesh.DeviceMesh] = None,
+        mesh: Optional[torch.distributed.device_mesh.DeviceMesh] = None,
     ):
         self._rank = rank
         self._world_size = world_size
         self._device = device
-        self._cp_mesh = cp_mesh
+        self._mesh = mesh
         if self.context_parallel_config is not None:
-            self.context_parallel_config.setup(rank, world_size, device, cp_mesh)
+            self.context_parallel_config.setup(rank, world_size, device, mesh)
 
 
 @dataclass(frozen=True)

src/diffusers/models/attention_dispatch.py

@@ -220,7 +220,7 @@ class _AttentionBackendRegistry:
     _backends = {}
     _constraints = {}
     _supported_arg_names = {}
-    _supports_context_parallel = {}
+    _supports_context_parallel = set()
     _active_backend = AttentionBackendName(DIFFUSERS_ATTN_BACKEND)
     _checks_enabled = DIFFUSERS_ATTN_CHECKS
 
@@ -237,7 +237,9 @@ class _AttentionBackendRegistry:
             cls._backends[backend] = func
             cls._constraints[backend] = constraints or []
             cls._supported_arg_names[backend] = set(inspect.signature(func).parameters.keys())
-            cls._supports_context_parallel[backend] = supports_context_parallel
+            if supports_context_parallel:
+                cls._supports_context_parallel.add(backend.value)
+
             return func
 
         return decorator
@@ -251,15 +253,12 @@ class _AttentionBackendRegistry:
         return list(cls._backends.keys())
 
     @classmethod
-    def _is_context_parallel_enabled(
-        cls, backend: AttentionBackendName, parallel_config: Optional["ParallelConfig"]
+    def _is_context_parallel_available(
+        cls,
+        backend: AttentionBackendName,
     ) -> bool:
-        supports_context_parallel = backend in cls._supports_context_parallel
-        is_degree_greater_than_1 = parallel_config is not None and (
-            parallel_config.context_parallel_config.ring_degree > 1
-            or parallel_config.context_parallel_config.ulysses_degree > 1
-        )
-        return supports_context_parallel and is_degree_greater_than_1
+        supports_context_parallel = backend.value in cls._supports_context_parallel
+        return supports_context_parallel
 
 
 @contextlib.contextmanager
@@ -306,14 +305,6 @@ def dispatch_attention_fn(
         backend_name = AttentionBackendName(backend)
         backend_fn = _AttentionBackendRegistry._backends.get(backend_name)
 
-    if parallel_config is not None and not _AttentionBackendRegistry._is_context_parallel_enabled(
-        backend_name, parallel_config
-    ):
-        raise ValueError(
-            f"Backend {backend_name} either does not support context parallelism or context parallelism "
-            f"was enabled with a world size of 1."
-        )
-
     kwargs = {
         "query": query,
         "key": key,

src/diffusers/models/modeling_utils.py

@@ -1484,59 +1484,71 @@ class ModelMixin(torch.nn.Module, PushToHubMixin):
         config: Union[ParallelConfig, ContextParallelConfig],
         cp_plan: Optional[Dict[str, ContextParallelModelPlan]] = None,
     ):
-        from ..hooks.context_parallel import apply_context_parallel
-        from .attention import AttentionModuleMixin
-        from .attention_processor import Attention, MochiAttention
-
         logger.warning(
             "`enable_parallelism` is an experimental feature. The API may change in the future and breaking changes may be introduced at any time without warning."
         )
 
+        if not torch.distributed.is_available() and not torch.distributed.is_initialized():
+            raise RuntimeError(
+                "torch.distributed must be available and initialized before calling `enable_parallelism`."
+            )
+
+        from ..hooks.context_parallel import apply_context_parallel
+        from .attention import AttentionModuleMixin
+        from .attention_dispatch import AttentionBackendName, _AttentionBackendRegistry
+        from .attention_processor import Attention, MochiAttention
+
         if isinstance(config, ContextParallelConfig):
             config = ParallelConfig(context_parallel_config=config)
 
-        if not torch.distributed.is_initialized():
-            raise RuntimeError("torch.distributed must be initialized before calling `enable_parallelism`.")
-
         rank = torch.distributed.get_rank()
         world_size = torch.distributed.get_world_size()
         device_type = torch._C._get_accelerator().type
         device_module = torch.get_device_module(device_type)
         device = torch.device(device_type, rank % device_module.device_count())
 
-        cp_mesh = None
+        attention_classes = (Attention, MochiAttention, AttentionModuleMixin)
+
         if config.context_parallel_config is not None:
-            cp_config = config.context_parallel_config
-            if cp_config.ring_degree < 1 or cp_config.ulysses_degree < 1:
-                raise ValueError("`ring_degree` and `ulysses_degree` must be greater than or equal to 1.")
-            if cp_config.ring_degree > 1 and cp_config.ulysses_degree > 1:
-                raise ValueError(
-                    "Unified Ulysses-Ring attention is not yet supported. Please set either `ring_degree` or `ulysses_degree` to 1."
-                )
-            if cp_config.ring_degree * cp_config.ulysses_degree > world_size:
-                raise ValueError(
-                    f"The product of `ring_degree` ({cp_config.ring_degree}) and `ulysses_degree` ({cp_config.ulysses_degree}) must not exceed the world size ({world_size})."
-                )
-            cp_mesh = torch.distributed.device_mesh.init_device_mesh(
-                device_type=device_type,
-                mesh_shape=(cp_config.ring_degree, cp_config.ulysses_degree),
-                mesh_dim_names=("ring", "ulysses"),
-            )
+            for module in self.modules():
+                if not isinstance(module, attention_classes):
+                    continue
 
-        config.setup(rank, world_size, device, cp_mesh=cp_mesh)
+                processor = module.processor
+                if processor is None or not hasattr(processor, "_attention_backend"):
+                    continue
 
-        if cp_plan is None and self._cp_plan is None:
+                attention_backend = processor._attention_backend
+                if attention_backend is None:
+                    attention_backend, _ = _AttentionBackendRegistry.get_active_backend()
+                else:
+                    attention_backend = AttentionBackendName(attention_backend)
+
+                if not _AttentionBackendRegistry._is_context_parallel_available(attention_backend):
+                    compatible_backends = sorted(_AttentionBackendRegistry._supports_context_parallel)
                     raise ValueError(
-                "`cp_plan` must be provided either as an argument or set in the model's `_cp_plan` attribute."
+                        f"Context parallelism is enabled but the attention processor '{processor.__class__.__name__}' "
+                        f"is using backend '{attention_backend.value}' which does not support context parallelism. "
+                        f"Please set a compatible attention backend: {compatible_backends} using `model.set_attention_backend()` before "
+                        f"calling `enable_parallelism()`."
                     )
-        cp_plan = cp_plan if cp_plan is not None else self._cp_plan
 
+                # All modules use the same attention processor and backend. We don't need to
+                # iterate over all modules after checking the first processor
+                break
+
+        mesh = None
         if config.context_parallel_config is not None:
-            apply_context_parallel(self, config.context_parallel_config, cp_plan)
+            cp_config = config.context_parallel_config
+            mesh = torch.distributed.device_mesh.init_device_mesh(
+                device_type=device_type,
+                mesh_shape=cp_config.mesh_shape,
+                mesh_dim_names=cp_config.mesh_dim_names,
+            )
 
+        config.setup(rank, world_size, device, mesh=mesh)
         self._parallel_config = config
 
-        attention_classes = (Attention, MochiAttention, AttentionModuleMixin)
         for module in self.modules():
             if not isinstance(module, attention_classes):
                 continue
@@ -1545,6 +1557,14 @@ class ModelMixin(torch.nn.Module, PushToHubMixin):
                 continue
             processor._parallel_config = config
 
+        if config.context_parallel_config is not None:
+            if cp_plan is None and self._cp_plan is None:
+                raise ValueError(
+                    "`cp_plan` must be provided either as an argument or set in the model's `_cp_plan` attribute."
+                )
+            cp_plan = cp_plan if cp_plan is not None else self._cp_plan
+            apply_context_parallel(self, config.context_parallel_config, cp_plan)
+
     @classmethod
     def _load_pretrained_model(
         cls,