[torch.compile] Speed up MOE handling in forward_context (#33184)

Signed-off-by: Richard Zou <zou3519@gmail.com>

tests/kernels/moe/test_moe.py

@@ -715,7 +715,7 @@ def test_mixtral_moe(
 
         # need to override the forward context for unittests, otherwise it assumes
         # we're running the model forward pass (the model specified in vllm_config)
-        get_forward_context().remaining_moe_layers = None
+        get_forward_context().all_moe_layers = None
 
         # Run forward passes for both MoE blocks
         hf_states, _ = hf_moe.forward(hf_inputs)

vllm/config/compilation.py

@@ -597,6 +597,10 @@ class CompilationConfig:
     Map from layer name to layer objects that need to be accessed outside
     model code, e.g., Attention, FusedMOE when dp_size>1."""
 
+    static_all_moe_layers: list[str] = field(default_factory=list, init=False)
+    """The names of all the MOE layers in the model
+    """
+
     # Attention ops; used for piecewise cudagraphs
     # Use PyTorch operator format: "namespace::name"
     _attention_ops: ClassVar[list[str]] = [

vllm/forward_context.py

@@ -217,9 +217,11 @@ class ForwardContext:
     # the graph.
     #
     # The workaround is to store a list of the strings that each of those
-    # custom ops needs, in reverse order, in the ForwardContext.
+    # custom ops needs in the ForwardContext (all_moe_layers)
+    # as well as a counter (moe_layer_index).
     # The ForwardContext object is alive for the duration of the forward pass.
-    # When the custom op needs the string, pop the string from this list.
+    # When the custom op needs a layer string, get the next string
+    # from all_moe_layers and increment the counter.
     #
     # This assumes that the custom operators will always be executed in
     # order and that torch.compile will not try to reorder these
@@ -233,7 +235,8 @@ class ForwardContext:
     #
     # If this value is None (like in some tests), then we end up baking the string
     # into the graph. Otherwise, the moe custom ops will pop a string from this list.
-    remaining_moe_layers: list[str] | None = None
+    all_moe_layers: list[str] | None = None
+    moe_layer_index: int = 0
 
     additional_kwargs: dict[str, Any] = field(default_factory=dict)
 
@@ -271,17 +274,9 @@ def create_forward_context(
     additional_kwargs: dict[str, Any] | None = None,
     skip_compiled: bool = False,
 ):
-    no_compile_layers = vllm_config.compilation_config.static_forward_context
-    from vllm.model_executor.layers.fused_moe.layer import FusedMoE
-
-    remaining_moe_layers = [
-        name for name, layer in no_compile_layers.items() if isinstance(layer, FusedMoE)
-    ]
-    remaining_moe_layers.reverse()
-
     return ForwardContext(
-        no_compile_layers=no_compile_layers,
-        remaining_moe_layers=remaining_moe_layers,
+        no_compile_layers=vllm_config.compilation_config.static_forward_context,
+        all_moe_layers=vllm_config.compilation_config.static_all_moe_layers,
         virtual_engine=virtual_engine,
         attn_metadata=attn_metadata,
         slot_mapping=slot_mapping or {},

vllm/model_executor/layers/fused_moe/layer.py

@@ -407,6 +407,7 @@ class FusedMoE(CustomOp):
         if prefix in compilation_config.static_forward_context:
             raise ValueError("Duplicate layer name: {}".format(prefix))
         compilation_config.static_forward_context[prefix] = self
+        compilation_config.static_all_moe_layers.append(prefix)
         self.layer_name = prefix
 
         self.enable_eplb = enable_eplb
@@ -1566,7 +1567,7 @@ class FusedMoE(CustomOp):
             # Can be unavailable or None in unittests
             if (
                 is_forward_context_available()
-                and get_forward_context().remaining_moe_layers is not None
+                and get_forward_context().all_moe_layers is not None
             ):
                 return "from_forward_context"
             return self.layer_name
@@ -1987,13 +1988,17 @@ class FusedMoE(CustomOp):
 def get_layer_from_name(layer_name: str) -> FusedMoE:
     forward_context: ForwardContext = get_forward_context()
     if layer_name == "from_forward_context":
-        if not forward_context.remaining_moe_layers:
+        all_moe_layers = forward_context.all_moe_layers
+        assert all_moe_layers is not None
+        moe_layer_index = forward_context.moe_layer_index
+        if moe_layer_index >= len(all_moe_layers):
             raise AssertionError(
-                "We expected the number of MOE layers in `remaining_moe_layers` "
+                "We expected the number of MOE layers in `all_moe_layers` "
                 "to be equal to the number of "
                 "{vllm.moe_forward, vllm.moe_forward_shared} calls."
             )
-        layer_name = forward_context.remaining_moe_layers.pop()
+        layer_name = all_moe_layers[moe_layer_index]
+        forward_context.moe_layer_index += 1
     self = cast(FusedMoE, forward_context.no_compile_layers[layer_name])
     return self