[BugFix] Make sure to allocate worst case MoE workspace during profile run in the DP + EP case (#27426)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>

vllm/envs.py

@@ -55,7 +55,7 @@ if TYPE_CHECKING:
     VLLM_CPU_SGL_KERNEL: bool = False
     VLLM_XLA_CACHE_PATH: str = os.path.join(VLLM_CACHE_ROOT, "xla_cache")
     VLLM_XLA_CHECK_RECOMPILATION: bool = False
-    VLLM_FUSED_MOE_CHUNK_SIZE: int = 64 * 1024
+    VLLM_FUSED_MOE_CHUNK_SIZE: int = 16 * 1024
     VLLM_ENABLE_FUSED_MOE_ACTIVATION_CHUNKING: bool = True
     VLLM_USE_RAY_COMPILED_DAG_CHANNEL_TYPE: Literal["auto", "nccl", "shm"] = "auto"
     VLLM_USE_RAY_COMPILED_DAG_OVERLAP_COMM: bool = False
@@ -785,7 +785,7 @@ environment_variables: dict[str, Callable[[], Any]] = {
     # Enable SPMD mode for TPU backend.
     "VLLM_XLA_USE_SPMD": lambda: bool(int(os.getenv("VLLM_XLA_USE_SPMD", "0"))),
     "VLLM_FUSED_MOE_CHUNK_SIZE": lambda: int(
-        os.getenv("VLLM_FUSED_MOE_CHUNK_SIZE", "32768")
+        os.getenv("VLLM_FUSED_MOE_CHUNK_SIZE", str(16 * 1024))
     ),
     # Control whether to use fused MoE activation chunking. Current chunking
     # logic is incompatible with torch.compile and causes IMA. See issue

vllm/model_executor/layers/fused_moe/modular_kernel.py

@@ -10,6 +10,9 @@ from typing import final
 import torch
 
 import vllm.envs as envs
+from vllm.config import get_current_vllm_config
+from vllm.forward_context import get_forward_context, is_forward_context_available
+from vllm.logger import init_logger
 from vllm.model_executor.layers.fused_moe.config import FusedMoEQuantConfig
 from vllm.model_executor.layers.fused_moe.utils import (
     _resize_cache,
@@ -26,6 +29,8 @@ from vllm.v1.worker.ubatching import (
     dbo_yield,
 )
 
+logger = init_logger(__name__)
+
 #
 # This file defines a set of base classes used to make MoE kernels more modular.
 # The goal is to be able to utilize different communication mechanisms with
@@ -798,6 +803,42 @@ class FusedMoEModularKernel(torch.nn.Module):
         buffers = self.shared_buffers[ubatch_idx]
         workspace_dtype = self.fused_experts.workspace_dtype(out_dtype)
 
+        # Force worst-case allocation in profiling run for
+        # "mk.FusedMoEModularKernel.Standard" formats where this is only bounded
+        # by `VLLM_FUSED_MOE_CHUNK_SIZE` and may not be seen during profiling with
+        # DP+EP due to the random token routing.
+        is_profile_run = (
+            is_forward_context_available()
+            and get_forward_context().attn_metadata is None
+        )
+        if is_profile_run and self.fused_experts.supports_chunking():
+            parallel_config = get_current_vllm_config().parallel_config
+            is_dp_ep = (
+                parallel_config.data_parallel_size > 1
+                and parallel_config.enable_expert_parallel
+            )
+            if is_dp_ep:
+                max_workspace_13, max_workspace_2, max_fused_out_shape = (
+                    self.fused_experts.workspace_shapes(
+                        envs.VLLM_FUSED_MOE_CHUNK_SIZE,
+                        N,
+                        K,
+                        top_k,
+                        global_num_experts,
+                        local_num_experts,
+                        expert_tokens_meta,
+                    )
+                )
+                buffers.workspace13.get(
+                    max_workspace_13, device=device, dtype=workspace_dtype
+                )
+                buffers.workspace2.get(
+                    max_workspace_2, device=device, dtype=workspace_dtype
+                )
+                buffers.fused_out.get(
+                    max_fused_out_shape, device=device, dtype=workspace_dtype
+                )
+
         # Get intermediate workspace shapes based off the chunked M size.
         workspace13_shape, workspace2_shape, _ = self.fused_experts.workspace_shapes(
             M_chunk,