fix[DP][v1]: Prevent hangs from mismatched worker configurations (#26218)

Signed-off-by: Ayush Satyam <ayushsatyam146@gmail.com>

vllm/config/parallel.py

@@ -336,6 +336,9 @@ class ParallelConfig:
         graph from input ids/embeddings to the final hidden states,
         excluding anything before input ids/embeddings and after
         the final hidden states.
+
+        This hash is also used for DP worker configuration validation
+        to prevent hangs from mismatched collective communication patterns.
         """
         factors: list[Any] = []
         factors.append(self.pipeline_parallel_size)
@@ -343,6 +346,12 @@ class ParallelConfig:
         factors.append(self.enable_expert_parallel)
         factors.append(self.data_parallel_size)
         factors.append(envs.VLLM_ALL2ALL_BACKEND)
+        factors.append(self.enable_eplb)
+        if self.enable_eplb:
+            factors.append(self.eplb_config.log_balancedness)
+            factors.append(self.eplb_config.window_size)
+            factors.append(self.eplb_config.step_interval)
+            factors.append(self.eplb_config.num_redundant_experts)
         return hashlib.sha256(str(factors).encode()).hexdigest()
 
     def __post_init__(self) -> None:

vllm/v1/engine/core.py

@@ -681,17 +681,21 @@ class EngineCoreProc(EngineCore):
             # external LB case for our colocated front-end to use (coordinator
             # only runs with rank 0).
             dp_stats_address = self.frontend_stats_publish_address
-            handshake_socket.send(
-                msgspec.msgpack.encode(
-                    {
+
+            # Include config hash for DP configuration validation
+            ready_msg = {
                 "status": "READY",
                 "local": local_client,
                 "headless": headless,
                 "num_gpu_blocks": num_gpu_blocks,
                 "dp_stats_address": dp_stats_address,
             }
+            if vllm_config.parallel_config.data_parallel_size > 1:
+                ready_msg["parallel_config_hash"] = (
+                    vllm_config.parallel_config.compute_hash()
                 )
-            )
+
+            handshake_socket.send(msgspec.msgpack.encode(ready_msg))
 
     @staticmethod
     def startup_handshake(

vllm/v1/engine/utils.py

@@ -73,6 +73,7 @@ class EngineHandshakeMetadata:
 
     addresses: EngineZmqAddresses
     parallel_config: dict[str, Union[int, str, list[int]]]
+    parallel_config_hash: Optional[str] = None
 
 
 class CoreEngineProcManager:
@@ -867,7 +868,8 @@ def wait_for_engine_startup(
                 )
 
         if status == "HELLO" and engine.state == CoreEngineState.NEW:
-            # Send init message with DP config info.
+            # Send init message with DP config info and config hash.
+            # The config hash ensures all DP workers have compatible configs.
             init_message = msgspec.msgpack.encode(
                 EngineHandshakeMetadata(
                     addresses=addresses,
@@ -880,6 +882,9 @@ def wait_for_engine_startup(
                             "data_parallel_size",
                         )
                     },
+                    parallel_config_hash=parallel_config.compute_hash()
+                    if parallel_config.data_parallel_size > 1
+                    else None,
                 )
             )
             handshake_socket.send_multipart((eng_identity, init_message), copy=False)
@@ -900,6 +905,23 @@ def wait_for_engine_startup(
             if addresses.frontend_stats_publish_address is None:
                 addresses.frontend_stats_publish_address = msg.get("dp_stats_address")
 
+            # Validate config hash consistency across DP workers
+            if parallel_config.data_parallel_size > 1:
+                worker_config_hash = msg.get("parallel_config_hash")
+                expected_hash = parallel_config.compute_hash()
+                if worker_config_hash != expected_hash:
+                    raise RuntimeError(
+                        f"Configuration mismatch detected for engine "
+                        f"{eng_index}. All DP workers must have identical "
+                        f"configurations for parameters that affect collective "
+                        f"communication (e.g., enable_eplb, "
+                        f"eplb_config.log_balancedness). "
+                        f"Worker hash: {worker_config_hash}, "
+                        f"Expected hash: {expected_hash}. "
+                        f"Please ensure all workers are started with the same "
+                        f"command-line arguments."
+                    )
+
             start_pending[0 if local else 1] -= 1
             engine.state = CoreEngineState.READY
         else: