[KVConnector] Auto-downgrade to PIECEWISE cudagraph mode for layerwise async ops (#31057)

Signed-off-by: Yashwant Bezawada <yashwant_b@me.com>

vllm/config/vllm.py

@@ -925,6 +925,33 @@ class VllmConfig:
                         CUDAGraphMode.FULL_DECODE_ONLY
                     )
 
+            # Check if KV connector requires PIECEWISE mode for CUDA graphs
+            if (
+                self.kv_transfer_config is not None
+                and self.kv_transfer_config.is_kv_transfer_instance
+                and self.compilation_config.cudagraph_mode.has_full_cudagraphs()
+            ):
+                # Lazy import to avoid circular dependencies
+                from vllm.distributed.kv_transfer.kv_connector.factory import (
+                    KVConnectorFactory,
+                )
+
+                connector_cls = KVConnectorFactory.get_connector_class(
+                    self.kv_transfer_config
+                )
+                if connector_cls.requires_piecewise_for_cudagraph(
+                    self.kv_transfer_config.kv_connector_extra_config
+                ):
+                    logger.warning_once(
+                        "KV connector %s requires PIECEWISE CUDA graph mode "
+                        "due to layerwise async operations that cannot be "
+                        "captured in CUDA graphs. "
+                        "Overriding cudagraph_mode from %s to PIECEWISE.",
+                        connector_cls.__name__,
+                        self.compilation_config.cudagraph_mode.name,
+                    )
+                    self.compilation_config.cudagraph_mode = CUDAGraphMode.PIECEWISE
+
             # disable cudagraph when enforce eager execution
             if self.model_config is not None and self.model_config.enforce_eager:
                 logger.info("Cudagraph is disabled under eager mode")

vllm/distributed/kv_transfer/kv_connector/v1/base.py

@@ -543,6 +543,28 @@ class KVConnectorBase_V1(ABC):
             )
         return None
 
+    @classmethod
+    def requires_piecewise_for_cudagraph(cls, extra_config: dict[str, Any]) -> bool:
+        """
+        Check if this connector requires PIECEWISE CUDA graph mode.
+
+        Connectors that use asynchronous layer-by-layer operations
+        (wait_for_layer_load/save_kv_layer) should override this method
+        to return True when those operations are enabled. These operations
+        cannot be captured in CUDA graphs and will be skipped during replay,
+        causing data races. PIECEWISE mode allows Python code to execute
+        between graph pieces, ensuring proper synchronization.
+
+        Args:
+            extra_config: The kv_connector_extra_config dict from
+                KVTransferConfig.
+
+        Returns:
+            True if this connector requires PIECEWISE CUDA graph mode,
+            False otherwise.
+        """
+        return False
+
     def get_finished_count(self) -> int | None:
         """
         Get the count of requests expected to complete send/receive operations

vllm/distributed/kv_transfer/kv_connector/v1/lmcache_connector.py

@@ -70,6 +70,16 @@ class LMCacheKVEvents(KVConnectorKVEvents):
 
 
 class LMCacheConnectorV1(KVConnectorBase_V1):
+    @classmethod
+    def requires_piecewise_for_cudagraph(cls, extra_config: dict[str, Any]) -> bool:
+        """
+        LMCache requires PIECEWISE CUDA graph mode when layerwise
+        operations are enabled. The wait_for_layer_load and save_kv_layer
+        methods perform actual async synchronization that cannot be
+        captured in CUDA graphs.
+        """
+        return extra_config.get("use_layerwise", False)
+
     def __init__(
         self,
         vllm_config: "VllmConfig",

vllm/distributed/kv_transfer/kv_connector/v1/multi_connector.py

@@ -112,6 +112,21 @@ class MultiConnector(KVConnectorBase_V1):
     - Save to all connectors.
     """
 
+    @classmethod
+    def requires_piecewise_for_cudagraph(cls, extra_config: dict[str, Any]) -> bool:
+        """
+        MultiConnector requires PIECEWISE CUDA graph mode if any of its
+        child connectors require it.
+        """
+        connectors_config = extra_config.get("connectors", [])
+        for conn_config in connectors_config:
+            temp_ktc = KVTransferConfig(**conn_config)
+            connector_cls = KVConnectorFactory.get_connector_class(temp_ktc)
+            child_extra_config = conn_config.get("kv_connector_extra_config", {})
+            if connector_cls.requires_piecewise_for_cudagraph(child_extra_config):
+                return True
+        return False
+
     def __init__(
         self,
         vllm_config: "VllmConfig",