[perf][connector] optimize build_connector_meta when host buffer transfer is not used (#37165)

Signed-off-by: youkaichao <youkaichao@gmail.com>

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

@@ -815,20 +815,12 @@ class NixlConnectorScheduler:
             # Only trigger 1 KV transfer per request.
             params["do_remote_prefill"] = False
 
-    def build_connector_meta(
+    def _build_save_meta(
         self,
+        meta: NixlConnectorMetadata,
         scheduler_output: SchedulerOutput,
-    ) -> KVConnectorMetadata:
-        meta = NixlConnectorMetadata()
-
-        # Loop through scheduled reqs and convert to ReqMeta.
-        for req_id, (req, block_ids) in self._reqs_need_recv.items():
-            assert req.kv_transfer_params is not None
-            meta.add_new_req_to_recv(
-                request_id=req_id,
-                local_block_ids=block_ids,
-                kv_transfer_params=req.kv_transfer_params,
-            )
+    ) -> None:
+        # only called when use_host_buffer is True to build the save metadata
 
         # NOTE: For the prefill side, there might be a chance that an early added
         # request is a chunked prefill, so we need to check if new blocks are added
@@ -858,6 +850,24 @@ class NixlConnectorScheduler:
                 # Therefore, only pop if `not is_partial`.
                 self._reqs_need_save.pop(req_id)
 
+    def build_connector_meta(
+        self,
+        scheduler_output: SchedulerOutput,
+    ) -> KVConnectorMetadata:
+        meta = NixlConnectorMetadata()
+
+        # Loop through scheduled reqs and convert to ReqMeta.
+        for req_id, (req, block_ids) in self._reqs_need_recv.items():
+            assert req.kv_transfer_params is not None
+            meta.add_new_req_to_recv(
+                request_id=req_id,
+                local_block_ids=block_ids,
+                kv_transfer_params=req.kv_transfer_params,
+            )
+
+        if self.use_host_buffer:
+            self._build_save_meta(meta, scheduler_output)
+
         meta.reqs_to_send = self._reqs_need_send
         meta.reqs_in_batch = self._reqs_in_batch
         meta.reqs_not_processed = self._reqs_not_processed