From 61485844fc5190721b1edf6bed9aa4d5567b70e0 Mon Sep 17 00:00:00 2001
From: Bangsheng Tang <5318912+bangshengtang@users.noreply.github.com>
Date: Mon, 17 Nov 2025 15:22:11 -0800
Subject: [PATCH] [BugFix] Corner case that could cause out-of-sync with
 external launcher mode and dp >1 (#28774)

---
 vllm/v1/worker/gpu_model_runner.py | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/vllm/v1/worker/gpu_model_runner.py b/vllm/v1/worker/gpu_model_runner.py
index 2a8ff746f1125..0102ca4739ad5 100644
--- a/vllm/v1/worker/gpu_model_runner.py
+++ b/vllm/v1/worker/gpu_model_runner.py
@@ -2663,6 +2663,18 @@ class GPUModelRunner(
                         return make_empty_encoder_model_runner_output(scheduler_output)
 
                 if not num_scheduled_tokens:
+                    if (
+                        self.parallel_config.distributed_executor_backend
+                        == "external_launcher"
+                        and self.parallel_config.data_parallel_size > 1
+                    ):
+                        # this is a corner case when both external launcher
+                        # and DP are enabled, num_scheduled_tokens could be
+                        # 0, and has_unfinished_requests in the outer loop
+                        # returns True. before returning early here we call
+                        # dummy run to ensure coordinate_batch_across_dp
+                        # is called into to avoid out of sync issues.
+                        self._dummy_run(1)
                     if not has_kv_transfer_group():
                         # Return empty ModelRunnerOutput if no work to do.
                         return EMPTY_MODEL_RUNNER_OUTPUT