[Bugfix][Core] Fix stuck chunked pipeline parallelism with async scheduling (#38726)

Signed-off-by: Jing Wang <jingwang96@qq.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

vllm/v1/worker/gpu_model_runner.py

@@ -3758,6 +3758,15 @@ class GPUModelRunner(
 
         return slot_mappings_by_gid, slot_mappings_by_layer
 
+    def _is_all_reqs_chunked_prefill(self) -> bool:
+        """Check if all scheduled requests are marked to discard sampled tokens.
+
+        This is true when `discard_request_mask` is set for every scheduled
+        request (e.g., for chunked prefill requests that are not the last
+        prefill chunk)."""
+        num_reqs = self.input_batch.num_reqs
+        return bool(self.discard_request_mask.np[:num_reqs].all())
+
     @torch.inference_mode()
     def execute_model(
         self,
@@ -4361,6 +4370,9 @@ class GPUModelRunner(
         assert sampled_token_ids.dim() == 2 and sampled_token_ids.shape[-1] == 1, (
             "PP+async expects sampled_token_ids to have shape [num_reqs, 1]"
         )
+        # Skip for chunked prefill: sampled tokens are dummy
+        # and will be discarded, no need to broadcast.
+        if not self._is_all_reqs_chunked_prefill():
             torch.distributed.broadcast(
                 sampled_token_ids, src=pp.rank, group=pp.device_group
             )
@@ -4372,6 +4384,8 @@ class GPUModelRunner(
         num_reqs = self.input_batch.num_reqs
         # `prev_sampled_token_ids` is expected to have shape [num_reqs, 1].
         recv = torch.empty((num_reqs, 1), dtype=torch.int32, device=self.device)
+        # skip for chunked prefill.
+        if not self._is_all_reqs_chunked_prefill():
             torch.distributed.broadcast(recv, src=pp.last_rank, group=pp.device_group)
         self.input_batch.prev_sampled_token_ids = recv