[DCP] Support Decode Context Parallel (DCP) for GQA with Flashinfer

tests/compile/test_fusions_e2e.py

@@ -74,7 +74,7 @@ class ModelBackendTestCase(NamedTuple):
         ModelBackendTestCase(
             model_name="Qwen/Qwen3-30B-A3B",
             model_kwargs=dict(max_model_len=1024),
-            backend=_Backend.TRITON_ATTN,
+            backend=AttentionBackendEnum.TRITON_ATTN,
             attention_fusions=0,
             allreduce_fusions=97,
         ),

tests/distributed/test_context_parallel.py

@@ -39,6 +39,7 @@ class ParallelSetup(NamedTuple):
 class CPTestOptions(NamedTuple):
     multi_node_only: bool
     load_format: str | None = None
+    attn_backend: str | None = None
 
 
 @dataclass
@@ -58,6 +59,7 @@ def detailed(
         multi_node_only: bool = False,
         runner: RunnerOption = "auto",
         load_format: str | None = None,
+        attn_backend: str | None = None,
     ):
         parallel_setups = []
         for eager_mode_val in [False]:
@@ -79,7 +81,9 @@ def detailed(
             distributed_backends=["mp"],
             runner=runner,
             test_options=CPTestOptions(
-                multi_node_only=multi_node_only, load_format=load_format
+                multi_node_only=multi_node_only,
+                load_format=load_format,
+                attn_backend=attn_backend,
             ),
         )
 
@@ -117,7 +121,7 @@ def _compare_cp_with_tp(
         chunked_prefill,
     ) = parallel_setup
 
-    multi_node_only, load_format = test_options
+    multi_node_only, load_format, attn_backend = test_options
 
     model_info = HF_EXAMPLE_MODELS.find_hf_info(model_id)
     model_info.check_transformers_version(on_fail="skip")
@@ -177,6 +181,13 @@ def _compare_cp_with_tp(
     if hf_overrides:
         common_args.extend(["--hf-overrides", json.dumps(hf_overrides)])
 
+    if not attn_backend:
+        cp_env = tp_env = {}
+    else:
+        cp_env = tp_env = {
+            "VLLM_ATTENTION_BACKEND": attn_backend,
+        }
+
     cp_args = [
         *common_args,
         "--tensor-parallel-size",
@@ -205,6 +216,8 @@ def _compare_cp_with_tp(
         model_id,
         cp_args,
         tp_args,
+        cp_env,
+        tp_env,
         method=method,
         max_wait_seconds=720,
     )

vllm/config/model.py

@@ -1182,6 +1182,14 @@ def verify_with_parallel_config(
                 f"but got {decode_context_parallel_size}"
             )
 
+            num_q_per_kv = total_num_attention_heads // total_num_kv_heads
+            assert num_q_per_kv % decode_context_parallel_size == 0, (
+                f"Total number of q per kv attn heads ({num_q_per_kv})"
+                " must be divisible by dcp world size when enable "
+                "decode context parallel for GQA "
+                f"({parallel_config.decode_context_parallel_size})."
+            )
+
     def get_sliding_window(self) -> int | None:
         """Get the sliding window size from the HF text config if present."""
         return getattr(self.hf_text_config, "sliding_window", None)

vllm/utils/flashinfer.py

@@ -248,6 +248,7 @@ def use_trtllm_attention(
     num_kv_heads: int,
     num_tokens: int,
     max_seq_len: int,
+    dcp_world_size: int,
     kv_cache_dtype: str,
     q_dtype: torch.dtype,
     is_prefill: bool,
@@ -261,6 +262,14 @@ def use_trtllm_attention(
     if force_use_trtllm is not None and not force_use_trtllm:
         return False
 
+    # Decode context parallel is not supported
+    if dcp_world_size > 1:
+        logger.warning_once(
+            "Trtllm does not support returning LSE and as a result"
+            "does not support DCP, reverting to FlashInfer"
+        )
+        return False
+
     # The platform is not supported
     if not supports_trtllm_attention():
         if force_use_trtllm: