[BugFix] Fix DCP Assert (AssertionError: DCP not support reorder_batch_threshold > 1 now.) (#28100)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>

vllm/v1/attention/backends/mla/common.py

@@ -545,6 +545,7 @@ class MLACommonMetadataBuilder(AttentionMetadataBuilder[M]):
         vllm_config: VllmConfig,
         device: torch.device,
         metadata_cls: type[M] | None = None,
+        supports_dcp_with_varlen: bool = False,
     ):
         self.metadata_cls = (
             metadata_cls if metadata_cls is not None else MLACommonMetadata
@@ -638,7 +639,7 @@ class MLACommonMetadataBuilder(AttentionMetadataBuilder[M]):
 
         supports_spec_decode = self.query_len_support != QueryLenSupport.SINGLE_ONLY
         self._init_reorder_batch_threshold(
-            self.reorder_batch_threshold, supports_spec_decode
+            self.reorder_batch_threshold, supports_spec_decode, supports_dcp_with_varlen
         )
 
         # Validate consistency between query_len_support and reorder_batch_threshold

vllm/v1/attention/backends/mla/flashattn_mla.py

@@ -81,7 +81,12 @@ class FlashAttnMLAMetadataBuilder(MLACommonMetadataBuilder[FlashAttnMLAMetadata]
         device: torch.device,
     ):
         super().__init__(
-            kv_cache_spec, layer_names, vllm_config, device, FlashAttnMLAMetadata
+            kv_cache_spec,
+            layer_names,
+            vllm_config,
+            device,
+            FlashAttnMLAMetadata,
+            supports_dcp_with_varlen=True,
         )
         self.max_num_splits = 0  # No upper bound on the number of splits.
         self.fa_aot_schedule = get_flash_attn_version() == 3

vllm/v1/attention/backends/utils.py

@@ -264,7 +264,10 @@ class AttentionMetadataBuilder(abc.ABC, Generic[M]):
         self.device = device
 
     def _init_reorder_batch_threshold(
-        self, reorder_batch_threshold: int = 1, supports_spec_as_decode: bool = False
+        self,
+        reorder_batch_threshold: int = 1,
+        supports_spec_as_decode: bool = False,
+        supports_dcp_with_varlen: bool = False,
     ) -> None:
         self.reorder_batch_threshold = reorder_batch_threshold
         if self.reorder_batch_threshold is not None and supports_spec_as_decode:
@@ -281,6 +284,12 @@ class AttentionMetadataBuilder(abc.ABC, Generic[M]):
                     1 + speculative_config.num_speculative_tokens,
                 )
 
+        if (
+            self.vllm_config.parallel_config.decode_context_parallel_size > 1
+            and not supports_dcp_with_varlen
+        ):
+            self.reorder_batch_threshold = 1
+
     @abstractmethod
     def build(
         self,