[Attention] Make CutlassMLA the default backend for SM100 (blackwell) (#21626)

Signed-off-by: Alexander Matveev <amatveev@redhat.com>
Signed-off-by: mgoin <mgoin64@gmail.com>
Co-authored-by: mgoin <mgoin64@gmail.com>

vllm/platforms/cuda.py

@@ -150,11 +150,28 @@ class CudaPlatformBase(Platform):
         # TODO(lucas): handle this more gracefully
         # Note: model_config may be None during testing
         if model_config is not None and model_config.use_mla:
-            # if `VLLM_ATTENTION_BACKEND` is not set and we are using MLA, then
-            # we default to FlashMLA backend, so we need to force the blocksize
-            # here
-            use_flashmla = (envs.VLLM_ATTENTION_BACKEND is None \
-                or envs.VLLM_ATTENTION_BACKEND == "FLASHMLA")
+            # If `VLLM_ATTENTION_BACKEND` is not set and we are using MLA,
+            # then we default to FlashMLA backend for non-blackwell GPUs,
+            # else we default to CutlassMLA. For each case, we force the
+            # required block_size.
+            use_flashmla = False
+            use_cutlass_mla = False
+
+            if envs.VLLM_ATTENTION_BACKEND is None:
+                # Default case
+                if cls.is_device_capability(100):
+                    # Blackwell => Force CutlassMLA.
+                    use_cutlass_mla = True
+                    envs.VLLM_ATTENTION_BACKEND = "CUTLASS_MLA_VLLM_V1"
+                else:
+                    # Not Blackwell
+                    use_flashmla = True
+            else:
+                # Forced case
+                use_flashmla = (envs.VLLM_ATTENTION_BACKEND == "FLASHMLA")
+                use_cutlass_mla = (
+                    envs.VLLM_ATTENTION_BACKEND == "CUTLASS_MLA_VLLM_V1")
+
             from vllm.attention.ops.flashmla import is_flashmla_supported
             if use_flashmla and is_flashmla_supported()[0] \
                 and cache_config.block_size != 64:
@@ -162,8 +179,6 @@ class CudaPlatformBase(Platform):
                 logger.info(
                     "Forcing kv cache block size to 64 for FlashMLA backend.")
 
-            use_cutlass_mla = (envs.VLLM_ATTENTION_BACKEND is not None \
-                and envs.VLLM_ATTENTION_BACKEND == "CUTLASS_MLA_VLLM_V1")
             if use_cutlass_mla and cache_config.block_size != 128:
                 cache_config.block_size = 128
                 logger.info("Forcing kv cache block size to 128 for "