[FlashInfer] Avoid FlashInfer block_size 16 + head_size 256 on blackwell (#27994)

Signed-off-by: Chen Zhang <zhangch99@outlook.com>

vllm/model_executor/models/config.py

@@ -7,6 +7,7 @@ from typing import TYPE_CHECKING
 import vllm.envs as envs
 from vllm.logger import init_logger
 from vllm.model_executor.models import ModelRegistry
+from vllm.platforms import current_platform
 from vllm.utils.math_utils import cdiv, round_up
 from vllm.utils.torch_utils import STR_DTYPE_TO_TORCH_DTYPE
 from vllm.v1.kv_cache_interface import FullAttentionSpec, MambaSpec, MLAAttentionSpec
@@ -356,6 +357,17 @@ class HybridAttentionMambaModelConfig(VerifyAndUpdateConfig):
             ).page_size_bytes
         else:
             kernel_block_alignment_size = 16
+            if (
+                current_platform.is_device_capability(100)
+                and model_config.get_head_size() == 256
+                and (
+                    envs.VLLM_ATTENTION_BACKEND is None
+                    or envs.VLLM_ATTENTION_BACKEND == "FLASHINFER"
+                )
+            ):
+                # https://github.com/flashinfer-ai/flashinfer/issues/1993 reports that`
+                # head size 256 and block size 16 is not supported on blackwell.
+                kernel_block_alignment_size = 32
             attn_page_size_1_token = FullAttentionSpec(
                 block_size=1,
                 num_kv_heads=model_config.get_num_kv_heads(parallel_config),

vllm/v1/attention/backends/flashinfer.py

@@ -402,6 +402,15 @@ class FlashInferMetadataBuilder(AttentionMetadataBuilder[FlashInferMetadata]):
         )
         self.paged_kv_last_page_len_np = self.paged_kv_last_page_len_cpu.numpy()
 
+        if self.head_dim == 256 and current_platform.is_device_capability(100):
+            # https://github.com/flashinfer-ai/flashinfer/issues/1993 reports that
+            # head size 256 and block size 16 is not supported on blackwell.
+            assert kv_cache_spec.block_size != 16, (
+                "There is a bug in FlashInfer "
+                "block_size 16 head size 256 support. Please avoid this combination by "
+                "passing --block-size 32 or --block-size 64."
+            )
+
     def _get_workspace_buffer(self):
         if self._workspace_buffer is None:
             buffer_size = FLASHINFER_WORKSPACE_BUFFER_SIZE