[bugfix] fix early import of flash attention (#12959)

Signed-off-by: youkaichao <youkaichao@gmail.com>

vllm/attention/backends/flash_attn.py

@@ -14,8 +14,8 @@ from vllm.attention.backends.abstract import (AttentionBackend, AttentionImpl,
                                               AttentionMetadataBuilder,
                                               AttentionType)
 from vllm.attention.backends.utils import (
-    PAD_SLOT_ID, VLLM_FLASH_ATTN_VERSION, CommonAttentionState,
-    compute_slot_mapping, compute_slot_mapping_start_idx,
+    PAD_SLOT_ID, CommonAttentionState, compute_slot_mapping,
+    compute_slot_mapping_start_idx, get_flash_attn_version,
     get_num_prefill_decode_query_kv_tokens, get_seq_len_block_table_args,
     is_all_cross_attn_metadata_set, is_all_encoder_attn_metadata_set,
     is_block_tables_empty)
@@ -640,6 +640,7 @@ class FlashAttentionImpl(AttentionImpl):
                 f"Head size {head_size} is not supported by FlashAttention. "
                 f"Supported head sizes are: {support_head_sizes}.")
         self.attn_type = attn_type
+        self.vllm_flash_attn_version = get_flash_attn_version()
 
     def forward(
         self,
@@ -759,7 +760,7 @@ class FlashAttentionImpl(AttentionImpl):
                     alibi_slopes=alibi_slopes,
                     softcap=logits_soft_cap,
                     out=prefill_output,
-                    fa_version=VLLM_FLASH_ATTN_VERSION,
+                    fa_version=self.vllm_flash_attn_version,
                 )
             else:
                 # prefix-enabled attention
@@ -782,7 +783,7 @@ class FlashAttentionImpl(AttentionImpl):
                     block_table=prefill_meta.block_tables,
                     softcap=logits_soft_cap,
                     out=prefill_output,
-                    fa_version=VLLM_FLASH_ATTN_VERSION,
+                    fa_version=self.vllm_flash_attn_version,
                 )
 
         if decode_meta := attn_metadata.decode_metadata:
@@ -811,7 +812,7 @@ class FlashAttentionImpl(AttentionImpl):
                     softcap=logits_soft_cap,
                     block_table=decode_meta.block_tables,
                     out=decode_output,
-                    fa_version=VLLM_FLASH_ATTN_VERSION,
+                    fa_version=self.vllm_flash_attn_version,
                 )
             else:
                 # Use flash_attn_with_kvcache for normal decoding.
@@ -832,7 +833,7 @@ class FlashAttentionImpl(AttentionImpl):
                     alibi_slopes=alibi_slopes,
                     softcap=logits_soft_cap,
                     out=decode_output.unsqueeze(1),
-                    fa_version=VLLM_FLASH_ATTN_VERSION,
+                    fa_version=self.vllm_flash_attn_version,
                 )
         return output
 

vllm/attention/backends/mla/utils.py

@@ -12,7 +12,7 @@ from vllm import envs
 from vllm.attention.backends.abstract import (AttentionLayer,
                                               AttentionMetadata,
                                               MLAAttentionImpl, T)
-from vllm.attention.backends.utils import VLLM_FLASH_ATTN_VERSION
+from vllm.attention.backends.utils import get_flash_attn_version
 from vllm.distributed import (get_tensor_model_parallel_world_size,
                               tensor_model_parallel_all_reduce)
 from vllm.model_executor.layers.linear import (ColumnParallelLinear,
@@ -181,6 +181,7 @@ class MLACommonImpl(MLAAttentionImpl[T], Generic[T]):
         self.q_proj = q_proj
         self.kv_b_proj = kv_b_proj
         self.o_proj = o_proj
+        self.vllm_flash_attn_version = get_flash_attn_version()
 
     def _v_up_proj_and_o_proj(self, x):
         if envs.VLLM_MLA_PERFORM_MATRIX_ABSORPTION:
@@ -515,7 +516,7 @@ class MLACommonImpl(MLAAttentionImpl[T], Generic[T]):
             max_seqlen_k=max_prefill_seq_len,
             softmax_scale=self.scale,
             causal=True,
-            fa_version=VLLM_FLASH_ATTN_VERSION,
+            fa_version=self.vllm_flash_attn_version,
         )
         attn_output = attn_output\
             .view(-1, self.num_heads, q.shape[-1])[..., :v.shape[-1]]\

vllm/attention/backends/utils.py

@@ -587,11 +587,11 @@ def get_num_prefill_decode_query_kv_tokens(
             num_decode_query_tokens)
 
 
-try:
+def get_flash_attn_version():
+    try:
         from vllm.vllm_flash_attn.flash_attn_interface import (
             fa_version_unsupported_reason, is_fa_version_supported)
 
-    def flash_attn_version():
         # if hopper default to FA3, otherwise stick to FA2 for now
         # TODO(lucas): profile FA3 on ampere to see if it makes sense to
         #  use FA3 as default for both
@@ -610,7 +610,5 @@ try:
 
         assert is_fa_version_supported(fa_version)
         return fa_version
-
-    VLLM_FLASH_ATTN_VERSION = flash_attn_version()
-except (ImportError, AssertionError):
-    VLLM_FLASH_ATTN_VERSION = None
+    except (ImportError, AssertionError):
+        return None

vllm/v1/attention/backends/flash_attn.py

@@ -10,7 +10,7 @@ import triton.language as tl
 
 from vllm.attention.backends.abstract import (AttentionBackend, AttentionImpl,
                                               AttentionMetadata, AttentionType)
-from vllm.attention.backends.utils import VLLM_FLASH_ATTN_VERSION
+from vllm.attention.backends.utils import get_flash_attn_version
 from vllm.logger import init_logger
 from vllm.utils import cdiv
 from vllm.vllm_flash_attn import flash_attn_varlen_func
@@ -132,6 +132,7 @@ class FlashAttentionImpl(AttentionImpl):
                                       "encoder/decoder cross-attention "
                                       "are not implemented for "
                                       "FlashAttentionImpl")
+        self.vllm_flash_attn_version = get_flash_attn_version()
 
     def forward(
         self,
@@ -205,7 +206,7 @@ class FlashAttentionImpl(AttentionImpl):
                 window_size=self.sliding_window,
                 block_table=attn_metadata.block_table,
                 softcap=self.logits_soft_cap,
-                fa_version=VLLM_FLASH_ATTN_VERSION,
+                fa_version=self.vllm_flash_attn_version,
             )
             return output
 
@@ -227,7 +228,7 @@ class FlashAttentionImpl(AttentionImpl):
             logits_soft_cap=self.logits_soft_cap,
             block_table=attn_metadata.block_table,
             common_prefix_len=attn_metadata.common_prefix_len,
-            fa_version=VLLM_FLASH_ATTN_VERSION,
+            fa_version=self.vllm_flash_attn_version,
         )
         return output