[Kernel] Use `out` arg in flash_attn_varlen_func (#10811)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

CMakeLists.txt

@@ -522,7 +522,7 @@ else()
   FetchContent_Declare(
           vllm-flash-attn
           GIT_REPOSITORY https://github.com/vllm-project/flash-attention.git
-          GIT_TAG fdf6d72b48aea41f4ae6a89139a453dae554abc8
+          GIT_TAG 04325b6798bcc326c86fb35af62d05a9c8c8eceb
           GIT_PROGRESS TRUE
           # Don't share the vllm-flash-attn build between build types
           BINARY_DIR ${CMAKE_BINARY_DIR}/vllm-flash-attn

tests/kernels/test_flash_attn.py

@@ -71,6 +71,7 @@ def ref_paged_attn(
     return torch.cat(outputs, dim=0)
 
 
+@pytest.mark.parametrize("use_out", [True, False])
 @pytest.mark.parametrize("kv_lens", [[1328, 18, 463], [1, 54, 293, 70]])
 @pytest.mark.parametrize("num_heads", NUM_HEADS)
 @pytest.mark.parametrize("head_size", HEAD_SIZES)
@@ -81,6 +82,7 @@ def ref_paged_attn(
 @pytest.mark.parametrize("sliding_window", [None, 256])
 @torch.inference_mode()
 def test_flash_attn_with_paged_kv(
+    use_out: bool,
     kv_lens: List[int],
     num_heads: Tuple[int, int],
     head_size: int,
@@ -116,17 +118,22 @@ def test_flash_attn_with_paged_kv(
                                  (num_seqs, max_num_blocks_per_seq),
                                  dtype=torch.int32)
 
+    q = query.unsqueeze(1)
+    out = torch.empty_like(q) if use_out else None
     output = flash_attn_with_kvcache(
-        q=query.unsqueeze(1),
+        q=q,
         k_cache=key_cache,
         v_cache=value_cache,
+        out=out,
         softmax_scale=scale,
         causal=True,
         block_table=block_tables,
         cache_seqlens=kv_lens_tensor,
         softcap=soft_cap if soft_cap is not None else 0,
         window_size=window_size,
-    ).squeeze(1)
+    )
+    output = output if not use_out else out
+    output = output.squeeze(1)
 
     ref_output = ref_paged_attn(query=query,
                                 key_cache=key_cache,
@@ -141,7 +148,10 @@ def test_flash_attn_with_paged_kv(
         f"{torch.max(torch.abs(output - ref_output))}"
 
 
-@pytest.mark.parametrize("seq_lens", [[(1, 1328), (5, 18), (129, 463)]])
+@pytest.mark.parametrize("use_out", [True, False])
+@pytest.mark.parametrize("seq_lens",
+                         [[(1, 1328), (5, 18),
+                           (129, 463)], [(1, 523), (1, 37), (1, 2011)]])
 @pytest.mark.parametrize("num_heads", NUM_HEADS)
 @pytest.mark.parametrize("head_size", HEAD_SIZES)
 @pytest.mark.parametrize("block_size", BLOCK_SIZES)
@@ -151,6 +161,7 @@ def test_flash_attn_with_paged_kv(
 @pytest.mark.parametrize("num_blocks", NUM_BLOCKS)
 @torch.inference_mode()
 def test_varlen_with_paged_kv(
+    use_out: bool,
     seq_lens: List[Tuple[int, int]],
     num_heads: Tuple[int, int],
     head_size: int,
@@ -197,10 +208,12 @@ def test_varlen_with_paged_kv(
                                  (num_seqs, max_num_blocks_per_seq),
                                  dtype=torch.int32)
 
+    out = torch.empty_like(query) if use_out else None
     output = flash_attn_varlen_func(
         q=query,
         k=key_cache,
         v=value_cache,
+        out=out,
         cu_seqlens_q=cu_query_lens,
         cu_seqlens_k=cu_kv_lens,
         max_seqlen_q=max_query_len,
@@ -211,6 +224,7 @@ def test_varlen_with_paged_kv(
         block_table=block_tables,
         softcap=soft_cap if soft_cap is not None else 0,
     )
+    output = output if not use_out else out
 
     ref_output = ref_paged_attn(
         query=query,

vllm/v1/attention/backends/flash_attn.py

@@ -205,10 +205,12 @@ def unified_v1_flash_attention(
         v_scale,
     )
 
-    attn_output = flash_attn_varlen_func(
+    # Compute attention and update output up to `num_actual_tokens`.
+    flash_attn_varlen_func(
         q=query[:num_actual_tokens],
         k=key_cache,
         v=value_cache,
+        out=output[:num_actual_tokens],
         cu_seqlens_q=attn_metadata.query_start_loc,
         max_seqlen_q=attn_metadata.max_query_len,
         cu_seqlens_k=attn_metadata.seq_start_loc,
@@ -220,8 +222,6 @@ def unified_v1_flash_attention(
         block_table=attn_metadata.block_table,
         softcap=logits_soft_cap,
     )
-    # TODO(woosuk): Remove this unnecessary copy.
-    output[:num_actual_tokens].copy_(attn_output)
 
 
 def unified_v1_flash_attention_fake(