[V1] Implement Cascade Attention

CMakeLists.txt

@@ -550,7 +550,7 @@ else()
   FetchContent_Declare(
           vllm-flash-attn
           GIT_REPOSITORY https://github.com/vllm-project/flash-attention.git
-          GIT_TAG 04325b6798bcc326c86fb35af62d05a9c8c8eceb
+          GIT_TAG 96266b1111111f3d11aabefaf3bacbab6a89d03c
           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_cascade_flash_attn.py

@@ -0,0 +1,182 @@
+from typing import List, Optional, Tuple
+
+import pytest
+import torch
+
+from vllm.platforms import current_platform
+from vllm.v1.attention.backends.flash_attn import (cascade_attention,
+                                                   merge_attn_states)
+from vllm.vllm_flash_attn import flash_attn_varlen_func
+
+NUM_HEADS = [(4, 4), (8, 2), (16, 2)]
+HEAD_SIZES = [128, 192, 256]
+BLOCK_SIZES = [16]
+DTYPES = [torch.float16, torch.bfloat16]
+
+
+@pytest.mark.parametrize("num_tokens", [1, 39, 16912])
+@pytest.mark.parametrize("num_heads", NUM_HEADS)
+@pytest.mark.parametrize("head_size", HEAD_SIZES)
+@pytest.mark.parametrize("dtype", DTYPES)
+@torch.inference_mode()
+def test_merge_kernel(
+    num_tokens: int,
+    num_heads: Tuple[int, int],
+    head_size: int,
+    dtype: torch.dtype,
+):
+    torch.set_default_device("cuda")
+    current_platform.seed_everything(0)
+    num_query_heads = num_heads[0]
+    num_kv_heads = num_heads[1]
+    assert num_query_heads % num_kv_heads == 0
+
+    # Prepare inputs.
+    prefix_output = torch.randn(num_tokens,
+                                num_query_heads,
+                                head_size,
+                                dtype=dtype)
+    suffix_output = torch.randn(num_tokens,
+                                num_query_heads,
+                                head_size,
+                                dtype=dtype)
+    prefix_lse = torch.randn(num_query_heads, num_tokens, dtype=torch.float32)
+    suffix_lse = torch.randn(num_query_heads, num_tokens, dtype=torch.float32)
+
+    # Run the kernel.
+    output = torch.empty(num_tokens, num_query_heads, head_size, dtype=dtype)
+    merge_attn_states(output, prefix_output, prefix_lse, suffix_output,
+                      suffix_lse)
+
+    # Reference implementation.
+    max_lse = torch.maximum(prefix_lse, suffix_lse)
+    p_lse = torch.exp(prefix_lse - max_lse)
+    s_lse = torch.exp(suffix_lse - max_lse)
+    p_scale = p_lse / (p_lse + s_lse)
+    s_scale = s_lse / (p_lse + s_lse)
+    p_scale = p_scale.transpose(0, 1).unsqueeze(2)
+    s_scale = s_scale.transpose(0, 1).unsqueeze(2)
+    ref_output = p_scale * prefix_output + s_scale * suffix_output
+    ref_output = ref_output.to(dtype)
+
+    # Compare the results.
+    torch.testing.assert_close(output, ref_output, atol=1e-2, rtol=1e-2)
+
+
+CASES = [
+    # Case 1. A general case.
+    ([(129, 871), (18, 280), (37, 988), (1023, 2304), (1, 257)], 256),
+    # Case 2. Flash-decoding case.
+    ([(1, 1023), (1, 879), (1, 778), (1, 1777)] * 100, 512),
+]
+
+
+@pytest.mark.parametrize("seq_lens_and_common_prefix", CASES)
+@pytest.mark.parametrize("num_heads", NUM_HEADS)
+@pytest.mark.parametrize("head_size", HEAD_SIZES)
+@pytest.mark.parametrize("dtype", DTYPES)
+@pytest.mark.parametrize("block_size", BLOCK_SIZES)
+@pytest.mark.parametrize("soft_cap", [None, 50])
+@pytest.mark.parametrize("num_blocks", [2048])
+@torch.inference_mode()
+def test_cascade(
+    seq_lens_and_common_prefix: Tuple[List[Tuple[int, int]], int],
+    num_heads: Tuple[int, int],
+    head_size: int,
+    dtype: torch.dtype,
+    block_size: int,
+    soft_cap: Optional[float],
+    num_blocks: int,
+) -> None:
+    torch.set_default_device("cuda")
+    current_platform.seed_everything(0)
+
+    window_size = (-1, -1)
+    scale = head_size**-0.5
+    num_query_heads = num_heads[0]
+    num_kv_heads = num_heads[1]
+    assert num_query_heads % num_kv_heads == 0
+    key_cache = torch.randn(num_blocks,
+                            block_size,
+                            num_kv_heads,
+                            head_size,
+                            dtype=dtype)
+    value_cache = torch.randn_like(key_cache)
+
+    seq_lens, common_prefix_len = seq_lens_and_common_prefix
+    num_seqs = len(seq_lens)
+    query_lens = [x[0] for x in seq_lens]
+    kv_lens = [x[1] for x in seq_lens]
+    max_query_len = max(query_lens)
+    max_kv_len = max(kv_lens)
+
+    total_num_query_tokens = sum(query_lens)
+    query = torch.randn(total_num_query_tokens,
+                        num_query_heads,
+                        head_size,
+                        dtype=dtype)
+    cu_query_lens = torch.tensor([0] + query_lens,
+                                 dtype=torch.int32).cumsum(dim=0,
+                                                           dtype=torch.int32)
+    cu_kv_lens = torch.tensor([0] + kv_lens,
+                              dtype=torch.int32).cumsum(dim=0,
+                                                        dtype=torch.int32)
+    max_num_blocks_per_seq = (max_kv_len + block_size - 1) // block_size
+    block_tables = torch.randint(0,
+                                 num_blocks,
+                                 (num_seqs, max_num_blocks_per_seq),
+                                 dtype=torch.int32)
+
+    assert common_prefix_len > 0
+    assert common_prefix_len % block_size == 0
+    num_common_kv_blocks = common_prefix_len // block_size
+    # Make sure the first `num_common_kv_blocks` blocks are the same.
+    block_tables[:, :num_common_kv_blocks] = \
+        block_tables[0, :num_common_kv_blocks]
+
+    # Run the regular attention.
+    ref_output = flash_attn_varlen_func(
+        q=query,
+        k=key_cache,
+        v=value_cache,
+        cu_seqlens_q=cu_query_lens,
+        cu_seqlens_k=cu_kv_lens,
+        max_seqlen_q=max_query_len,
+        max_seqlen_k=max_kv_len,
+        softmax_scale=scale,
+        causal=True,
+        window_size=window_size,
+        block_table=block_tables,
+        softcap=soft_cap if soft_cap is not None else 0,
+    )
+
+    # Run cascade attention.
+    assert all(common_prefix_len < kv_len for kv_len in kv_lens)
+    cu_prefix_query_lens = torch.tensor([0, total_num_query_tokens],
+                                        dtype=torch.int32)
+    cu_prefix_kv_lens = torch.tensor([0, common_prefix_len], dtype=torch.int32)
+    cu_suffix_kv_lens = (
+        cu_kv_lens -
+        torch.arange(num_seqs + 1, dtype=torch.int32) * common_prefix_len)
+    output = torch.empty_like(query)
+    cascade_attention(
+        output=output,
+        query=query,
+        key_cache=key_cache,
+        value_cache=value_cache,
+        cu_query_lens=cu_query_lens,
+        max_query_len=max_query_len,
+        cu_prefix_query_lens=cu_prefix_query_lens,
+        cu_prefix_kv_lens=cu_prefix_kv_lens,
+        cu_suffix_kv_lens=cu_suffix_kv_lens,
+        max_kv_len=max_kv_len,
+        softmax_scale=scale,
+        alibi_slopes=None,
+        sliding_window=window_size,
+        logits_soft_cap=soft_cap if soft_cap is not None else 0,
+        block_table=block_tables,
+        common_prefix_len=common_prefix_len,
+    )
+
+    # Compare the results.
+    torch.testing.assert_close(output, ref_output, atol=1e-2, rtol=1e-2)