[Kernel] Use flash-attn for decoding

tests/kernels/test_flash_attn.py

@@ -0,0 +1,205 @@
+from typing import List, Optional, Tuple
+
+import pytest
+import torch
+from vllm_flash_attn import flash_attn_varlen_func, flash_attn_with_kvcache
+
+NUM_HEADS = [(16, 16), (32, 8), (64, 8)]
+HEAD_SIZES = [128, 256]
+BLOCK_SIZES = [16, 32]
+DTYPES = [torch.float16, torch.bfloat16]
+
+
+def ref_paged_attn(
+    query: torch.Tensor,
+    key_cache: torch.Tensor,
+    value_cache: torch.Tensor,
+    query_lens: List[int],
+    kv_lens: List[int],
+    block_tables: torch.Tensor,
+    scale: float,
+    sliding_window: Optional[int] = None,
+) -> torch.Tensor:
+    num_seqs = len(query_lens)
+    block_tables = block_tables.cpu().numpy()
+    _, block_size, num_kv_heads, head_size = key_cache.shape
+
+    outputs = []
+    start_idx = 0
+    for i in range(num_seqs):
+        query_len = query_lens[i]
+        kv_len = kv_lens[i]
+        q = query[start_idx:start_idx + query_len]
+        q *= scale
+
+        num_kv_blocks = (kv_len + block_size - 1) // block_size
+        block_indices = block_tables[i, :num_kv_blocks]
+
+        k = key_cache[block_indices].view(-1, num_kv_heads, head_size)
+        k = k[:kv_len]
+        v = value_cache[block_indices].view(-1, num_kv_heads, head_size)
+        v = v[:kv_len]
+
+        if q.shape[1] != k.shape[1]:
+            k = torch.repeat_interleave(k, q.shape[1] // k.shape[1], dim=1)
+            v = torch.repeat_interleave(v, q.shape[1] // v.shape[1], dim=1)
+        attn = torch.einsum("qhd,khd->hqk", q, k).float()
+        mask = torch.triu(torch.ones(query_len, kv_len),
+                          diagonal=kv_len - query_len + 1).bool()
+        if sliding_window is not None:
+            sliding_window_mask = torch.triu(torch.ones(query_len, kv_len),
+                                             diagonal=kv_len -
+                                             (query_len + sliding_window) +
+                                             1).bool().logical_not()
+            mask |= sliding_window_mask
+        attn.masked_fill_(mask, float("-inf"))
+        attn = torch.softmax(attn, dim=-1).to(v.dtype)
+        out = torch.einsum("hqk,khd->qhd", attn, v)
+
+        outputs.append(out)
+        start_idx += query_len
+
+    return torch.cat(outputs, dim=0)
+
+
+@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)
+@pytest.mark.parametrize("block_size", BLOCK_SIZES)
+@pytest.mark.parametrize("dtype", DTYPES)
+@torch.inference_mode
+def test_flash_attn_with_paged_kv(
+    kv_lens: List[Tuple[int, int]],
+    num_heads: Tuple[int, int],
+    head_size: int,
+    dtype: torch.dtype,
+    block_size: int,
+) -> None:
+    torch.set_default_device("cuda")
+    torch.cuda.manual_seed_all(0)
+    num_blocks = 128
+    num_seqs = len(kv_lens)
+    num_query_heads = num_heads[0]
+    num_kv_heads = num_heads[1]
+    assert num_query_heads % num_kv_heads == 0
+    max_kv_len = max(kv_lens)
+    scale = head_size**-0.5
+
+    query = torch.randn(num_seqs, num_query_heads, head_size, dtype=dtype)
+    key_cache = torch.randn(num_blocks,
+                            block_size,
+                            num_kv_heads,
+                            head_size,
+                            dtype=dtype)
+    value_cache = torch.randn_like(key_cache)
+    kv_lens_tensor = torch.tensor(kv_lens, 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)
+
+    output = flash_attn_with_kvcache(
+        q=query.unsqueeze(1),
+        k_cache=key_cache,
+        v_cache=value_cache,
+        softmax_scale=scale,
+        causal=True,
+        block_table=block_tables,
+        cache_seqlens=kv_lens_tensor,
+    ).squeeze(1)
+
+    ref_output = ref_paged_attn(
+        query=query,
+        key_cache=key_cache,
+        value_cache=value_cache,
+        query_lens=[1] * num_seqs,
+        kv_lens=kv_lens,
+        block_tables=block_tables,
+        scale=scale,
+    )
+    assert torch.allclose(output, ref_output, atol=1e-2, rtol=1e-2), \
+        f"{torch.max(torch.abs(output - ref_output))}"
+
+
+@pytest.mark.parametrize("seq_lens", [[(1, 1328), (5, 18), (129, 463)]])
+@pytest.mark.parametrize("num_heads", NUM_HEADS)
+@pytest.mark.parametrize("head_size", HEAD_SIZES)
+@pytest.mark.parametrize("block_size", BLOCK_SIZES)
+@pytest.mark.parametrize("sliding_window", [None])
+@pytest.mark.parametrize("dtype", DTYPES)
+@torch.inference_mode
+def test_varlen_with_paged_kv(
+    seq_lens: List[Tuple[int, int]],
+    num_heads: Tuple[int, int],
+    head_size: int,
+    sliding_window: Optional[int],
+    dtype: torch.dtype,
+    block_size: int,
+) -> None:
+    torch.set_default_device("cuda")
+    torch.cuda.manual_seed_all(0)
+    num_blocks = 128
+    num_seqs = len(seq_lens)
+    query_lens = [x[0] for x in seq_lens]
+    kv_lens = [x[1] for x in seq_lens]
+    num_query_heads = num_heads[0]
+    num_kv_heads = num_heads[1]
+    assert num_query_heads % num_kv_heads == 0
+    max_query_len = max(query_lens)
+    max_kv_len = max(kv_lens)
+    window_size = ((sliding_window,
+                    sliding_window) if sliding_window is not None else
+                   (-1, -1))
+    scale = head_size**-0.5
+
+    query = torch.randn(sum(query_lens),
+                        num_query_heads,
+                        head_size,
+                        dtype=dtype)
+    key_cache = torch.randn(num_blocks,
+                            block_size,
+                            num_kv_heads,
+                            head_size,
+                            dtype=dtype)
+    value_cache = torch.randn_like(key_cache)
+    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)
+
+    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,
+    )
+
+    ref_output = ref_paged_attn(
+        query=query,
+        key_cache=key_cache,
+        value_cache=value_cache,
+        query_lens=query_lens,
+        kv_lens=kv_lens,
+        block_tables=block_tables,
+        scale=scale,
+        sliding_window=sliding_window,
+    )
+    assert torch.allclose(output, ref_output, atol=1e-2, rtol=1e-2), \
+        f"{torch.max(torch.abs(output - ref_output))}"