example_tilelang_nsa_fwd.py

# ruff: noqa
import torch
from reference import naive_nsa
import tilelang
from tilelang import language as T
import tilelang.testing

tilelang.testing.set_random_seed(0)


@tilelang.jit(
    out_idx=[-1],
    pass_configs={
        tilelang.PassConfigKey.TL_ENABLE_FAST_MATH: True,
        tilelang.PassConfigKey.TL_DISABLE_TMA_LOWER: True,
        tilelang.PassConfigKey.TL_DISABLE_WARP_SPECIALIZED: True,
    })
def native_sparse_attention(batch,
                            heads,
                            seq_len,
                            dim,
                            is_causal,
                            scale=None,
                            block_size=64,
                            groups=1,
                            selected_blocks=16):
    if scale is None:
        scale = (1.0 / dim)**0.5 * 1.44269504  # log2(e)
    else:
        scale = scale * 1.44269504  # log2(e)

    head_kv = heads // groups
    q_shape = [batch, seq_len, heads, dim]
    kv_shape = [batch, seq_len, head_kv, dim]
    block_indices_shape = [batch, seq_len, head_kv, selected_blocks]
    block_indices_dtype = "int32"
    dtype = "float16"
    accum_dtype = "float"
    block_S = block_size
    block_T = min(128, tilelang.math.next_power_of_2(dim))

    NK = tilelang.cdiv(dim, block_T)
    NV = tilelang.cdiv(dim, block_T)
    assert NK == 1, "The key dimension can not be larger than 256"

    S = selected_blocks
    G = groups
    BS = block_S
    BK = BV = block_T
    num_stages = 2
    threads = 32

    @T.prim_func
    def native_sparse_attention(
            Q: T.Tensor(q_shape, dtype),
            K: T.Tensor(kv_shape, dtype),
            V: T.Tensor(kv_shape, dtype),
            BlockIndices: T.Tensor(block_indices_shape, block_indices_dtype),
            Output: T.Tensor(q_shape, dtype),
    ):
        with T.Kernel(seq_len, NV, batch * head_kv, threads=threads) as (bx, by, bz):
            Q_shared = T.alloc_shared([G, BK], dtype)
            K_shared = T.alloc_shared([BS, BK], dtype)
            V_shared = T.alloc_shared([BS, BV], dtype)
            O_shared = T.alloc_shared([G, BV], dtype)

            acc_s = T.alloc_fragment([G, BS], accum_dtype)
            acc_s_cast = T.alloc_fragment([G, BS], dtype)
            acc_o = T.alloc_fragment([G, BV], accum_dtype)
            scores_max = T.alloc_fragment([G], accum_dtype)
            scores_max_prev = T.alloc_fragment([G], accum_dtype)
            scores_scale = T.alloc_fragment([G], accum_dtype)
            scores_sum = T.alloc_fragment([G], accum_dtype)
            logsum = T.alloc_fragment([G], accum_dtype)

            i_t, i_v, i_bh = bx, by, bz
            i_b, i_h = i_bh // head_kv, i_bh % head_kv

            NS = S
            T.copy(Q[i_b, i_t, i_h * G:(i_h + 1) * G, :], Q_shared)

            T.fill(acc_o, 0)
            T.fill(logsum, 0)
            T.fill(scores_max, -T.infinity(accum_dtype))

            for i in T.Pipelined(NS, num_stages=num_stages):
                i_s = BlockIndices[i_b, i_t, i_h, i] * BS
                if i_s <= i_t and i_s >= 0:
                    # [BS, BK]
                    T.copy(K[i_b, i_s:i_s + BS, i_h, :], K_shared)

                    if is_causal:
                        for i, j in T.Parallel(G, BS):
                            acc_s[i, j] = T.if_then_else(i_t >= (i_s + j), 0,
                                                         -T.infinity(acc_s.dtype))
                    else:
                        T.clear(acc_s)

                    T.gemm(
                        Q_shared,
                        K_shared,
                        acc_s,
                        transpose_B=True,
                        policy=T.GemmWarpPolicy.FullRow)

                    # Softmax
                    T.copy(scores_max, scores_max_prev)
                    T.fill(scores_max, -T.infinity(accum_dtype))
                    T.reduce_max(acc_s, scores_max, dim=1, clear=True)
                    for i in T.Parallel(G):
                        scores_scale[i] = T.exp2(scores_max_prev[i] * scale - scores_max[i] * scale)
                    for i, j in T.Parallel(G, BS):
                        acc_s[i, j] = T.exp2(acc_s[i, j] * scale - scores_max[i] * scale)
                    T.reduce_sum(acc_s, scores_sum, dim=1)
                    for i in T.Parallel(G):
                        logsum[i] = logsum[i] * scores_scale[i] + scores_sum[i]
                    T.copy(acc_s, acc_s_cast)

                    # Rescale
                    for i, j in T.Parallel(G, BV):
                        acc_o[i, j] *= scores_scale[i]

                    # V * softmax(Q * K)
                    T.copy(V[i_b, i_s:i_s + BS, i_h, i_v * BV:(i_v + 1) * BV], V_shared)
                    T.gemm(acc_s_cast, V_shared, acc_o, policy=T.GemmWarpPolicy.FullRow)

            for i, j in T.Parallel(G, BV):
                acc_o[i, j] /= logsum[i]
            T.copy(acc_o, O_shared)
            T.copy(O_shared, Output[i_b, i_t, i_h * G:(i_h + 1) * G, i_v * BV:(i_v + 1) * BV])

    return native_sparse_attention


def main():
    B, SEQ_LEN, H, HQ, D, S, block_size, dtype, scale = 2, 64, 1, 16, 32, 1, 32, torch.float16, 0.1

    kernel = native_sparse_attention(
        batch=B,
        heads=HQ,
        seq_len=SEQ_LEN,
        dim=D,
        is_causal=True,
        block_size=block_size,
        groups=HQ // H,
        selected_blocks=S,
        scale=scale,
    )
    print(kernel.get_kernel_source())
    torch.random.manual_seed(0)
    Q = torch.randn((B, SEQ_LEN, HQ, D), dtype=dtype, device='cuda').requires_grad_(True)
    K = torch.randn((B, SEQ_LEN, H, D), dtype=dtype, device='cuda').requires_grad_(True)
    V = torch.randn((B, SEQ_LEN, H, D), dtype=dtype, device='cuda').requires_grad_(True)
    g_slc = torch.ones((B, SEQ_LEN, HQ), dtype=dtype, device='cuda').requires_grad_(True)
    g_swa = torch.ones((B, SEQ_LEN, HQ), dtype=dtype, device='cuda').requires_grad_(True)
    DO = torch.randn((B, SEQ_LEN, HQ, D), dtype=dtype, device='cuda')

    block_indices = torch.full((B, SEQ_LEN, H, S), SEQ_LEN, dtype=torch.long, device='cuda')
    block_counts = torch.zeros((B, SEQ_LEN, H), dtype=torch.long, device='cuda')
    for b in range(B):
        for t in range(SEQ_LEN):
            for h in range(H):
                i_i = torch.randperm(max(1, (t // block_size)))[:S]
                block_indices[b, t, h, :len(i_i)] = i_i
                block_counts[b, t, h] = (block_indices[b, t, h] != SEQ_LEN).sum().item()
    block_indices = block_indices.sort(-1)[0]

    out = kernel(Q, K, V, block_indices.to(torch.int32))

    ref = naive_nsa(
        q=Q,
        k=K,
        v=V,
        g_slc=g_slc,
        g_swa=g_swa,
        block_indices=block_indices,
        block_counts=block_counts,
        block_size=block_size,
        scale=scale,
    )

    print("out", out)
    print("ref", ref)
    torch.testing.assert_close(ref, out, atol=1e-2, rtol=1e-2)


if __name__ == "__main__":
    main()