test_sparse_mla_backends.py

# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
"""Unit tests for the FlashMLA sparse backend utilities."""

import math
from types import MethodType, SimpleNamespace

import numpy as np
import pytest
import torch

from tests.v1.attention.test_mla_backends import (
    BATCH_SPECS,
    BatchSpec,
    MockAttentionLayer,
    create_and_prepopulate_kv_cache,
)
from tests.v1.attention.utils import (
    create_common_attn_metadata,
    create_standard_kv_cache_spec,
    create_vllm_config,
)
from vllm import _custom_ops as ops
from vllm.attention.ops import flashmla
from vllm.model_executor.layers.linear import ColumnParallelLinear
from vllm.utils import cdiv
from vllm.v1.attention.backends.mla.flashmla_sparse import FlashMLASparseBackend
from vllm.v1.attention.backends.mla.indexer import split_prefill_chunks

SPARSE_BACKEND_BATCH_SPECS = {
    name: BATCH_SPECS[name]
    for name in [
        "mixed_small",
        "mixed_medium",
        "small_prefill",
        "medium_prefill",
        "single_prefill",
    ]
}

SPARSE_BACKEND_BATCH_SPECS["large_q_prefill"] = BatchSpec(
    seq_lens=[1024] * 2, query_lens=[256] * 2
)
SPARSE_BACKEND_BATCH_SPECS["large_q_pure_prefill"] = BatchSpec(
    seq_lens=[256] * 2, query_lens=[256] * 2
)


def _dequantize_fp8_ds_mla_entry(
    cache_slice: torch.Tensor, kv_lora_rank: int, rope_dim: int, dtype: torch.dtype
) -> tuple[torch.Tensor, torch.Tensor]:
    """Dequantize a single fp8_ds_mla cache entry back to latent + rope."""

    # The first kv_lora_rank bytes store FP8 latent values with one scale per
    # 128 element tile written as float32 right after the latent payload.
    scales = cache_slice.view(torch.float32)[kv_lora_rank // 4 : kv_lora_rank // 4 + 4]
    latent = torch.empty(kv_lora_rank, dtype=torch.float16, device=cache_slice.device)
    for tile_idx in range(4):
        tile_start = tile_idx * 128
        tile_end = tile_start + 128
        ops.convert_fp8(
            latent[tile_start:tile_end],
            cache_slice[tile_start:tile_end],
            float(scales[tile_idx].item()),
            kv_dtype="fp8",
        )
    latent = latent.to(dtype)

    rope_offset = kv_lora_rank // 2 + 8
    rope_vals = cache_slice.view(dtype)[rope_offset : rope_offset + rope_dim]
    return latent, rope_vals.clone()


def _quantize_dequantize_fp8_ds_mla(
    kv_c: torch.Tensor, k_pe: torch.Tensor, block_size: int, scale: torch.Tensor
) -> tuple[torch.Tensor, torch.Tensor]:
    """Round-trip kv_c/k_pe though the fp8_ds_mla cache layout."""

    if kv_c.numel() == 0:
        return kv_c.clone(), k_pe.clone()

    kv_lora_rank = kv_c.shape[-1]
    rope_dim = k_pe.shape[-1]
    num_tokens = kv_c.shape[0]
    num_blocks = max(1, math.ceil(num_tokens / block_size))
    entry_size = kv_lora_rank + 4 * 4 + 2 * rope_dim

    tmp_cache = torch.zeros(
        num_blocks, block_size, entry_size, dtype=torch.uint8, device=kv_c.device
    )
    slot_mapping = torch.arange(num_tokens, dtype=torch.long, device=kv_c.device)

    ops.concat_and_cache_mla(
        kv_c, k_pe, tmp_cache, slot_mapping, kv_cache_dtype="fp8_ds_mla", scale=scale
    )

    dequant_kv_c = torch.empty_like(kv_c)
    dequant_k_pe = torch.empty_like(k_pe)

    for token_idx in range(num_tokens):
        slot = slot_mapping[token_idx].item()
        block_idx = slot // block_size
        block_offset = slot % block_size
        cache_slice = tmp_cache[block_idx, block_offset]
        latent, rope_vals = _dequantize_fp8_ds_mla_entry(
            cache_slice, kv_lora_rank, rope_dim, kv_c.dtype
        )
        dequant_kv_c[token_idx] = latent
        dequant_k_pe[token_idx] = rope_vals

    return dequant_kv_c, dequant_k_pe


@pytest.mark.parametrize("batch_name", list(SPARSE_BACKEND_BATCH_SPECS.keys()))
@pytest.mark.parametrize("kv_cache_dtype", ["fp8_ds_mla", "auto"])
def test_sparse_backend_decode_correctness(dist_init, batch_name, kv_cache_dtype):
    if not torch.cuda.is_available():
        pytest.skip("CUDA is required for sparse MLA decode test")

    device = torch.device("cuda")
    dtype = torch.bfloat16

    batch_spec = SPARSE_BACKEND_BATCH_SPECS[batch_name]

    # Model hyper-parameters (kept intentionally small for the unit test)
    num_heads = 128
    kv_lora_rank = 512
    qk_nope_head_dim = 128
    qk_rope_head_dim = 64
    v_head_dim = 128
    head_size = kv_lora_rank + qk_rope_head_dim
    topk_tokens = 2048

    max_seqlen = max(batch_spec.seq_lens)
    total_cache_tokens = sum(batch_spec.seq_lens)
    block_size = 64

    vllm_config = create_vllm_config(
        model_name="deepseek-ai/DeepSeek-V2-Lite-Chat",
        max_model_len=max_seqlen,
        num_gpu_blocks=max(2048, cdiv(total_cache_tokens, block_size) + 1),
        block_size=block_size,
        hf_config_override={
            "index_topk": topk_tokens,
            "attn_module_list_cfg": [{"topk_tokens": topk_tokens}],
        },
    )
    model_config = vllm_config.model_config
    model_config.hf_text_config = SimpleNamespace(
        q_lora_rank=None,
        kv_lora_rank=kv_lora_rank,
        qk_nope_head_dim=qk_nope_head_dim,
        qk_rope_head_dim=qk_rope_head_dim,
        v_head_dim=v_head_dim,
        model_type="deepseek_v2",
    )
    model_config.dtype = dtype
    model_config.get_num_attention_heads = MethodType(
        lambda self, parallel_config: num_heads, model_config
    )
    model_config.get_num_kv_heads = MethodType(
        lambda self, parallel_config: 1, model_config
    )
    model_config.get_head_size = MethodType(lambda self: head_size, model_config)
    model_config.get_sliding_window = MethodType(lambda self: None, model_config)

    kv_cache_spec = create_standard_kv_cache_spec(vllm_config)

    torch.manual_seed(0)

    scale = 1.0 / math.sqrt(head_size)

    # Shared MLA projection weights to keep reference and backend in sync
    W_UK = torch.randn(
        kv_lora_rank, num_heads, qk_nope_head_dim, dtype=dtype, device=device
    )
    W_UV = torch.randn(kv_lora_rank, num_heads, v_head_dim, dtype=dtype, device=device)

    # Build synthetic decode-only workload
    seq_lens = batch_spec.seq_lens
    query_lens = batch_spec.query_lens

    all_q_vllm, all_kv_c_vllm, all_k_pe_vllm = [], [], []
    kv_c_contexts, k_pe_contexts = [], []
    reference_outputs = []

    kv_cache_scale = torch.tensor(1.0, dtype=torch.float32, device=device)

    for i in range(batch_spec.batch_size):
        s_len = seq_lens[i]
        q_len = query_lens[i]
        ctx_len = s_len - q_len

        q_c = torch.rand(
            q_len,
            num_heads,
            qk_nope_head_dim + qk_rope_head_dim,
            dtype=dtype,
            device=device,
        )
        kv_c_full = torch.rand(s_len, kv_lora_rank, dtype=dtype, device=device)
        k_pe_full = torch.rand(s_len, 1, qk_rope_head_dim, dtype=dtype, device=device)

        kv_c_full, k_pe_full = _quantize_dequantize_fp8_ds_mla(
            kv_c_full,
            k_pe_full.squeeze(1),
            block_size=vllm_config.cache_config.block_size,
            scale=kv_cache_scale,
        )

        q_nope, q_pe = q_c.split([qk_nope_head_dim, qk_rope_head_dim], dim=-1)
        ql_nope = torch.einsum("qnh,lnh->qnl", q_nope, W_UK)
        q_mqa = torch.cat([ql_nope, q_pe], dim=-1)

        k_mqa = torch.cat([kv_c_full, k_pe_full], dim=-1)
        k_mqa = k_mqa.unsqueeze(1).expand(-1, num_heads, -1)
        v_mqa = kv_c_full.unsqueeze(1).expand(-1, num_heads, -1)

        attn_mask = torch.ones(q_len, s_len, dtype=torch.bool, device=device)
        causal_mask = torch.tril(torch.ones(q_len, q_len, device=device))
        attn_mask[:, ctx_len:] = causal_mask

        q_sdpa_in = q_mqa.unsqueeze(0).transpose(1, 2)
        k_sdpa_in = k_mqa.unsqueeze(0).transpose(1, 2)
        v_sdpa_in = v_mqa.unsqueeze(0).transpose(1, 2)

        sdpa_out = torch.nn.functional.scaled_dot_product_attention(
            q_sdpa_in, k_sdpa_in, v_sdpa_in, attn_mask=attn_mask, scale=scale
        )
        sdpa_out = sdpa_out.transpose(1, 2).squeeze(0)

        sdpa_out = torch.einsum("qnl,lnv->qnv", sdpa_out, W_UV)
        reference_outputs.append(sdpa_out.flatten(start_dim=-2))

        all_q_vllm.append(q_c)
        all_kv_c_vllm.append(kv_c_full[ctx_len:])
        all_k_pe_vllm.append(k_pe_full[ctx_len:])
        kv_c_contexts.append(kv_c_full[: ctx_len + 1])
        k_pe_contexts.append(k_pe_full[: ctx_len + 1])

    query_vllm = torch.cat(all_q_vllm, dim=0)
    kv_c_vllm = torch.cat(all_kv_c_vllm, dim=0)
    k_pe_vllm = torch.cat(all_k_pe_vllm, dim=0)
    sdpa_reference = torch.cat(reference_outputs, dim=0)

    vllm_config.cache_config.cache_dtype = kv_cache_dtype
    vllm_config.model_config.hf_config.index_topk = topk_tokens

    common_attn_metadata = create_common_attn_metadata(
        batch_spec,
        vllm_config.cache_config.block_size,
        device,
        arange_block_indices=True,
    )

    kv_cache = create_and_prepopulate_kv_cache(
        kv_c_contexts=kv_c_contexts,
        k_pe_contexts=k_pe_contexts,
        block_size=vllm_config.cache_config.block_size,
        head_size=head_size,
        dtype=dtype,
        device=device,
        num_blocks=vllm_config.cache_config.num_gpu_blocks,
        common_attn_metadata=common_attn_metadata,
        randomize_blocks=False,
        kv_cache_dtype=vllm_config.cache_config.cache_dtype,
        scale=kv_cache_scale,
    )

    builder_cls = FlashMLASparseBackend.get_builder_cls()
    builder = builder_cls(kv_cache_spec, ["placeholder"], vllm_config, device)
    metadata = builder.build(
        common_prefix_len=0, common_attn_metadata=common_attn_metadata
    )

    starts = np.asarray(common_attn_metadata.query_start_loc_cpu, dtype=np.int32)
    seg_lengths = np.diff(starts)
    positions = np.arange(starts[-1], dtype=np.int32) - np.repeat(
        starts[:-1], seg_lengths
    )
    seq_lengths = np.asarray(common_attn_metadata.seq_lens_cpu, dtype=np.int32)
    prefix_lengths = seq_lengths - seg_lengths
    positions += np.repeat(prefix_lengths, seg_lengths)

    pos_gpu = torch.as_tensor(positions, device=device, dtype=torch.int32)
    topk = metadata.topk_tokens
    debug_indices = torch.arange(topk, device=device, dtype=torch.int32).unsqueeze(0)
    token_positions = pos_gpu.unsqueeze(1)
    causal_mask = debug_indices <= token_positions
    debug_indices = torch.where(
        causal_mask, debug_indices, torch.full_like(debug_indices, -1)
    )

    # FlashMLASparseImpl now reads top-k indices from the indexer-provided
    # buffer, so emulate that contract with a simple namespace mock.
    debug_indices = debug_indices.expand(metadata.num_actual_tokens, -1).clone()
    mock_indexer = SimpleNamespace(topk_indices_buffer=debug_indices)

    ok, reason = flashmla.is_flashmla_sparse_supported()
    if not ok:
        pytest.skip(reason)

    kv_b_proj_weight = torch.cat([W_UK, W_UV], dim=-1)
    kv_b_proj_weight = kv_b_proj_weight.view(
        kv_lora_rank, num_heads * (qk_nope_head_dim + v_head_dim)
    )

    mock_kv_b_proj = ColumnParallelLinear(
        input_size=kv_lora_rank,
        output_size=num_heads * (qk_nope_head_dim + v_head_dim),
        bias=False,
    ).to(device=device, dtype=dtype)
    mock_kv_b_proj.weight = torch.nn.Parameter(kv_b_proj_weight.T.contiguous())

    impl_cls = FlashMLASparseBackend.get_impl_cls()
    impl = impl_cls(
        num_heads=num_heads,
        head_size=head_size,
        scale=scale,
        num_kv_heads=1,
        alibi_slopes=None,
        sliding_window=None,
        kv_cache_dtype=vllm_config.cache_config.cache_dtype,
        logits_soft_cap=None,
        attn_type="decoder",
        kv_sharing_target_layer_name=None,
        q_lora_rank=None,
        kv_lora_rank=kv_lora_rank,
        qk_nope_head_dim=qk_nope_head_dim,
        qk_rope_head_dim=qk_rope_head_dim,
        qk_head_dim=qk_nope_head_dim + qk_rope_head_dim,
        v_head_dim=v_head_dim,
        kv_b_proj=mock_kv_b_proj,
        indexer=mock_indexer,
    )

    impl.process_weights_after_loading(dtype)

    layer = MockAttentionLayer(device)
    out_buffer = torch.empty(
        metadata.num_actual_tokens, num_heads * v_head_dim, dtype=dtype, device=device
    )

    with torch.inference_mode():
        backend_output = impl.forward(
            layer,
            query_vllm,
            kv_c_vllm,
            k_pe_vllm,
            kv_cache,
            metadata,
            output=out_buffer,
        )

    assert backend_output.shape == sdpa_reference.shape
    assert backend_output.dtype == sdpa_reference.dtype
    assert torch.isfinite(backend_output).all()

    torch.testing.assert_close(backend_output, sdpa_reference, rtol=0.5, atol=0.5)


@pytest.mark.parametrize(
    "seq_lens,max_buf,start,expected",
    [
        # Basic split: totals per chunk ≤ max_buf
        (torch.tensor([2, 3, 4, 2]), 5, 0, [(0, 2), (2, 3), (3, 4)]),
        # Non-zero start index
        (torch.tensor([2, 3, 4, 2]), 5, 1, [(1, 2), (2, 3), (3, 4)]),
        # Exact fits should split between items when adding the next would
        # overflow
        (torch.tensor([5, 5, 5]), 5, 0, [(0, 1), (1, 2), (2, 3)]),
        # All requests fit in a single chunk
        (torch.tensor([1, 1, 1]), 10, 0, [(0, 3)]),
        # Large buffer with non-zero start
        (torch.tensor([4, 4, 4]), 100, 1, [(1, 3)]),
    ],
)
def test_split_prefill_chunks(seq_lens, max_buf, start, expected):
    out = split_prefill_chunks(seq_lens, max_buf, start)
    assert out == expected