[Attention] MLA decode optimizations

csrc/cache.h

@@ -28,6 +28,11 @@ void reshape_and_cache_flash(torch::Tensor& key, torch::Tensor& value,
                              const std::string& kv_cache_dtype,
                              torch::Tensor& k_scale, torch::Tensor& v_scale);
 
+void concat_and_cache_mla(torch::Tensor& kv_c, torch::Tensor& k_pe,
+                          torch::Tensor& kv_cache, torch::Tensor& slot_mapping,
+                          const std::string& kv_cache_dtype,
+                          torch::Tensor& scale);
+
 // Just for unittest
 void convert_fp8(torch::Tensor& dst_cache, torch::Tensor& src_cache,
                  const double scale, const std::string& kv_cache_dtype);

csrc/cache_kernels.cu

@@ -245,6 +245,51 @@ __global__ void reshape_and_cache_flash_kernel(
     }
   }
 }
+
+template <typename scalar_t, typename cache_t, Fp8KVCacheDataType kv_dt>
+__global__ void concat_and_cache_mla_kernel(
+    const scalar_t* __restrict__ kv_c,  // [num_tokens, kv_lora_rank]
+    const scalar_t* __restrict__ k_pe,  // [num_tokens, pe_dim]
+    cache_t* __restrict__ kv_cache,  // [num_blocks, block_size, (kv_lora_rank
+                                     // + pe_dim)]
+    const int64_t* __restrict__ slot_mapping,  // [num_tokens]
+    const int block_stride,                    //
+    const int kv_c_stride,                     //
+    const int k_pe_stride,                     //
+    const int kv_lora_rank,                    //
+    const int pe_dim,                          //
+    const int block_size,                      //
+    const float* scale                         //
+) {
+  const int64_t token_idx = blockIdx.x;
+  const int64_t slot_idx = slot_mapping[token_idx];
+  // NOTE: slot_idx can be -1 if the token is padded
+  if (slot_idx < 0) {
+    return;
+  }
+  const int64_t block_idx = slot_idx / block_size;
+  const int64_t block_offset = slot_idx % block_size;
+
+  auto copy = [&](const scalar_t* __restrict__ src, cache_t* __restrict__ dst,
+                  int src_stride, int dst_stride, int size, int offset) {
+    for (int i = threadIdx.x; i < size; i += blockDim.x) {
+      const int64_t src_idx = token_idx * src_stride + i;
+      const int64_t dst_idx = block_idx * block_stride +
+                              block_offset * (kv_lora_rank + pe_dim) + i +
+                              offset;
+      if constexpr (kv_dt == Fp8KVCacheDataType::kAuto) {
+        dst[dst_idx] = src[src_idx];
+      } else {
+        dst[dst_idx] =
+            fp8::scaled_convert<cache_t, scalar_t, kv_dt>(src[src_idx], *scale);
+      }
+    }
+  };
+
+  copy(kv_c, kv_cache, kv_c_stride, block_stride, kv_lora_rank, 0);
+  copy(k_pe, kv_cache, k_pe_stride, block_stride, pe_dim, kv_lora_rank);
+}
+
 }  // namespace vllm
 
 // KV_T is the stored data type of kv-cache.
@@ -343,6 +388,56 @@ void reshape_and_cache_flash(
                              CALL_RESHAPE_AND_CACHE_FLASH);
 }
 
+// KV_T is the stored data type of kv-cache.
+// CACHE_T is the data type of key and value tensors.
+// KV_DTYPE is the real data type of kv-cache.
+#define CALL_CONCAT_AND_CACHE_MLA(KV_T, CACHE_T, KV_DTYPE)             \
+  vllm::concat_and_cache_mla_kernel<KV_T, CACHE_T, KV_DTYPE>           \
+      <<<grid, block, 0, stream>>>(                                    \
+          reinterpret_cast<KV_T*>(kv_c.data_ptr()),                    \
+          reinterpret_cast<KV_T*>(k_pe.data_ptr()),                    \
+          reinterpret_cast<CACHE_T*>(kv_cache.data_ptr()),             \
+          slot_mapping.data_ptr<int64_t>(), block_stride, kv_c_stride, \
+          k_pe_stride, kv_lora_rank, pe_dim, block_size,               \
+          reinterpret_cast<const float*>(scale.data_ptr()));
+
+void concat_and_cache_mla(
+    torch::Tensor& kv_c,          // [num_tokens, kv_lora_rank]
+    torch::Tensor& k_pe,          // [num_tokens, pe_dim]
+    torch::Tensor& kv_cache,      // [num_blocks, block_size, (kv_lora_rank +
+                                  // pe_dim)]
+    torch::Tensor& slot_mapping,  // [num_tokens] or [num_actual_tokens]
+    const std::string& kv_cache_dtype, torch::Tensor& scale) {
+  // NOTE(woosuk): In vLLM V1, key.size(0) can be different from
+  // slot_mapping.size(0) because of padding for CUDA graphs.
+  // In vLLM V0, key.size(0) is always equal to slot_mapping.size(0) because
+  // both include padding.
+  // In vLLM V1, however, key.size(0) can be larger than slot_mapping.size(0)
+  // since key includes padding for CUDA graphs, while slot_mapping does not.
+  // In this case, slot_mapping.size(0) represents the actual number of tokens
+  // before padding.
+  // For compatibility with both cases, we use slot_mapping.size(0) as the
+  // number of tokens.
+  int num_tokens = slot_mapping.size(0);
+  int kv_lora_rank = kv_c.size(1);
+  int pe_dim = k_pe.size(1);
+  int block_size = kv_cache.size(1);
+
+  TORCH_CHECK(kv_cache.size(2) == kv_lora_rank + pe_dim);
+
+  int kv_c_stride = kv_c.stride(0);
+  int k_pe_stride = k_pe.stride(0);
+  int block_stride = kv_cache.stride(0);
+
+  dim3 grid(num_tokens);
+  dim3 block(std::min(kv_lora_rank, 512));
+  const at::cuda::OptionalCUDAGuard device_guard(device_of(kv_c));
+  const cudaStream_t stream = at::cuda::getCurrentCUDAStream();
+
+  DISPATCH_BY_KV_CACHE_DTYPE(kv_c.dtype(), kv_cache_dtype,
+                             CALL_CONCAT_AND_CACHE_MLA);
+}
+
 namespace vllm {
 
 template <typename Tout, typename Tin, Fp8KVCacheDataType kv_dt>

csrc/torch_bindings.cpp

@@ -463,6 +463,15 @@ TORCH_LIBRARY_EXPAND(CONCAT(TORCH_EXTENSION_NAME, _cache_ops), cache_ops) {
   cache_ops.impl("reshape_and_cache_flash", torch::kCUDA,
                  &reshape_and_cache_flash);
 
+  // Concat kv_c and k_pe and cache them.
+  cache_ops.def(
+      "concat_and_cache_mla(Tensor kv_c, Tensor k_pe,"
+      "                     Tensor! kv_cache,"
+      "                     Tensor slot_mapping,"
+      "                     str kv_cache_dtype,"
+      "                     Tensor scale) -> ()");
+  cache_ops.impl("concat_and_cache_mla", torch::kCUDA, &concat_and_cache_mla);
+
   // Convert the key and value cache to fp8 data type.
   cache_ops.def(
       "convert_fp8(Tensor! dst_cache, Tensor src_cache, float scale, "

tests/kernels/test_triton_decode_attention.py

@@ -0,0 +1,89 @@
+import pytest
+import torch
+
+from vllm.attention.ops.triton_decode_attention import decode_attention_fwd
+
+
+def cdiv(a, b):
+    return (a + b - 1) // b
+
+
+@pytest.mark.parametrize("B", [3, 5])
+@pytest.mark.parametrize("L", [1027, 1025])
+@pytest.mark.parametrize("H_Q", [32])
+@pytest.mark.parametrize("H_KV", [32, 8])
+@pytest.mark.parametrize("D_QK", [128, 192, 576])
+@pytest.mark.parametrize("D_V", [128, 512])
+@pytest.mark.parametrize("CACHE_SIZE", [16384])
+@pytest.mark.parametrize("PAGE_SIZE", [1, 16])
+def test_decode_attention(B, L, H_Q, H_KV, D_QK, D_V, CACHE_SIZE, PAGE_SIZE):
+    assert CACHE_SIZE % PAGE_SIZE == 0
+    dtype = torch.bfloat16
+    seq_len = L  # This represents the number of tokens already in the sequence
+    sm_scale = 1.0 / (D_QK**0.5)
+    num_kv_splits = 8
+
+    num_pages_per_batch = cdiv(seq_len, PAGE_SIZE)
+    req_to_page = torch.randint(0,
+                                CACHE_SIZE // PAGE_SIZE,
+                                (B, num_pages_per_batch, 1),
+                                device="cuda")
+    req_to_token = req_to_page * PAGE_SIZE
+    req_to_token = req_to_token.expand(B, num_pages_per_batch, PAGE_SIZE)
+    req_to_token = req_to_token + torch.arange(PAGE_SIZE, device="cuda").view(
+        1, 1, -1)
+    req_to_token = req_to_token.view(B, -1)
+    req_to_token = req_to_token[:, :seq_len].contiguous()
+
+    # q represents the new token being generated, one per batch
+    q = torch.randn(B, H_Q, D_QK, dtype=dtype, device="cuda")
+
+    # k_buffer and v_buffer represent all previous tokens
+    # Page size is 1.
+    k_buffer = torch.randn(CACHE_SIZE, H_KV, D_QK, dtype=dtype, device="cuda")
+    v_buffer = torch.randn(CACHE_SIZE, H_KV, D_V, dtype=dtype, device="cuda")
+
+    # o will have the same shape as q
+    o = torch.zeros(B, H_Q, D_V, dtype=dtype, device="cuda")
+
+    b_seq_len = torch.full((B, ), seq_len, device="cuda")
+
+    attn_logits = torch.empty(
+        (B, H_Q, num_kv_splits, D_V + 1),
+        dtype=torch.float32,
+        device="cuda",
+    )
+
+    # Call the original implementation.
+    decode_attention_fwd(
+        q,
+        k_buffer,
+        v_buffer,
+        o,
+        req_to_token,
+        b_seq_len,
+        attn_logits,
+        num_kv_splits,
+        sm_scale,
+    )
+
+    # Page size can be larger than 1.
+    k_buffer = k_buffer.view(CACHE_SIZE // PAGE_SIZE, PAGE_SIZE, H_KV, D_QK)
+    v_buffer = v_buffer.view(CACHE_SIZE // PAGE_SIZE, PAGE_SIZE, H_KV, D_V)
+
+    o1 = torch.zeros_like(o)
+
+    decode_attention_fwd(
+        q,
+        k_buffer,
+        v_buffer,
+        o1,
+        req_to_page,
+        b_seq_len,
+        attn_logits,
+        num_kv_splits,
+        sm_scale,
+        PAGE_SIZE,
+    )
+
+    assert torch.allclose(o, o1)

tests/plugins/vllm_add_dummy_platform/vllm_add_dummy_platform/dummy_platform.py

@@ -5,5 +5,5 @@ class DummyPlatform(CudaPlatform):
     device_name = "DummyDevice"
 
     def get_attn_backend_cls(self, backend_name, head_size, dtype,
-                             kv_cache_dtype, block_size, use_v1):
+                             kv_cache_dtype, block_size, use_v1, use_mla):
         return "vllm_add_dummy_platform.dummy_attention_backend.DummyAttentionBackend"  # noqa E501

vllm/_custom_ops.py

@@ -980,6 +980,19 @@ def reshape_and_cache_flash(
                                                    v_scale)
 
 
+def concat_and_cache_mla(
+    kv_c: torch.Tensor,
+    k_pe: torch.Tensor,
+    kv_cache: torch.Tensor,
+    slot_mapping: torch.Tensor,
+    kv_cache_dtype: str,
+    scale: torch.Tensor,
+) -> None:
+    torch.ops._C_cache_ops.concat_and_cache_mla(kv_c, k_pe, kv_cache,
+                                                slot_mapping, kv_cache_dtype,
+                                                scale)
+
+
 def copy_blocks(key_caches: List[torch.Tensor],
                 value_caches: List[torch.Tensor],
                 block_mapping: torch.Tensor) -> None:

vllm/attention/backends/abstract.py

@@ -268,9 +268,25 @@ def __init__(
     def forward(
         self,
         layer: AttentionLayer,
-        query: torch.Tensor,
-        key: torch.Tensor,
-        value: torch.Tensor,
+        query: torch.Tensor,  # For MLA hidden_states_or_cq
+        key: torch.Tensor,  # For MLA kv_c_normed
+        value: torch.Tensor,  # For MLA k_pe
+        kv_cache: torch.Tensor,
+        attn_metadata: T,
+        output: Optional[torch.Tensor] = None,
+    ) -> torch.Tensor:
+        raise NotImplementedError
+
+
+class MLAAttentionImpl(AttentionImpl):
+
+    @abstractmethod
+    def forward(
+        self,
+        layer: AttentionLayer,
+        hidden_states_or_cq: torch.Tensor,
+        kv_c_normed: torch.Tensor,
+        k_pe: torch.Tensor,
         kv_cache: torch.Tensor,
         attn_metadata: T,
         output: Optional[torch.Tensor] = None,