[Kernel] FlashMLA integration (vllm-project#13747)

Signed-off-by: Lucas Wilkinson <[email protected]>
Signed-off-by: Lucas Wilkinson <[email protected]>

Author: LucasWilkinson

CMakeLists.txt

@@ -575,77 +575,8 @@ if(VLLM_GPU_LANG STREQUAL "HIP")
     WITH_SOABI)
 endif()
 
-# vllm-flash-attn currently only supported on CUDA
-if (NOT VLLM_GPU_LANG STREQUAL "CUDA")
-  return()
+# For CUDA we also build and ship some external projects.
+if (VLLM_GPU_LANG STREQUAL "CUDA")
+    include(cmake/external_projects/flashmla.cmake)
+    include(cmake/external_projects/vllm_flash_attn.cmake)
 endif ()
-
-# vLLM flash attention requires VLLM_GPU_ARCHES to contain the set of target
-# arches in the CMake syntax (75-real, 89-virtual, etc), since we clear the
-# arches in the CUDA case (and instead set the gencodes on a per file basis)
-# we need to manually set VLLM_GPU_ARCHES here.
-if(VLLM_GPU_LANG STREQUAL "CUDA")
-  foreach(_ARCH ${CUDA_ARCHS})
-    string(REPLACE "." "" _ARCH "${_ARCH}")
-    list(APPEND VLLM_GPU_ARCHES "${_ARCH}-real")
-  endforeach()
-endif()
-
-#
-# Build vLLM flash attention from source
-#
-# IMPORTANT: This has to be the last thing we do, because vllm-flash-attn uses the same macros/functions as vLLM.
-# Because functions all belong to the global scope, vllm-flash-attn's functions overwrite vLLMs.
-# They should be identical but if they aren't, this is a massive footgun.
-#
-# The vllm-flash-attn install rules are nested under vllm to make sure the library gets installed in the correct place.
-# To only install vllm-flash-attn, use --component _vllm_fa2_C (for FA2) or --component _vllm_fa3_C (for FA3).
-# If no component is specified, vllm-flash-attn is still installed.
-
-# If VLLM_FLASH_ATTN_SRC_DIR is set, vllm-flash-attn is installed from that directory instead of downloading.
-# This is to enable local development of vllm-flash-attn within vLLM.
-# It can be set as an environment variable or passed as a cmake argument.
-# The environment variable takes precedence.
-if (DEFINED ENV{VLLM_FLASH_ATTN_SRC_DIR})
-  set(VLLM_FLASH_ATTN_SRC_DIR $ENV{VLLM_FLASH_ATTN_SRC_DIR})
-endif()
-
-if(VLLM_FLASH_ATTN_SRC_DIR)
-  FetchContent_Declare(
-          vllm-flash-attn SOURCE_DIR
-          ${VLLM_FLASH_ATTN_SRC_DIR}
-          BINARY_DIR ${CMAKE_BINARY_DIR}/vllm-flash-attn
-  )
-else()
-  FetchContent_Declare(
-          vllm-flash-attn
-          GIT_REPOSITORY https://github.com/vllm-project/flash-attention.git
-          GIT_TAG 720c94869cf2e0ff5a706e9c7f1dce0939686ade
-          GIT_PROGRESS TRUE
-          # Don't share the vllm-flash-attn build between build types
-          BINARY_DIR ${CMAKE_BINARY_DIR}/vllm-flash-attn
-  )
-endif()
-
-
-# Fetch the vllm-flash-attn library
-FetchContent_MakeAvailable(vllm-flash-attn)
-message(STATUS "vllm-flash-attn is available at ${vllm-flash-attn_SOURCE_DIR}")
-
-# Copy over the vllm-flash-attn python files (duplicated for fa2 and fa3, in
-# case only one is built, in the case both are built redundant work is done)
-install(
-  DIRECTORY ${vllm-flash-attn_SOURCE_DIR}/vllm_flash_attn/
-  DESTINATION vllm_flash_attn
-  COMPONENT _vllm_fa2_C
-  FILES_MATCHING PATTERN "*.py"
-)
-
-install(
-  DIRECTORY ${vllm-flash-attn_SOURCE_DIR}/vllm_flash_attn/
-  DESTINATION vllm_flash_attn
-  COMPONENT _vllm_fa3_C
-  FILES_MATCHING PATTERN "*.py"
-)
-
-# Nothing after vllm-flash-attn, see comment about macros above

cmake/external_projects/flashmla.cmake

@@ -0,0 +1,66 @@
+include(FetchContent)
+
+# If FLASH_MLA_SRC_DIR is set, flash-mla is installed from that directory 
+# instead of downloading.
+# It can be set as an environment variable or passed as a cmake argument.
+# The environment variable takes precedence.
+if (DEFINED ENV{FLASH_MLA_SRC_DIR})
+  set(FLASH_MLA_SRC_DIR $ENV{FLASH_MLA_SRC_DIR})
+endif()
+
+if(FLASH_MLA_SRC_DIR)
+  FetchContent_Declare(
+        flashmla 
+        SOURCE_DIR ${FLASH_MLA_SRC_DIR}
+        CONFIGURE_COMMAND ""
+        BUILD_COMMAND ""
+  )
+else()
+  FetchContent_Declare(
+        flashmla
+        GIT_REPOSITORY https://github.com/vllm-project/FlashMLA.git
+        GIT_TAG 575f7724b9762f265bbee5889df9c7d630801845
+        GIT_PROGRESS TRUE
+        CONFIGURE_COMMAND ""
+        BUILD_COMMAND ""
+  )
+endif()
+
+
+FetchContent_MakeAvailable(flashmla)
+message(STATUS "FlashMLA is available at ${flashmla_SOURCE_DIR}")
+
+# The FlashMLA kernels only work on hopper and require CUDA 12.3 or later.
+# Only build FlashMLA kernels if we are building for something compatible with 
+# sm90a
+cuda_archs_loose_intersection(FLASH_MLA_ARCHS "9.0a" "${CUDA_ARCHS}")
+if(${CMAKE_CUDA_COMPILER_VERSION} VERSION_GREATER 12.3 AND FLASH_MLA_ARCHS)
+    set(FlashMLA_SOURCES
+        ${flashmla_SOURCE_DIR}/csrc/flash_api.cpp
+        ${flashmla_SOURCE_DIR}/csrc/flash_fwd_mla_bf16_sm90.cu
+        ${flashmla_SOURCE_DIR}/csrc/flash_fwd_mla_fp16_sm90.cu
+        ${flashmla_SOURCE_DIR}/csrc/flash_fwd_mla_metadata.cu)
+
+    set(FlashMLA_INCLUDES
+        ${flashmla_SOURCE_DIR}/csrc/cutlass/include
+        ${flashmla_SOURCE_DIR}/csrc/include)
+
+    set_gencode_flags_for_srcs(
+        SRCS "${FlashMLA_SOURCES}"
+        CUDA_ARCHS "${FLASH_MLA_ARCHS}")
+
+    define_gpu_extension_target(
+        _flashmla_C
+        DESTINATION vllm
+        LANGUAGE ${VLLM_GPU_LANG}
+        SOURCES ${FlashMLA_SOURCES}
+        COMPILE_FLAGS ${VLLM_GPU_FLAGS}
+        ARCHITECTURES ${VLLM_GPU_ARCHES}
+        INCLUDE_DIRECTORIES ${FlashMLA_INCLUDES}
+        USE_SABI 3
+        WITH_SOABI)
+else()
+    # Create an empty target for setup.py when not targeting sm90a systems
+    add_custom_target(_flashmla_C)
+endif()
+

cmake/external_projects/vllm_flash_attn.cmake

@@ -0,0 +1,67 @@
+# vLLM flash attention requires VLLM_GPU_ARCHES to contain the set of target
+# arches in the CMake syntax (75-real, 89-virtual, etc), since we clear the
+# arches in the CUDA case (and instead set the gencodes on a per file basis)
+# we need to manually set VLLM_GPU_ARCHES here.
+if(VLLM_GPU_LANG STREQUAL "CUDA")
+  foreach(_ARCH ${CUDA_ARCHS})
+    string(REPLACE "." "" _ARCH "${_ARCH}")
+    list(APPEND VLLM_GPU_ARCHES "${_ARCH}-real")
+  endforeach()
+endif()
+
+#
+# Build vLLM flash attention from source
+#
+# IMPORTANT: This has to be the last thing we do, because vllm-flash-attn uses the same macros/functions as vLLM.
+# Because functions all belong to the global scope, vllm-flash-attn's functions overwrite vLLMs.
+# They should be identical but if they aren't, this is a massive footgun.
+#
+# The vllm-flash-attn install rules are nested under vllm to make sure the library gets installed in the correct place.
+# To only install vllm-flash-attn, use --component _vllm_fa2_C (for FA2) or --component _vllm_fa3_C (for FA3).
+# If no component is specified, vllm-flash-attn is still installed.
+
+# If VLLM_FLASH_ATTN_SRC_DIR is set, vllm-flash-attn is installed from that directory instead of downloading.
+# This is to enable local development of vllm-flash-attn within vLLM.
+# It can be set as an environment variable or passed as a cmake argument.
+# The environment variable takes precedence.
+if (DEFINED ENV{VLLM_FLASH_ATTN_SRC_DIR})
+  set(VLLM_FLASH_ATTN_SRC_DIR $ENV{VLLM_FLASH_ATTN_SRC_DIR})
+endif()
+
+if(VLLM_FLASH_ATTN_SRC_DIR)
+  FetchContent_Declare(
+          vllm-flash-attn SOURCE_DIR 
+          ${VLLM_FLASH_ATTN_SRC_DIR}
+          BINARY_DIR ${CMAKE_BINARY_DIR}/vllm-flash-attn
+  )
+else()
+  FetchContent_Declare(
+          vllm-flash-attn
+          GIT_REPOSITORY https://github.com/vllm-project/flash-attention.git
+          GIT_TAG 720c94869cf2e0ff5a706e9c7f1dce0939686ade
+          GIT_PROGRESS TRUE
+          # Don't share the vllm-flash-attn build between build types
+          BINARY_DIR ${CMAKE_BINARY_DIR}/vllm-flash-attn
+  )
+endif()
+
+
+# Fetch the vllm-flash-attn library
+FetchContent_MakeAvailable(vllm-flash-attn)
+message(STATUS "vllm-flash-attn is available at ${vllm-flash-attn_SOURCE_DIR}")
+
+# Copy over the vllm-flash-attn python files (duplicated for fa2 and fa3, in
+# case only one is built, in the case both are built redundant work is done)
+install(
+  DIRECTORY ${vllm-flash-attn_SOURCE_DIR}/vllm_flash_attn/
+  DESTINATION vllm_flash_attn
+  COMPONENT _vllm_fa2_C
+  FILES_MATCHING PATTERN "*.py"
+)
+
+install(
+  DIRECTORY ${vllm-flash-attn_SOURCE_DIR}/vllm_flash_attn/
+  DESTINATION vllm_flash_attn
+  COMPONENT _vllm_fa3_C
+  FILES_MATCHING PATTERN "*.py"
+)

setup.py

@@ -324,6 +324,7 @@ def run(self) -> None:
             files_to_copy = [
                 "vllm/_C.abi3.so",
                 "vllm/_moe_C.abi3.so",
+                "vllm/_flashmla_C.abi3.so",
                 "vllm/vllm_flash_attn/_vllm_fa2_C.abi3.so",
                 "vllm/vllm_flash_attn/_vllm_fa3_C.abi3.so",
                 "vllm/vllm_flash_attn/flash_attn_interface.py",
@@ -613,6 +614,11 @@ def _read_requirements(filename: str) -> List[str]:
         # FA3 requires CUDA 12.0 or later
         ext_modules.append(
             CMakeExtension(name="vllm.vllm_flash_attn._vllm_fa3_C"))
+    if envs.VLLM_USE_PRECOMPILED or get_nvcc_cuda_version() >= Version("12.3"):
+        # Optional since this doesn't get built (produce an .so file) when
+        # not targeting a hopper system
+        ext_modules.append(
+            CMakeExtension(name="vllm._flashmla_C", optional=True))
     ext_modules.append(CMakeExtension(name="vllm.cumem_allocator"))
 
 if _build_custom_ops():

tests/kernels/test_flashmla.py

@@ -0,0 +1,132 @@
+# Adapted from: https://github.com/deepseek-ai/FlashMLA/blob/main/tests/test_flash_mla.py
+# SPDX-License-Identifier: Apache-2.0
+import math
+import random
+
+import pytest
+import torch
+import triton
+
+from vllm.attention.ops.flashmla import (flash_mla_with_kvcache,
+                                         get_mla_metadata,
+                                         is_flashmla_supported)
+
+
+def cal_diff(x: torch.Tensor, y: torch.Tensor, name: str) -> None:
+    x, y = x.double(), y.double()
+    cos_diff = 1 - 2 * (x * y).sum().item() / max(
+        (x * x + y * y).sum().item(), 1e-12)
+    assert cos_diff < 1e-5
+
+FLASH_MLA_UNSUPPORTED_REASON = is_flashmla_supported()[1] \
+    if not is_flashmla_supported()[0] else "FlashMLA is supported"
+
+
+@pytest.mark.skipif(not is_flashmla_supported()[0],
+                    reason=FLASH_MLA_UNSUPPORTED_REASON)
+@pytest.mark.parametrize("b", [128])
+@pytest.mark.parametrize("s_q", [1, 2])
+@pytest.mark.parametrize("mean_sk", [4096, 8192])
+@pytest.mark.parametrize("h_q", [16, 32, 64, 128])
+@pytest.mark.parametrize("h_kv", [1])
+@pytest.mark.parametrize("d", [576])
+@pytest.mark.parametrize("dv", [512])
+@pytest.mark.parametrize("block_size", [64])
+@pytest.mark.parametrize("causal", [True])
+@pytest.mark.parametrize("varlen", [False, True])
+@torch.inference_mode()
+def test_flash_mla(b, s_q, mean_sk, h_q, h_kv, d, dv, block_size, causal,
+                   varlen):
+    # TODO: parametrize using pytest
+    dtype = torch.bfloat16
+    device = torch.device("cuda:0")
+    torch.set_default_dtype(dtype)
+    torch.set_default_device(device)
+    torch.cuda.set_device(device)
+    torch.manual_seed(0)
+    random.seed(0)
+
+    print(f"{b=}, {s_q=}, {mean_sk=}, {h_q=}, {h_kv=}, "
+          f"{d=}, {dv=}, {causal=}, {varlen=}")
+
+    cache_seqlens = torch.full((b, ), mean_sk, dtype=torch.int32)
+    if varlen:
+        for i in range(b):
+            cache_seqlens[i] = max(random.normalvariate(mean_sk, mean_sk / 2),
+                                   s_q)
+    total_seqlens = cache_seqlens.sum().item()
+    max_seqlen = cache_seqlens.max().item()
+    max_seqlen_pad = triton.cdiv(max_seqlen, 256) * 256
+
+    q = torch.randn(b, s_q, h_q, d)
+    block_table = torch.arange(b * max_seqlen_pad // block_size,
+                               dtype=torch.int32).view(
+                                   b, max_seqlen_pad // block_size)
+    blocked_k = torch.randn(block_table.numel(), block_size, h_kv, d)
+    for i in range(b):
+        blocked_k.view(b, max_seqlen_pad, h_kv,
+                       d)[i, cache_seqlens[i].item():] = float("nan")
+    blocked_v = blocked_k[..., :dv]
+
+    tile_scheduler_metadata, num_splits = get_mla_metadata(
+        cache_seqlens, s_q * h_q // h_kv, h_kv)
+
+    def flash_mla():
+        return flash_mla_with_kvcache(
+            q,
+            blocked_k,
+            block_table,
+            cache_seqlens,
+            dv,
+            tile_scheduler_metadata,
+            num_splits,
+            causal=causal,
+        )
+
+    def scaled_dot_product_attention(query, key, value, is_causal=False):
+        query = query.float()
+        key = key.float()
+        value = value.float()
+        key = key.repeat_interleave(h_q // h_kv, dim=0)
+        value = value.repeat_interleave(h_q // h_kv, dim=0)
+        attn_weight = query @ key.transpose(-2, -1) / math.sqrt(query.size(-1))
+        if is_causal:
+            s_q = query.shape[-2]
+            s_k = key.shape[-2]
+            attn_bias = torch.zeros(s_q, s_k, dtype=query.dtype)
+            temp_mask = torch.ones(s_q, s_k,
+                                   dtype=torch.bool).tril(diagonal=s_k - s_q)
+            attn_bias.masked_fill_(temp_mask.logical_not(), float("-inf"))
+            attn_bias.to(query.dtype)
+            attn_weight += attn_bias
+        lse = attn_weight.logsumexp(dim=-1)
+        attn_weight = torch.softmax(attn_weight, dim=-1, dtype=torch.float32)
+        return attn_weight @ value, lse
+
+    def ref_mla():
+        out = torch.empty(b, s_q, h_q, dv, dtype=torch.float32)
+        lse = torch.empty(b, h_q, s_q, dtype=torch.float32)
+        for i in range(b):
+            begin = i * max_seqlen_pad
+            end = begin + cache_seqlens[i]
+            ref_O, LSE = scaled_dot_product_attention(
+                q[i].transpose(0, 1),
+                blocked_k.view(-1, h_kv, d)[begin:end].transpose(0, 1),
+                blocked_v.view(-1, h_kv, dv)[begin:end].transpose(0, 1),
+                is_causal=causal,
+            )
+            out[i] = ref_O.transpose(0, 1)
+            lse[i] = LSE
+        return out, lse
+
+    out_flash, lse_flash = flash_mla()
+    out_torch, lse_torch = ref_mla()
+    cal_diff(out_flash, out_torch, "out")
+    cal_diff(lse_flash, lse_torch, "lse")
+
+    t = triton.testing.do_bench(flash_mla, fast_flush=False)
+    FLOPS = s_q * total_seqlens * h_q * (d + dv) * 2
+    bytes = (total_seqlens * h_kv * d + b * s_q * h_q * d +
+             b * s_q * h_q * dv) * (torch.finfo(dtype).bits // 8)
+    print(f"{t:.3f} ms, {FLOPS / 10 ** 9 / t:.0f} "
+          f"TFLOPS, {bytes / 10 ** 6 / t:.0f} GB/s")