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[Build] Compile FlashAttention on arm64 platforms #79488

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Merged
SigureMo merged 3 commits into from
Jul 17, 2026
+3 −2
Merged

[Build] Compile FlashAttention on arm64 platforms #79488

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469767f
Update GPU third-party conditions
liuhao2638 Jul 16, 2026
9034ce9
ci: rerun PR checks
liuhao2638 Jul 17, 2026
abe4c2e
fix flashattn cache on arm builds
liuhao2638 Jul 17, 2026
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2 changes: 0 additions & 2 deletions cmake/third_party.cmake
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Original file line number Diff line number Diff line change
Expand Up @@ -349,7 +349,6 @@ include(external/glog) # download, build, install glog

########################### include third_party according to flags ###############################
if(WITH_GPU
AND NOT WITH_ARM
AND NOT WIN32
AND NOT APPLE)
if(${CMAKE_CUDA_COMPILER_VERSION} GREATER_EQUAL 11.0)
Expand Down Expand Up @@ -601,7 +600,6 @@ if(WITH_ROCM)
endif()

if(WITH_GPU

@risemeup1111 risemeup1111 Jul 16, 2026

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P1 优先级：P1

处理要求：请针对该评论修复并提交新的 commit。

这里放开 ARM 后，CUDA ARM 构建会进入 FlashAttention 外部依赖块，但 cmake/external/flashattn.cmake 的缓存下载路径只由 NVCC_ARCH_BIN 和 FLASHATTN_TAG 组成（https://.../flash-attention/cu${FA_NVCC_ARCH_BIN}/flashattn_libs_${FLASHATTN_TAG}.tar），没有区分 aarch64/x86_64。下载成功时还会直接复制缓存里的 .so 并设置 SKIP_BUILD_FA ON，因此 ARM 机器可能复用 x86_64 产物，后续链接、打包或运行会失败。

请在启用 ARM 的同时补齐主机架构处理：要么让缓存 key/URL 包含 host arch 并提供对应的 aarch64 缓存包；要么在 WITH_ARM 时强制关闭缓存、走本机源码编译。修复形态例如：

# cmake/external/flashattn.cmake
if(WITH_ARM)
  set(WITH_FA_BUILD_WITH_CACHE OFF)
endif()

@risemeup1111 risemeup1111 Jul 16, 2026

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P1 优先级：P1

处理要求：请针对该评论修复并提交新的 commit。

复查当前 head 469767fb2b22bdb0cd69b08de949feb0e4045baa 后，b0339c695d890caa01ac9f035fb08b1bfe50f597..469767fb2b22bdb0cd69b08de949feb0e4045baa 在相关文件没有差异；cmake/external/flashattn.cmake 仍然使用 cu${FA_NVCC_ARCH_BIN}/flashattn_libs_${FLASHATTN_TAG}.tar 作为缓存路径，并在下载成功后复制 .so、设置 SKIP_BUILD_FA ON。因此原问题没有被修复：ARM 构建仍可能复用未区分 host arch 的缓存产物。

请继续按上面的建议补齐 host arch 缓存隔离，或在 WITH_ARM 下关闭 FlashAttention 缓存并走本机编译。

@liuhao2638 liuhao2638 Jul 16, 2026

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CI中没有arm的编译拦截，所以这一条可以豁免

@risemeup1111 risemeup1111 Jul 16, 2026

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P1 优先级：P1

处理要求：请针对该评论修复并提交新的 commit。

这里不能因为 CI 里没有 ARM 编译拦截就豁免。这个问题不是“CI 是否覆盖 ARM”，而是本 PR 放开 ARM 后，真实的 ARM64 GPU 构建会进入这段 FlashAttention 逻辑；当前缓存 URL 仍不区分 host arch，下载成功时会直接复制缓存 .so 并 SKIP_BUILD_FA ON，因此 ARM 构建/打包存在误用非 ARM 产物的风险。缺少 ARM CI 只能说明这个风险不会被自动发现，不能说明代码路径是安全的。

如果只是豁免 ARM CI 验证，可以在测试计划里说明；但代码层面仍需要二选一处理：为 FlashAttention 缓存增加 host arch 维度并提供对应产物，或在 WITH_ARM 下关闭缓存走本机编译，例如：

if(WITH_ARM)
  set(WITH_FA_BUILD_WITH_CACHE OFF)
endif()

@liuhao2638 liuhao2638 Jul 16, 2026

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我们永远不会在CI之外使用编译cache

@risemeup1111 risemeup1111 Jul 16, 2026

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P1 优先级：P1

处理要求：请针对该评论修复并提交新的 commit。

我理解你说的是使用约定，但当前仓库没有把“编译 cache 只在 CI 使用”这个约束写进构建逻辑：CMakeLists.txt 里 WITH_FA_BUILD_WITH_CACHE 默认是 ON，cmake/external/flashattn.cmake 也只判断这个 CMake 变量，没有判断是否处于 CI 环境。因此任意源码构建只要没有显式关掉该选项，WITH_GPU + WITH_ARM 仍会进入下载缓存并复制 .so 的路径。

如果项目约定确实是 cache 只服务 CI，请把这个约束编码到构建系统里，例如把默认值改为 OFF，只在 CI 脚本中显式打开；或者至少在 ARM 下强制关闭：

if(WITH_ARM)
  set(WITH_FA_BUILD_WITH_CACHE OFF)
endif()

这样代码行为才和“CI-only cache”的约定一致，也能避免后续手动/发布 ARM 构建误用缓存产物。

@liuhao2638 liuhao2638 Jul 17, 2026

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Done. 已在 commit abe4c2ea21 中修复：WITH_ARM 下关闭 FlashAttention 缓存复用，避免 ARM64 GPU 构建下载/复用未区分 host arch 的缓存包，改为走本机源码编译。

@risemeup1111 risemeup1111 Jul 17, 2026

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已确认，abe4c2ea21 中在 WITH_ARM 下关闭了 FlashAttention cache，原先担心的 ARM 复用非 ARM 缓存包问题已消除。

@PaddlePaddle-bot PaddlePaddle-bot Jul 16, 2026

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🔴 Bug 这里允许 ARM64 CUDA 构建进入 FlashAttention external 逻辑，但 cmake/external/flashattn.cmake 的缓存包 URL 只按 NVCC_ARCH_BIN 拼接，没有区分宿主 CPU 架构；默认 WITH_FA_BUILD_WITH_CACHE=ON 时，一旦该 URL 命中缓存，就会复制缓存里的 .so 并设置 SKIP_BUILD_FA=ON，ARM64 构建不会从源码产出 aarch64 的 libflashattn.so。

这会让 ARM64 wheel/安装包携带非本机架构的 FlashAttention DSO，后续 dynload 失败或运行时不可用。

建议修复方式：在放开 ARM 前，让 FlashAttention 缓存路径包含宿主架构并提供对应 aarch64 产物；或者在 WITH_ARM/CMAKE_SYSTEM_PROCESSOR=aarch64 时强制关闭缓存并走源码构建，例如给 external 传 -DSKIP_BUILD_FA=OFF/禁用 WITH_FA_BUILD_WITH_CACHE，确保安装目录中的 .so 由当前 ARM64 toolchain 构建。

AND NOT WITH_ARM
AND NOT WIN32
AND NOT APPLE)
if(${CMAKE_CUDA_COMPILER_VERSION} GREATER_EQUAL 12.3
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