Fix cpuinfo init on Linux without CPU sysfs lists

[tianleiwu]

Description

Fixes ONNX Runtime startup on Linux ARM64 environments where /sys/devices/system/cpu/possible and /sys/devices/system/cpu/present are unavailable, such as AWS Lambda ARM64/Graviton and restricted build sandboxes.

There are two related failure modes:

1. PosixEnv may be constructed before ORT's default logger is registered. If cpuinfo_initialize() fails during that early construction path, the existing LOGS_DEFAULT(INFO) call can terminate with Attempt to use DefaultLogger but none has been registered.
2. The bundled pytorch/cpuinfo code treats missing Linux CPU possible/present sysfs cpulists as fatal on ARM Linux. The max-count helpers return UINT32_MAX, which wraps to 0 after 1 + UINT32_MAX in ARM Linux initialization and prevents cpuinfo from reaching the later /proc/cpuinfo and getauxval() based detection paths.

Root Cause

The immediate import crash is caused by unsafe early logging in onnxruntime/core/platform/posix/env.cc. Python bindings can reference Env::Default() during module load before logging is initialized, so a cpuinfo initialization failure must not use LOGS_DEFAULT() unless a default logger exists.

The cpuinfo initialization failure is more subtle. A count-only fallback is not enough: after cpuinfo computes max possible/present CPU counts, it calls cpuinfo_linux_detect_possible_processors() and cpuinfo_linux_detect_present_processors() to set CPUINFO_LINUX_FLAG_POSSIBLE and CPUINFO_LINUX_FLAG_PRESENT on each processor. ARM Linux initialization later marks processors valid only if those flags are set. If only the count fallback is provided, valid_processors can remain zero and cpuinfo can proceed into an invalid partial initialization state.

Fix

• Make PosixEnv logging safe when cpuinfo initialization fails before a default logger exists:
  • use logging::LoggingManager::HasDefaultLogger() before LOGS_DEFAULT()
  • fall back to std::cerr when no logger is registered
• Add a cpuinfo patch for Linux missing sysfs CPU cpulists:
  • fallback max possible/present processor detection to sysconf(_SC_NPROCESSORS_ONLN) - 1
  • fallback present/possible processor flag detection by marking CPUs 0..nproc-1
  • preserve existing sysfs parsing behavior when the cpulist files are available
• Wire the cpuinfo patch into the existing cpuinfo FetchContent flow for Linux and existing ARM64/ARM64EC patch path.
• Add a simulation test that validates:
  • safe early logging without a registered default logger
  • sysconf(_SC_NPROCESSORS_ONLN) count and present/possible flag fallback behavior
  • hiding /sys/devices/system/cpu/{possible,present} via LD_PRELOAD
  • optional ORT import with hidden sysfs when a built ORT package is importable

Testing

Ran from a clean branch/worktree:

python onnxruntime/test/common/test_cpuinfo_sysfs_fallback.py

Result:

• safe logging simulation: PASS
• sysconf count + flag fallback simulation: PASS
• LD_PRELOAD sysfs-hiding simulation: PASS
• ORT import integration: SKIP (onnxruntime.capi not built/importable in this workspace)

Also validated the cpuinfo patch directly:

cd build/cu128/Release/_deps/pytorch_cpuinfo-src
patch --dry-run -p1 < /path/to/cmake/patches/cpuinfo/fix_missing_sysfs_fallback.patch

And syntax-checked patched src/linux/processors.c in a temporary tree with cpuinfo headers.

Related Issue

Fixes #10038.

[Copilot]

Pull request overview

Fixes ONNX Runtime startup failures on Linux ARM64 environments where /sys/devices/system/cpu/{possible,present} are unavailable by (1) making early cpuinfo-init logging safe before a default logger exists, and (2) patching the bundled pytorch/cpuinfo to fall back to sysconf(_SC_NPROCESSORS_ONLN) for both CPU counts and per-CPU present/possible flags.

Changes:

• Guard LOGS_DEFAULT(...) usage in PosixEnv so cpuinfo init failures won’t crash when logging hasn’t been initialized yet.
• Patch pytorch/cpuinfo Linux processor detection to provide robust sysfs-missing fallbacks (counts + flags).
• Add a standalone simulation script to validate the early-logging and sysfs-missing behaviors (incl. LD_PRELOAD sysfs hiding).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
onnxruntime/core/platform/posix/env.cc	Avoids crashing during early PosixEnv construction by falling back to std::cerr when no default logger exists.
cmake/external/onnxruntime_external_deps.cmake	Wires in the new cpuinfo patch during FetchContent dependency setup (Linux + ARM64/ARM64EC patch flow).
cmake/patches/cpuinfo/fix_missing_sysfs_fallback.patch	Adds sysconf(_SC_NPROCESSORS_ONLN)-based fallbacks for max CPU count and present/possible flags when sysfs cpulists are missing.
onnxruntime/test/common/test_cpuinfo_sysfs_fallback.py	Adds a manual/simulation validation script (compiles small programs + LD_PRELOAD shim).

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[yuslepukhin]