Reapply "A few fixes in the threadpool semaphore. Unify Windows/Unix implementation of LIFO policy." (#125193)

[VSadov]

Re: #125193

TODO: need to confirm that NuGet restore performance is ok with the updated change.

[dotnet-policy-service]

Tagging subscribers to this area: @agocke, @VSadov
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Reapplies and reworks the ThreadPool LIFO semaphore changes (previously reverted due to NuGet restore regressions) by unifying the blocking/wake implementation across Windows and Unix using OS compare-and-wait primitives (WaitOnAddress / futex) with a monitor fallback.

Changes:

• Adds low-level compare-and-wait interop for Windows (WaitOnAddress) and Linux (futex) and wires them through System.Native/CoreLib.
• Replaces the prior per-OS LowLevelLifoSemaphore implementations with a unified managed implementation using a LIFO stack of per-thread blockers plus updated spin/backoff behavior.
• Adjusts worker dispatch heuristics (missed-steal handling) and configuration plumbing (cooperative blocking env var alias).

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/native/libs/System.Native/pal_threading.h	Adds exported futex-related entrypoints to the System.Native PAL surface.
src/native/libs/System.Native/pal_threading.c	Implements Linux futex wait/wake wrappers; provides non-Linux stubs.
src/native/libs/System.Native/entrypoints.c	Registers the new futex entrypoints for managed interop.
src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolWorkQueue.cs	Adds a 1ms sleep before requesting workers when a steal was missed.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.WorkerThread.cs	Switches the worker wait to the new LowLevelLifoSemaphore.Wait(timeout, activeThreadCount) signature; removes old spin-limit wiring at the call site.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.Blocking.cs	Adds the DOTNET_ThreadPool_CooperativeBlocking env var alias for cooperative blocking.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelThreadBlocker.cs	Introduces a portable blocker abstraction (futex/WaitOnAddress or LowLevelMonitor fallback).
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs	Replaces OS-specific semaphore core with a unified managed LIFO implementation + updated spin heuristic and wake accounting.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Windows.cs	Removes the prior Windows IOCP-based LIFO semaphore implementation.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Unix.cs	Removes the prior Unix WaitSubsystem-based semaphore implementation.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Windows.cs	Adds Windows WaitOnAddress-based compare-and-wait wrapper.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Unix.cs	Adds Unix futex wrapper (currently Linux-only per comments).
src/libraries/System.Private.CoreLib/src/System/Threading/Backoff.cs	Updates exponential backoff to return spin count and reduces max backoff.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Wires new threading and interop files into CoreLib build (adds/removes Compile items).
src/libraries/Common/src/Interop/Windows/Mincore/Interop.WaitOnAddress.cs	Adds LibraryImport declarations for WaitOnAddress/WakeByAddressSingle.
src/libraries/Common/src/Interop/Windows/Kernel32/Interop.CriticalSection.cs	Adds SuppressGCTransition on LeaveCriticalSection.
src/libraries/Common/src/Interop/Windows/Interop.Libraries.cs	Adds the Synch API-set library constant for WaitOnAddress imports.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.LowLevelMonitor.cs	Adds SuppressGCTransition on LowLevelMonitor_Release.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.Futex.cs	Adds LibraryImport declarations for futex wait/wake entrypoints.
src/coreclr/tools/aot/ILCompiler/reproNative/reproNative.vcxproj	Adds Synchronization.lib to link set for NativeAOT repro project.
src/coreclr/nativeaot/BuildIntegration/WindowsAPIs.txt	Allows WaitOnAddress/WakeByAddressSingle through the NativeAOT Windows API allowlist.
src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Windows.targets	Adds Synchronization.lib to NativeAOT SDK library list.
docs/coding-guidelines/interop-guidelines.md	Updates interop guideline examples to match casing/structure and adds Synch library mention.

---

You can also share your feedback on Copilot code review. Take the survey.

[Copilot]

Pull request overview

Reapplies and extends the threadpool semaphore/LIFO-policy changes that were previously reverted due to NuGet restore performance regression, by unifying the Windows/Unix implementation around a shared managed LIFO waiter stack and adding low-level wait/wake primitives (Linux futex, Windows WaitOnAddress) plus supporting interop/AOT wiring.

Changes:

• Add Linux futex exports in System.Native and corresponding managed interop; add Windows WaitOnAddress interop and link inputs for NativeAOT.
• Replace platform-specific LowLevelLifoSemaphore implementations with a unified managed implementation built on LowLevelThreadBlocker.
• Adjust threadpool behavior around missed steals (including a brief delay) and tweak a blocking config switch plumbing.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/native/libs/System.Native/pal_threading.h	Adds futex-related PALEXPORT declarations.
src/native/libs/System.Native/pal_threading.c	Implements Linux futex wait/wake syscalls (with non-Linux stubs).
src/native/libs/System.Native/entrypoints.c	Exposes futex entrypoints via DllImportEntry.
src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolWorkQueue.cs	Adds a delay before requesting a worker when missed steals occur.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.WorkerThread.cs	Switches semaphore construction/Wait signature; minor comment fix.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.Blocking.cs	Adds env-var name to cooperative blocking config lookup.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelThreadBlocker.cs	Introduces a portable blocker using futex/WaitOnAddress or monitor fallback.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs	Reworks semaphore into a single managed implementation with LIFO waiter stack + spin heuristic.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Windows.cs	Removes prior Windows IOCP-based implementation.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Unix.cs	Removes prior Unix WaitSubsystem-based implementation.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Windows.cs	Adds Windows WaitOnAddress/WakeByAddressSingle wrapper.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Unix.cs	Adds Linux futex wrapper (Linux-only).
src/libraries/System.Private.CoreLib/src/System/Threading/Backoff.cs	Changes exponential backoff to return spin count and reduces cap.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Wires new threading/interop files into CoreLib build and removes old semaphore OS-specific files.
src/libraries/Common/src/Interop/Windows/Mincore/Interop.WaitOnAddress.cs	Adds LibraryImport for WaitOnAddress/WakeByAddressSingle.
src/libraries/Common/src/Interop/Windows/Kernel32/Interop.CriticalSection.cs	Adds SuppressGCTransition to LeaveCriticalSection.
src/libraries/Common/src/Interop/Windows/Interop.Libraries.cs	Adds Libraries.Synch constant for the synch api-set.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.LowLevelMonitor.cs	Adds SuppressGCTransition to LowLevelMonitor_Release.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.Futex.cs	Adds LibraryImport declarations for System.Native futex exports.
src/coreclr/tools/aot/ILCompiler/reproNative/reproNative.vcxproj	Links Synchronization.lib for WaitOnAddress/WakeByAddressSingle.
src/coreclr/nativeaot/BuildIntegration/WindowsAPIs.txt	Adds WaitOnAddress/WakeByAddressSingle to the NativeAOT Windows API list.
src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Windows.targets	Adds Synchronization.lib to SDK native libraries for NativeAOT.
docs/coding-guidelines/interop-guidelines.md	Updates interop naming/examples (e.g., Mincore, Synch).

---

You can also share your feedback on Copilot code review. Take the survey.