Suppress wake-up preemption on Linux in LowLevelLifoSemaphore (parallel to Windows SetThreadPriorityBoost)

[Copilot]

Note

AI-generated draft.

This change implements the Linux counterpart to the existing Windows wake-preemption suppression used around LowLevelLifoSemaphore.Block. The goal is the same on both platforms: a worker waking from park should rejoin queue processing without receiving a transient wake-up scheduling advantage over already-running workers.

• LowLevelLifoSemaphore integration

  • Replaced the Windows-only SetThreadPriorityBoost block in Block with platform-agnostic SuppressWakePreemption() / RestoreWakePreemption(...) helpers.
  • Kept suppression scoped exactly to the _blocker.TimedWait(...) window.

• Linux wake-preemption mechanism (System.Native)

  • Added SystemNative_SuppressWakePreemption() — sets the calling thread to SCHED_BATCH before the wait.
  • Added SystemNative_RestoreWakePreemption() — restores the thread to the default SCHED_OTHER (priority 0) after the wait.
  • Both functions are parameterless; no previous state is captured or restored. Threadpool threads are expected to run at default scheduler settings, and reverting to defaults is the correct behavior even if a user changed the thread's scheduler settings.
  • Non-Linux Unix targets compile the functions as no-ops.

• Managed interop and wrapper

  • Added Unix interop surface: Interop.WakePreemption.cs.
  • Added LowLevelLifoSemaphore.WakePreemption.cs with WakePreemptionScope:
    • Windows: wraps SetThreadPriorityBoost(true/false).
    • Linux: sets SCHED_BATCH on suppress and restores SCHED_OTHER on restore; WakePreemptionScope carries a single bool Suppressed field.
    • Other platforms: no-op.
  • Added Debug.Assert on the return values of both SuppressWakePreemption and RestoreWakePreemption interop calls to catch unexpected syscall failures in debug builds. Failures are otherwise ignored as benign.

• Entry point and project plumbing

  • Registered new native exports in entrypoints.c (unconditional).
  • Wired new interop and wrapper source files into System.Private.CoreLib.Shared.projitems.

if (blockerNode != null)
{
    WakePreemptionScope wakePreemptionScope = SuppressWakePreemption();
    try
    {
        while (!blockerNode._blocker.TimedWait(timeoutMs))
        {
            if (TryRemove(blockerNode))
                return false;

            timeoutMs = 10;
        }
    }
    finally
    {
        RestoreWakePreemption(wakePreemptionScope);
    }
}

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[Copilot]

Pull request overview

This PR extends LowLevelLifoSemaphore.Block to suppress OS “wakeup advantage” during the blocking wait window, generalizing the existing Windows behavior (priority-boost suppression) and adding a Unix/Linux-native mechanism via System.Native.

Changes:

• Added SystemNative_SuppressWakePreemption / SystemNative_RestoreWakePreemption to System.Native and wired them into entrypoints.c.
• Introduced Unix interop (Interop.WakePreemption.cs) and a managed wrapper (LowLevelLifoSemaphore.WakePreemption.cs) to scope suppression/restoration.
• Updated LowLevelLifoSemaphore.Block to use the new platform-agnostic suppression helpers instead of Windows-only SetThreadPriorityBoost directly.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/native/libs/System.Native/pal_threading.h	Declares new wake-preemption suppression/restoration exports.
src/native/libs/System.Native/pal_threading.c	Implements suppression/restoration (Linux uses sched_*; other targets are stubbed).
src/native/libs/System.Native/entrypoints.c	Registers new exports in the System.Native entrypoint table.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.WakePreemption.cs	Adds LibraryImport declarations for the new System.Native APIs.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.WakePreemption.cs	Adds managed WakePreemptionScope + suppress/restore helpers (Windows/Linux/no-op).
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs	Uses the new suppression scope around _blocker.TimedWait(...) and updates the explanatory comment.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Wires the new interop + managed helper file into the build.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

src/native/libs/System.Native/entrypoints.c:262

• SystemNative_SuppressWakePreemption / SystemNative_RestoreWakePreemption are added to the s_sysNative entrypoint table unconditionally, but pal_threading_wasi.c (used for WASI per System.Native/CMakeLists.txt) does not define these functions. Because DllImportEntry takes the address of the symbol, this will cause a link failure for WASI static builds. The simplest fix is to put these entries under the existing #if !defined(TARGET_WASI) guard with the futex entries.

    DllImportEntry(SystemNative_LowLevelFutex_WakeByAddressSingle)
#endif
    DllImportEntry(SystemNative_SuppressWakePreemption)
    DllImportEntry(SystemNative_RestoreWakePreemption)
    DllImportEntry(SystemNative_LoadLibrary)

[VSadov]

@copilot there are failures on musl. Can that be fixed?

[Copilot]

@copilot there are failures on musl. Can that be fixed?

Fixed in 7aac3f78.

The musl failures were due to debug assert aborts when sched_setscheduler returned ENOSYS. I updated SuppressWakePreemption assert logic to treat ENOSYS as an expected non-asserting outcome while still asserting on other unexpected errors.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/native/libs/System.Native/entrypoints.c:262

• These new DllImportEntry(SystemNative_SuppressWakePreemption) / ...RestoreWakePreemption entries are unconditionally taking the address of the symbols (see minipal/entrypoints.h), but the WASI build uses pal_threading_wasi.c (CMakeLists.txt) which does not define these functions. This will cause undefined-symbol link failures for WASI static builds unless the entries are guarded out (similar to the futex entries) or WASI stubs are added.

#if !defined(TARGET_WASI)
    DllImportEntry(SystemNative_LowLevelFutex_WaitOnAddress)
    DllImportEntry(SystemNative_LowLevelFutex_WaitOnAddressTimeout)
    DllImportEntry(SystemNative_LowLevelFutex_WakeByAddressSingle)
#endif
    DllImportEntry(SystemNative_SuppressWakePreemption)
    DllImportEntry(SystemNative_RestoreWakePreemption)
    DllImportEntry(SystemNative_LoadLibrary)

[VSadov]

@copilot i think we do not want to exclude musl. Can we bypass libc and do syscall directly?

[Copilot]

@copilot i think we do not want to exclude musl. Can we bypass libc and do syscall directly?

Yes — done in 47a711eb.

musl deliberately stubs sched_setscheduler to always fail with ENOSYS (it doesn't implement the POSIX process-wide semantics). The kernel interface, however, is per-thread, which is exactly what we want here, so the native helper now issues syscall(SYS_sched_setscheduler, 0, policy, &param) directly with a local struct matching the kernel's single-member sched_param. SCHED_BATCH is also defined locally if the libc headers don't provide it.

With that, musl behaves the same as glibc, so I removed the ENOSYS exemption from the Debug.Assert — any failure now asserts.

Verified locally: the direct syscall succeeds for both SCHED_BATCH and SCHED_OTHER, and native libs + CoreLib build clean.

Note

This comment was generated by GitHub Copilot.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/native/libs/System.Native/entrypoints.c:261

• SystemNative_SuppressWakePreemption / SystemNative_RestoreWakePreemption are exported unconditionally, but System.Native uses pal_threading_wasi.c for WASI builds (see src/native/libs/System.Native/CMakeLists.txt), and that file does not define these symbols. This will cause a WASI link failure similar to why futex exports are guarded out for TARGET_WASI.

    DllImportEntry(SystemNative_LowLevelFutex_WakeByAddressSingle)
#endif
    DllImportEntry(SystemNative_SuppressWakePreemption)
    DllImportEntry(SystemNative_RestoreWakePreemption)