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2026-05-02-cyclictest-musl-scheduler.md

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2026-05-02 cyclictest-musl scheduler syscall investigation

Background

The cyclictest-musl OS COMP test failed with:

unable to get scheduler parameters

The log showed sys_get_mempolicy (stub) immediately before the failure, so the first suspicion was that the NUMA compatibility stub might affect cyclictest.

Findings

sys_get_mempolicy was not the direct cause. It returned success and execution continued into scheduler capability checks.

The failing message comes from rt-tests check_privs() when libc sched_getparam(0, &old_param) returns an error.

Kernel logs showed sched_getaffinity was called and returned successfully, but there were no kernel-side logs for:

  • sched_getparam
  • sched_getscheduler
  • sched_setparam
  • sched_setscheduler

This meant the calls were failing before entering the kernel.

I extracted and inspected the LoongArch64 musl loader from the generated rootfs:

/lib64/ld-musl-loongarch-lp64d.so.1

Its scheduler entry points were local ENOSYS stubs. For example, sched_getparam loaded -38 (ENOSYS) into a0 and returned through __syscall_ret, without issuing a syscall.

I also checked sdcard-la.img:/musl/lib/libc.so as requested. It is a LoongArch64 musl shared object, but its implementations of these functions are also ENOSYS stubs, so it cannot directly fix the issue.

Changes

Kernel scheduler compatibility

Updated:

  • pulse_syscalls/src/impls/task/schedule.rs
  • pulse_syscalls/src/handler.rs

Added or improved compatibility for:

  • sched_getaffinity
  • sched_setaffinity
  • sched_getscheduler
  • sched_setscheduler
  • sched_getparam
  • sched_setparam
  • sched_get_priority_max
  • sched_get_priority_min
  • sched_rr_get_interval
  • sched_setattr
  • sched_getattr

The compatibility behavior reports a fixed RT-style scheduler view sufficient for cyclictest privilege and parameter checks. It does not implement real per-task RT scheduling semantics.

LoongArch64 musl loader handling

Updated:

  • build_img.sh

The image builder now handles LoongArch64 musl scheduler stubs as follows:

  1. Check whether /musl/lib/libc.so exists in the staged rootfs.
  2. If that bundled musl object does not contain the scheduler ENOSYS stubs, install it as /lib64/ld-musl-loongarch-lp64d.so.1.
  3. If the bundled object is also a stub, keep the Alpine loader and apply a compatibility patch to only the affected scheduler wrapper bytes.

In the current test image, /musl/lib/libc.so is also a stub, so the fallback patch path is used.

Verification

Ran:

make test

Result: build and rootfs generation completed successfully.

The LoongArch64 rootfs build printed:

[loongarch64] Patching musl scheduler ENOSYS stubs

After rebuilding, I extracted rootfs-loongarch64.img:/lib64/ld-musl-loongarch-lp64d.so.1 and confirmed by disassembly that:

  • sched_getparam issues syscall 121
  • sched_getscheduler issues syscall 120
  • sched_setparam issues syscall 118
  • sched_setscheduler issues syscall 119

Attempting to run the LoongArch64 QEMU command locally failed because sdcard-la.img is owned by root:root and QEMU could not open it:

Could not open 'sdcard-la.img': Permission denied

No permission workaround was attempted.