nxnative-easy/docs/reference/threads.md

Threads

Guest threads, their synchronization and how the host schedules them. Each entry: symptom, cause, fix, the games it was found on. The symptom index lists every symptom.

Thread creation and cores

Starting a higher-priority thread on the caller's core

• Symptom: a parent returns from StartThread before a newly runnable higher-priority child pinned to its core finishes its work (deterministic regression, not attributed to a game failure).
• Cause: StartThread signalled the child's pthread but did not request the priority handoff already used when waking a waiter. Atmosphère's KThread::Run makes the child runnable; scheduler priority rules then apply. It does not promise that an equal-priority or other-core child has entered its function before the call returns.
• Fix: note_runnable records the same handoff for starts and wakes (hle_thread.c). No unconditional child-entry handshake or timeout.
• Check: make -C dm/host/tests GAME=010038B015560000 preempt delays the child's host dispatch by 1 ms and 20 ms. With DM_PREEMPT=0, both delayed starts failed before the fix. Afterward both DM_PREEMPT=0 and 1 pass; the broader mode already supplied the handoff before this correction. Equal-priority, other-core, migratable, host-helper, suspended and host-GPU-work children do not hold up the caller.
• Found on: the threading audit of FINAL FANTASY TACTICS; its affected workers are lower priority and initially migratable, so this correction does not remove its local wake cut.

Core mask changes move the ideal core into the new mask

• Symptom: per-core job queues never drained.
• Cause: a SetThreadCoreMask that pins a thread to other cores (ideal core −3 = keep) left the ideal core outside the new mask, so GetCurrentProcessorNumber reported a core the thread cannot run on.
• Fix: the host moves the ideal core into the new mask, as Horizon does, so GetCurrentProcessorNumber reports a core the thread can run on.

Ideal core −2 resolves to the process default core

• Symptom: crash indexing a per-core table with a huge offset (index −2).
• Cause: threads created with ideal core −2 (use process default) were recorded as core −2, and GetCurrentProcessorNumber returned −2.
• Fix: −2 resolves to the process default core (0), and a negative core is never returned (hle_thread.c).
• Found on: Super Mario Bros. Wonder, in its font engine (main+0x107dbc0).

Free thread count (svcGetInfo type 24)

• Symptom: the main thread waits forever on an event right after nnMain starts, with only three or four threads created; DM_THR=1 shows no CreateThread for the thread it waits on.
• Cause: Unity's thread start (main+0x3e1a50) creates a thread only when nn::os::GetFreeThreadCount() is at least 1. That is svcGetInfo type 24 (FreeThreadCount, Horizon 11 and later), which the host answered 0 like every type it did not know, so the thread was never asked for. The kernel answers the thread resource limit less the threads alive (Atmosphère kern_svc_info.cpp); pm gives an application 96 threads (pm_spec.cpp, 256 with a boost).
• Fix: the host answers 96 less the game's threads that have not exited (thr_limit_used; guest threads the host spawns for itself are left out).
• Found on: Monster Train 2 (Unity 2022.3.54f1 on SDK 18.3.5: main+0x3e7288 starts a thread and waits for it to signal main+0x539cf80). With the fix the game creates 40 threads and reaches its title.
• Note: of the 20 games extracted on this machine, only this main imports the function.

Pausing threads and reading their context for garbage collectors

• Symptom: in garbage-collected titles (Unity / IL2CPP), live objects freed, with their finalizers releasing native resources.
• Cause: the Boehm collector pauses each thread, reads its registers for roots and scans its stack. SetThreadActivity (0x32) and GetThreadContext3 (0x33) were placeholders that only reported success: threads kept running and the context was empty, so roots held only in registers were missed.
• Fix: both really pause and report registers (hle_thread.c). A thread inside an SVC counts as paused at once, with the gate frame as its context (thr_svc_enter / thr_svc_leave around every gate SVC; it waits at the SVC exit while paused). A thread in guest code gets SIGUSR2, and the handler saves the ucontext and waits there. A thread in host code is retried every 100 µs. The context is Horizon's 0x320-byte ThreadContext.
• Found on: Unity / IL2CPP titles in general.

Same-core wake handoffs

• Symptom: crashes around movie changes in the game's own player threads, about one run in two: SignalEvent → MultiWaitImpl::NotifyAndWakeupThread on a null MultiWait at address 0x20; WaitAny walking freed holders; a worker reading a freed object at main+0x225338.
• Cause: Horizon schedules the highest-priority runnable thread on a core (Mesosphere's scheduler). A lower-priority thread that wakes a higher-priority thread pinned to its core cannot continue until that work blocks. The host's pthreads run in parallel. The earlier approximation also confused host dispatch state with guest readiness: wake() cleared the guest wait before the pthread resumed, but thread_info could still report that pthread waiting. The caller then continued too soon. Its 5 ms timeout opened the same hole under a longer host delay; the recorded yield_pending flag was unused unless the profile enabled checks after every thread SVC.
• Fix: every game's thread SVC now honours yield_pending. Before returning, it waits for the higher-priority peers pinned to the caller's core to block, sleep, exit, be suspended, or enter explicit host GPU work (svc_thread / dm_yield_to_core_peers in hle_thread.c). Guest runnable state is authoritative, including the wake-to-pthread-resume interval; a host dispatch snapshot cannot release the handoff, and there is no timeout that lets unfinished work through. Host-created helpers and threads that can run on another core are excluded.
• Check: make -C dm/host/tests GAME=010093801237C000 preempt delays a woken pthread's dispatch by 1 ms and 20 ms through real event, mutex and condition-variable SVCs. The worker's payload and exit must precede the signaller's return. The original implementation failed both delays; changing readiness alone fixed 1 ms but still failed 20 ms. Equal-priority, other-core, migratable, host-helper, suspended and host-GPU-work peers do not block the caller.
• Found on: Metroid Dread, whose per-build movie request-ring gates and core_preempt on setting are now removed.
• Scope: these are cooperative thread-SVC handoffs, not a complete asynchronous guest scheduler. The optional core_preempt profile key (DM_PREEMPT=0 / 1 overrides it) additionally checks already-runnable peers after every thread SVC; it defaults off. Historically that broader policy moved Super Mario Bros. Wonder's first frame from 0.6 s to 2.9 s by waiting for busy decompression threads. Wake-triggered handoffs apply with either setting.

Simultaneous crashes of several threads

• Symptom: several threads crash at once and the dumps interleave, then the process hangs instead of exiting.
• Cause: Unity's job threads tend to crash together.
• Fix: crash.c lets only the first crashing thread print (crash_gate); the others stop in pause(), and a 15 s alarm ends the process if the dump itself stalls.
• Found on: Unity titles in general.

Synchronization and address arbitration

SignalToAddress type 2 with count ≤ 0

• Symptom: none seen; found while investigating Fire Emblem Engage's demo crash, which it did not change (that was the address-space shortage, Size-aligned regions exhaust the alias address space).
• Cause: for SignalToAddress type 2 (SignalAndModifyByWaitingCountIfEqual) with count ≤ 0 and waiters present, the new value is value − 2, not value − 1 (mesosphere KAddressArbiter; Ryujinx and yuzu agree). The host had −1.
• Fix: the host writes value − 2, the specified behaviour.

WaitForAddress type 3: 64-bit wait if equal

• Symptom: a 64-bit wait could return at once or block when it should not (not met in a game).
• Cause: Horizon 18 added a 64-bit "wait if equal" arbitration type, which SDK 19 exports as nn::os::WaitForAddress(const long*, long); the host used to fall into its 32-bit "wait if less than" test.
• Fix: type 3 compares the 64-bit value (hle_thread.c).
• Found on: Pokémon Legends: Z-A (SDK 19.3.11): Z-A does not call it, but the SDK has it.

Late PhysX task destructor after the unlock

• Symptom: a PhysX worker crashes in a task's removeReference with a null task manager, right as the game switches to 30 fps (pc=main+0x10ccf28, lr=main+0x13c236c); on the iPhone and in 2 of 5 Mac runs, in every Mac run with 12 busy loops loading the CPU.
• Cause: the game publishes completion before task teardown ends, then reuses the same task. Its main thread rebuilds each world's completion task before simulate; the worker sets done, notifies and unlocks inside run, then calls release, whose destructor clears mTm. Another core may rebuild it in between. This is a guest lifetime race: an unlock cannot protect subsequent destructor writes. PhysX's task contract places recycling in release, after run, and its PxLightCpuTask destructor clears mTm. There is no measured Switch timing guarantee that makes the ordering safe. The game's offsets, affinities and replay evidence are in its note.
• Fix: a per-build cut makes that one release a ret (its patches.py); the destructor frees nothing and the step rewrites every field.
• Check: dm/tools/check_thread_cuts.py replays the original construction, release and removeReference instructions with a forced late release: the original faults at the recorded null-manager load; normal release-before-rebuild succeeds; the registered cut preserves the new task. This controlled replay has no host mutex or graphics implementation in its failure path.
• Found on: The Legend of Zelda: Link's Awakening. How it was found is in 05.

Yields and spin waits

The yield back-off (DM_YIELD_BACKOFF) and the rewriter's spin waits (patch_nso.py "Spin waits", dm_spin_wait in hle_thread.c), with their measurements, are in development.md ("Yield loops", "Spin loops").

Timer slack in guest sleeps (sleep_precise)

• Symptom: a 30 Hz game thread runs at 27 Hz, and whatever it paces comes out slow (cutscenes at 36 ms per frame instead of 33.3; gameplay was fine at 60).
• Cause: macOS timer slack: nanosleep and mach_wait_until overshoot by about 10% at any QoS (measured: 33 ms → 36.4 ms, 5 ms → 6.1 ms, 1 ms → 1.26 ms). Dread's sound thread (main+0xf95b0) reads a millisecond clock, updates, then sleeps "33 ms minus this round's work", so on the host its period was 36.9 ms. The cutscene loop renders a frame whenever the audio play position, which that thread refreshes, moves (main+0x119a3d0; the position is played samples ÷ sample rate of the cutscene's stream), so cutscenes ran at about 27 fps.
• Fix: SleepThread sleeps three quarters of the remaining time per step until within 20 µs (sleep_precise in hle_thread.c).
• Found on: Metroid Dread: cutscenes at 33.3 ms, the sound thread at 33.2 ms. Super Mario Bros. Wonder's frame-time p95 also went from 19.7 to 18.4 ms.
• Note: timed waits (WaitSynchronization, condition variables) still use the host's own timeouts.

Sleeps under 20 µs take the yield path

• Symptom: a guest thread keeps a core at 100% with billions of SleepThread calls.
• Cause: sleep_precise returns at once for anything under 20 µs (its nanosleep would overshoot), so a loop of such sleeps never sleeps.
• Fix: sleeps under 20 µs take the yield path and its back-off (plain yields for the first 2 ms of a run, then short sleeps).
• Found on: Super Mario Galaxy: Hagi polls with SleepThread(100 ns) (main+0x517c), 1.4 billion calls in 200 s, and the render thread nearly as busy. The two threads went from 199 s and 171 s of CPU per 200 s to 5 s and 19 s per 120 s, at the same frame rate.

Yields that make progress return at once (thr_yield)

• Symptom: a loading screen sits for a minute with the CPU nearly idle; one thread yields thousands of times a second, every yield a back-off sleep (the power line's top yielder, with as many back-off sleeps).
• Cause: the thread was working, not waiting: its loop calls nn::os::YieldThread once per item, and the back-off made each yield a 250 µs sleep.
• Fix: a yield whose caller's x19–x28 changed since the last one is progress (thr_yield in hle_thread.c; the SDK reaches the SVC through tail branches, so those registers are the loop's own): it returns at once with no host call, and the run starts again. Polls keep those registers (Three Houses' limiter, Galaxy 2's Hagi sleep poll at main+0x11ec); counters move them.
• Found on: Animal Crossing: New Horizons: a black screen with the plane icon for 77 s after the opening movie, SaveProc at about 2,600 yields a second, its grid walks over the new island (main+0x157d020) yielding once per cell; also about 40 s of the same at boot. The island now shows about 10 s after the movie, as with DM_YIELD_BACKOFF=0. Polls that keep their registers: Fire Emblem: Three Houses, Super Mario Galaxy 2.
• Check: check such a change on The Legend of Zelda: Breath of the Wild outdoors: its PlacementMgr and RadarMgr turned out to yield per item too, and a sched_yield on each of their 200,000 yields a second cost a third of a core more than the back-off had; with no host call, CPU energy fell instead (numbers in development.md "Yield loops").
• Note: a temporary log of which of x19–x28 move at each yield, per thread, is what told the two kinds apart.

Events and notifications

One auto-clear event shared by a pool's workers

• Symptom: the main thread waits forever on a job's done event at boot while every worker sleeps, two of them on one event (main+0x1426c waiting on Job::m_doneEvent); 4 of 6 Mac runs.
• Cause: the engine puts a job in worker k's private queue but signals one shared SDK auto-clear event. Its condition wake chooses by priority, then FIFO; it has no target-worker identity. The wrong worker checks only its own queue and sleeps again. This is a guest assumption about waiter order, exposed by host timing. Host pick / wseq agrees with Atmosphère's condition-tree comparator and Signal, including the historic 0.19.0 implementation. This describes the SDK's condition-based auto-clear event; kernel readable events instead notify all synchronization waiters. Game offsets and observations belong in its note.
• Fix: the game's own wait wrapper (main+0x12500, {event, pending}) signals the event once more when it takes a nonzero pending word after waking, and nothing set it: a build cut makes the signal set it first (its patches.py), so a worker woken in another's place passes the wake on; 6 of 6 runs boot.
• Found on: FINAL FANTASY TACTICS - The Ivalice Chronicles.
• Check: t_preempt checks priority/FIFO selection and mutex reacquisition through real condition SVCs. dm/tools/check_thread_cuts.py runs the game's actual signal and wait wrappers with both worker arrival orders: only the original wrong-worker cases strand a job. The game's note records the Mac boot comparison with the corrected host.
• Note: an unconditional StartThread child-entry handshake changed the timing but did not eliminate the hang and is not a kernel guarantee. The separate same-core start handoff applies only when priority and affinity require it.

The oe notification event and its queue

• Symptom: abort in os::WaitSystemEvent under oe::PopNotificationMessage (The Legend of Zelda: Link's Awakening); one guest thread at about 1000 ms/s on the iPhone only, the phone hot, CPU energy 5–7 W (Monster Hunter Rise's MessageSuperviseThread, priority 32, in the [nvn] threads lines).
• Cause: the host keeps the notifications in a queue of its own. The blocking pop waits on the system event oe::Initialize would have created, and the host's Initialize is a no-op. Monster Hunter Rise's thread waits on oe::GetNotificationMessageEvent() with os::WaitAny, which does not clear an event, then pops notifications until the queue is empty. The host only ever signalled its notification event, so after the first focus change (iOS changes focus at launch; the Mac does not) it stayed set and the loop never blocked again.
• Fix: the host now implements the blocking pop on its notification queue (a condition variable, then the TryPop path), and the event follows the queue as the am service's does: set while a notification is queued, cleared when the last one is popped (hle_oe.c mq_ev_sync, hle_clear_system_event).
• Found on: The Legend of Zelda: Link's Awakening (SDK 8.2.0, the blocking pop); Monster Hunter Rise (the event).
• Check: Mac repro DM_OE_SCRIPT=focus=2@5,focus=1@6 (the script also starts at the game's first GetNotificationMessageEvent, since such a game pops only once the event fires): before 995 ms/s, CPU 109%, 6.97 W; after, the thread is idle and CPU energy is 0.05 W.