diff --git a/docs/README-n3ds.md b/docs/README-n3ds.md index 66e194d0b..e9e7c7d01 100644 --- a/docs/README-n3ds.md +++ b/docs/README-n3ds.md @@ -25,3 +25,4 @@ cmake --install build - SDL2main should be used to ensure ROMFS is enabled. - By default, the extra L2 cache and higher clock speeds of the New 2/3DS lineup are enabled. If you wish to turn it off, use `osSetSpeedupEnable(false)` in your main function. - `SDL_GetBasePath` returns the romfs root instead of the executable's directory. +- The Nintendo 3DS uses a cooperative threading model on a single core, meaning a thread will never yield unless done manually through the `SDL_Delay` functions, or blocking waits (`SDL_LockMutex`, `SDL_SemWait`, `SDL_CondWait`, `SDL_WaitThread`). To avoid starving other threads, `SDL_SemTryWait` and `SDL_SemWaitTimeout` will yield if they fail to acquire the semaphore, see https://github.com/libsdl-org/SDL/pull/6776 for more information. diff --git a/src/thread/n3ds/SDL_syssem.c b/src/thread/n3ds/SDL_syssem.c index 1a4109be5..e7cbacb8d 100644 --- a/src/thread/n3ds/SDL_syssem.c +++ b/src/thread/n3ds/SDL_syssem.c @@ -27,14 +27,16 @@ #include <3ds.h> #include "SDL_thread.h" +#include "SDL_timer.h" + +int WaitOnSemaphoreFor(SDL_sem *sem, Uint32 timeout); struct SDL_semaphore { LightSemaphore semaphore; }; -SDL_sem * -SDL_CreateSemaphore(Uint32 initial_value) +SDL_sem *SDL_CreateSemaphore(Uint32 initial_value) { SDL_sem *sem; @@ -59,9 +61,7 @@ SDL_CreateSemaphore(Uint32 initial_value) */ void SDL_DestroySemaphore(SDL_sem *sem) { - if (sem) { - SDL_free(sem); - } + SDL_free(sem); } int SDL_SemTryWait(SDL_sem *sem) @@ -70,35 +70,49 @@ int SDL_SemTryWait(SDL_sem *sem) return SDL_InvalidParamError("sem"); } - return SDL_SemWaitTimeout(sem, 0); + if (LightSemaphore_TryAcquire(&sem->semaphore, 1) != 0) { + /* If we failed, yield to avoid starvation on busy waits */ + svcSleepThread(1); + return SDL_MUTEX_TIMEDOUT; + } + + return 0; } int SDL_SemWaitTimeout(SDL_sem *sem, Uint32 timeout) { - int retval; - if (sem == NULL) { return SDL_InvalidParamError("sem"); } if (timeout == SDL_MUTEX_MAXWAIT) { LightSemaphore_Acquire(&sem->semaphore, 1); - retval = 0; - } else { - int return_code = LightSemaphore_TryAcquire(&sem->semaphore, 1); - if (return_code != 0) { - for (u32 i = 0; i < timeout; i++) { - svcSleepThread(1000000LL); - return_code = LightSemaphore_TryAcquire(&sem->semaphore, 1); - if (return_code == 0) { - break; - } - } - } - retval = return_code != 0 ? SDL_MUTEX_TIMEDOUT : 0; + return 0; } - return retval; + if (LightSemaphore_TryAcquire(&sem->semaphore, 1) != 0) { + return WaitOnSemaphoreFor(sem, timeout); + } + + return 0; +} + +int WaitOnSemaphoreFor(SDL_sem *sem, Uint32 timeout) +{ + Uint64 stop_time = SDL_GetTicks64() + timeout; + Uint64 current_time = SDL_GetTicks64(); + while (current_time < stop_time) { + if (LightSemaphore_TryAcquire(&sem->semaphore, 1) == 0) { + return 0; + } + /* 100 microseconds seems to be the sweet spot */ + svcSleepThread(100000LL); + current_time = SDL_GetTicks64(); + } + + /* If we failed, yield to avoid starvation on busy waits */ + svcSleepThread(1); + return SDL_MUTEX_TIMEDOUT; } int SDL_SemWait(SDL_sem *sem) @@ -106,8 +120,7 @@ int SDL_SemWait(SDL_sem *sem) return SDL_SemWaitTimeout(sem, SDL_MUTEX_MAXWAIT); } -Uint32 -SDL_SemValue(SDL_sem *sem) +Uint32 SDL_SemValue(SDL_sem *sem) { if (sem == NULL) { SDL_InvalidParamError("sem"); diff --git a/src/thread/n3ds/SDL_systhread.c b/src/thread/n3ds/SDL_systhread.c index caf2df7bb..926dd3488 100644 --- a/src/thread/n3ds/SDL_systhread.c +++ b/src/thread/n3ds/SDL_systhread.c @@ -49,8 +49,9 @@ static void ThreadEntry(void *arg) int SDL_SYS_CreateThread(SDL_Thread *thread) { - s32 priority = N3DS_THREAD_PRIORITY_MEDIUM; + s32 priority; size_t stack_size = GetStackSize(thread->stacksize); + svcGetThreadPriority(&priority, CUR_THREAD_HANDLE); thread->handle = threadCreate(ThreadEntry, thread, diff --git a/test/testsem.c b/test/testsem.c index 2b36a3a82..8d8a2493c 100644 --- a/test/testsem.c +++ b/test/testsem.c @@ -210,6 +210,8 @@ TestOverheadContended(SDL_bool try_wait) } /* Make sure threads consumed everything */ while (SDL_SemValue(sem)) { + /* Friendlier with cooperative threading models */ + SDL_Delay(1); } } end_ticks = SDL_GetTicks();