On hardware the kernel fills a new thread's stack with interrupts on, so
a better thread that wakes during it runs before the call returns, the
time it takes doesn't count towards the call, and worse threads get
nothing (pspautotests threads/scheduling/preemptsyscall). PPSSPP ate the
whole cost at once and only rescheduled at the end.
__KernelBusyDelayResult() models such a syscall: the caller waits, an
idle thread stands in for it while nothing better wants the CPU, and its
remaining cycles only count down while that's the case. When done it
goes back ahead of threads of its own priority, having never given up
the CPU. sceKernelCreateThread uses it for the stack fill, unless a
thread event handler is about to run.
Booting 75 games against master shows no difference.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
