Kernel: Add priority boost code for starved threads

src/core/hle/kernel/thread.cpp

@@ -143,6 +143,29 @@ void ArbitrateAllThreads(u32 address) {
     }
 }
 
+/// Boost low priority threads (temporarily) that have been starved
+static void PriorityBoostStarvedThreads() {
+    u64 current_ticks = CoreTiming::GetTicks();
+
+    for (auto& thread : thread_list) {
+        // TODO(bunnei): Threads that have been waiting to be scheduled for `boost_ticks` (or
+        // longer) will have their priority temporarily adjusted to 1 higher than the highest
+        // priority thread to prevent thread starvation. This general behavior has been verified
+        // on hardware. However, this is almost certainly not perfect, and the real CTR OS scheduler
+        // should probably be reversed to verify this.
+
+        const u64 boost_timeout = 2000000; // Boost threads that have been ready for > this long
+
+        u64 delta = current_ticks - thread->last_running_ticks;
+
+        if (thread->status == THREADSTATUS_READY && delta > boost_timeout) {
+            const s32 priority = std::max(ready_queue.get_first()->current_priority - 1, 
+                                 static_cast<unsigned int>(0));
+            thread->BoostPriority(priority);
+        }
+    }
+}
+
 /**
  * Switches the CPU's active thread context to that of the specified thread
  * @param new_thread The thread to switch to
@@ -177,6 +200,10 @@ static void SwitchContext(Thread* new_thread) {
 
         ready_queue.remove(new_thread->current_priority, new_thread);
         new_thread->status = THREADSTATUS_RUNNING;
+
+        // Restores thread to its nominal priority if it has been temporarily changed
+        new_thread->current_priority = new_thread->nominal_priority;
+
 
         if (previous_process != current_thread->owner_process) {
             Kernel::g_current_process = current_thread->owner_process;
@@ -513,6 +540,8 @@ bool HaveReadyThreads() {
 }
 
 void Reschedule() {
+    PriorityBoostStarvedThreads();
+
     Thread* cur = GetCurrentThread();
     Thread* next = PopNextReadyThread();