From 7b9f428b23e1761e7b6c177d2e8eb9219ac6b7f6 Mon Sep 17 00:00:00 2001
From: bunnei <bunneidev@gmail.com>
Date: Mon, 23 Mar 2015 23:55:21 -0400
Subject: Thread: Implement priority boost for starved threads.

SVC: Return correct error code on invalid CreateThread processor ID.

SVC: Assert when creating a thread with an invalid userland priority.
---
 src/core/hle/kernel/thread.cpp | 48 +++++++++++++++++++++++++++++++-----------
 1 file changed, 36 insertions(+), 12 deletions(-)

(limited to 'src/core/hle/kernel/thread.cpp')

diff --git a/src/core/hle/kernel/thread.cpp b/src/core/hle/kernel/thread.cpp
index be1aed61..3a1e15ac 100644
--- a/src/core/hle/kernel/thread.cpp
+++ b/src/core/hle/kernel/thread.cpp
@@ -140,6 +140,29 @@ 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 && !thread->idle) {
+            const s32 boost_priority = std::max(ready_queue.get_first()->current_priority - 1, 0);
+            ready_queue.move(thread, thread->current_priority, boost_priority);
+            thread->current_priority = boost_priority;
+        }
+    }
+}
+
 /** 
  * Switches the CPU's active thread context to that of the specified thread
  * @param new_thread The thread to switch to
@@ -151,6 +174,7 @@ static void SwitchContext(Thread* new_thread) {
 
     // Save context for previous thread
     if (previous_thread) {
+        previous_thread->last_running_ticks = CoreTiming::GetTicks();
         Core::g_app_core->SaveContext(previous_thread->context);
 
         if (previous_thread->status == THREADSTATUS_RUNNING) {
@@ -168,6 +192,9 @@ 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;
+
         Core::g_app_core->LoadContext(new_thread->context);
     } else {
         current_thread = nullptr;
@@ -364,7 +391,8 @@ ResultVal<SharedPtr<Thread>> Thread::Create(std::string name, VAddr entry_point,
     thread->status = THREADSTATUS_DORMANT;
     thread->entry_point = entry_point;
     thread->stack_top = stack_top;
-    thread->initial_priority = thread->current_priority = priority;
+    thread->nominal_priority = thread->current_priority = priority;
+    thread->last_running_ticks = CoreTiming::GetTicks();
     thread->processor_id = processor_id;
     thread->wait_set_output = false;
     thread->wait_all = false;
@@ -400,18 +428,11 @@ static void ClampPriority(const Thread* thread, s32* priority) {
 void Thread::SetPriority(s32 priority) {
     ClampPriority(this, &priority);
 
-    if (current_priority == priority) {
-        return;
-    }
+    // If thread was ready, adjust queues
+    if (status == THREADSTATUS_READY)
+        ready_queue.move(this, current_priority, priority);
 
-    if (status == THREADSTATUS_READY) {
-        // If thread was ready, adjust queues
-        ready_queue.remove(current_priority, this);
-        ready_queue.prepare(priority);
-        ready_queue.push_back(priority, this);
-    }
-    
-    current_priority = priority;
+    nominal_priority = current_priority = priority;
 }
 
 SharedPtr<Thread> SetupIdleThread() {
@@ -440,6 +461,9 @@ SharedPtr<Thread> SetupMainThread(u32 stack_size, u32 entry_point, s32 priority)
 
 void Reschedule() {
     Thread* prev = GetCurrentThread();
+
+    PriorityBoostStarvedThreads();
+
     Thread* next = PopNextReadyThread();
     HLE::g_reschedule = false;
 
-- 
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