Kernel: Optimize scheduler idling

Now after each interrupt we will ask the scheduler to reschedule
if the current thread is the idle thread. This allows semaphore
unblocking to be practically instant when there is only one thread
executing.

Now disk reading is back to ~3 MB/s for single threaded process

kernel/arch/i386/IDT.cpp

@@ -3,6 +3,7 @@
 #include <kernel/InterruptController.h>
 #include <kernel/kmalloc.h>
 #include <kernel/Panic.h>
+#include <kernel/Scheduler.h>
 
 #define INTERRUPT_HANDLER____(i, msg)													\
 	static void interrupt ## i ()														\
@@ -132,6 +133,8 @@ found:
 		// NOTE: Scheduler sends PIT eoi's
 		if (irq != PIT_IRQ)
 			InterruptController::get().eoi(irq);
+
+		Kernel::Scheduler::get().reschedule_if_idling();
 	}
 
 	extern "C" void handle_irq_common();

kernel/arch/x86_64/IDT.cpp

@@ -3,6 +3,7 @@
 #include <kernel/InterruptController.h>
 #include <kernel/kmalloc.h>
 #include <kernel/Panic.h>
+#include <kernel/Scheduler.h>
 
 #define REGISTER_ISR_HANDLER(i) register_interrupt_handler(i, isr ## i)
 #define REGISTER_IRQ_HANDLER(i) register_interrupt_handler(IRQ_VECTOR_BASE + i, irq ## i)
@@ -129,6 +130,8 @@ namespace IDT
 		// NOTE: Scheduler sends PIT eoi's
 		if (irq != PIT_IRQ)
 			InterruptController::get().eoi(irq);
+
+		Kernel::Scheduler::get().reschedule_if_idling();
 	}
 
 	static void flush_idt()

kernel/include/kernel/Scheduler.h

@@ -16,6 +16,7 @@ namespace Kernel
 
 		void start();
 		void reschedule();
+		void reschedule_if_idling();
 
 		BAN::ErrorOr<void> add_thread(Thread*);
 

kernel/kernel/Scheduler.cpp

@@ -70,6 +70,20 @@ namespace Kernel
 		ASSERT_NOT_REACHED();
 	}
 
+	void Scheduler::reschedule_if_idling()
+	{
+		VERIFY_CLI();
+
+		if (m_active_threads.empty() || &current_thread() != m_idle_thread)
+			return;
+		
+		if (save_current_thread())
+			return;
+		m_current_thread = m_active_threads.begin();
+		execute_current_thread();
+		ASSERT_NOT_REACHED();
+	}
+
 	void Scheduler::wake_threads()
 	{
 		VERIFY_CLI();