From e322826347f383ec8ca6ae14326c761a1d7bef8a Mon Sep 17 00:00:00 2001
From: Bananymous <bananymousosq@gmail.com>
Date: Mon, 3 Apr 2023 01:51:05 +0300
Subject: [PATCH] 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 +++
 kernel/arch/x86_64/IDT.cpp        |  3 +++
 kernel/include/kernel/Scheduler.h |  1 +
 kernel/kernel/Scheduler.cpp       | 14 ++++++++++++++
 4 files changed, 21 insertions(+)

diff --git a/kernel/arch/i386/IDT.cpp b/kernel/arch/i386/IDT.cpp
index 3efd51024e..ba5511c4ab 100644
--- a/kernel/arch/i386/IDT.cpp
+++ b/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();
diff --git a/kernel/arch/x86_64/IDT.cpp b/kernel/arch/x86_64/IDT.cpp
index 5fa5005422..c4b0572d32 100644
--- a/kernel/arch/x86_64/IDT.cpp
+++ b/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()
diff --git a/kernel/include/kernel/Scheduler.h b/kernel/include/kernel/Scheduler.h
index 61dffeb7ad..fd7fcc1ed1 100644
--- a/kernel/include/kernel/Scheduler.h
+++ b/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*);
 
diff --git a/kernel/kernel/Scheduler.cpp b/kernel/kernel/Scheduler.cpp
index 7b65c2f9c2..df089066db 100644
--- a/kernel/kernel/Scheduler.cpp
+++ b/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();