From db684b8653d93f4a374d8d692bb0afb31db67987 Mon Sep 17 00:00:00 2001
From: Danny Holman <dholman@gymli.org>
Date: Thu, 28 Mar 2024 22:00:16 -0500
Subject: kernel: add a basic thread scheduler

Add a basic thread scheduler. This should allow the kernel to schedule
threads according to the round robin algorithm.

Signed-off-by: Danny Holman <dholman@gymli.org>
---
 Makefile                         |  1 +
 arch/i386/kernel/switch_thread.s | 28 ++++++++++++++++++
 arch/i386/make.config            |  2 ++
 include/kernel/sched.h           | 27 +++++++++++++++++
 kernel/sched.c                   | 64 ++++++++++++++++++++++++++++++++++++++++
 5 files changed, 122 insertions(+)
 create mode 100644 arch/i386/kernel/switch_thread.s
 create mode 100644 include/kernel/sched.h
 create mode 100644 kernel/sched.c

diff --git a/Makefile b/Makefile
index 874f614..043e883 100644
--- a/Makefile
+++ b/Makefile
@@ -29,6 +29,7 @@ KERNEL_OBJS=$(KERNEL_ARCH_OBJS) \
 	    kernel/string.o \
 	    kernel/io.o \
 	    kernel/panic.o \
+	    kernel/sched.o \
 
 OBJS=$(ARCHDIR)/boot/crti.o \
      $(ARCHDIR)/crtbegin.o \
diff --git a/arch/i386/kernel/switch_thread.s b/arch/i386/kernel/switch_thread.s
new file mode 100644
index 0000000..f1168d5
--- /dev/null
+++ b/arch/i386/kernel/switch_thread.s
@@ -0,0 +1,28 @@
+.global switch_thread
+.type switch_thread, @function
+switch_thread:
+        pushl %ebp
+        movl %esp, %ebp
+
+        pushl %ebx
+        pushl %esi
+        pushl %edi
+        pushl %ebp
+
+        movl 8(%ebp), %esi
+        movl 12(%ebp), %edi
+
+        movl %esp, (%edi);
+        movl %cr3, %ecx
+        movl %ecx, 4(%edi)
+
+        movl (%esi), %esp
+        movl 4(%esi), %ecx
+        movl %ecx, %cr3
+
+        popl %ebp
+        popl %edi
+        popl %esi
+        popl %ebx
+        popl %ebp
+        ret
diff --git a/arch/i386/make.config b/arch/i386/make.config
index 8499f1a..d4d98a7 100644
--- a/arch/i386/make.config
+++ b/arch/i386/make.config
@@ -17,3 +17,5 @@ KERNEL_ARCH_OBJS=$(ARCHDIR)/boot/boot.o \
                  $(ARCHDIR)/kernel/paging.o \
                  $(ARCHDIR)/kernel/pmem.o \
                  $(ARCHDIR)/kernel/kmalloc.o \
+                 $(ARCHDIR)/kernel/jump_userspace.o \
+                 $(ARCHDIR)/kernel/switch_thread.o \
diff --git a/include/kernel/sched.h b/include/kernel/sched.h
new file mode 100644
index 0000000..fef36b8
--- /dev/null
+++ b/include/kernel/sched.h
@@ -0,0 +1,27 @@
+#ifndef KERNEL_SCHED_H
+#define KERNEL_SCHED_H
+
+#include <kernel/asm.h>
+#include <kernel/data/list.h>
+
+#define THREAD_READY    0
+#define THREAD_RUNNING  1
+#define THREAD_WAIT     2
+
+struct task_block {
+        unsigned int pid;
+        unsigned int parent_pid;
+        int nice;
+        unsigned int num_threads;
+        struct thread_block *threads;
+        struct list_head list;
+};
+
+void sched_init(void);
+void schedule_next(void);
+
+struct task_block* init_task(int nice, unsigned int ppid);
+void schedule_thread(struct thread_block *thread);
+void switch_thread(struct thread_block *old, struct thread_block *new);
+
+#endif
diff --git a/kernel/sched.c b/kernel/sched.c
new file mode 100644
index 0000000..8aab675
--- /dev/null
+++ b/kernel/sched.c
@@ -0,0 +1,64 @@
+#include <kernel/sched.h>
+#include <kernel/container_of.h>
+#include <kernel/data/list.h>
+#include <kernel/string.h>
+#include <kernel/kmalloc.h>
+#include <stddef.h>
+
+static struct list_head *ready_queue;
+static struct list_head *wait_queue;
+
+static struct task_block *cur = NULL;
+static long int next_id = 1;
+
+static void _enqueue(struct list_head *queue, struct task_block *task) {
+        struct list_head *temp = queue;
+        while (temp->next != NULL)
+                temp = temp->next;
+        list_add(&task->list, temp);
+}
+
+static struct task_block* _dequeue(struct list_head *queue) {
+        if (queue->next == NULL)
+                return NULL;
+        struct list_head *temp = queue->next;
+        list_del(temp);
+        struct task_block *ret = container_of(temp, struct task_block, list);
+        return ret;
+}
+
+void sched_init(void) {
+        ready_queue = kmalloc(sizeof(struct list_head));
+        wait_queue = kmalloc(sizeof(struct list_head));
+        struct task_block *boot_task = init_task(0, 0);
+        boot_task->threads->state = THREAD_RUNNING;
+        cur = boot_task;
+
+        switch_thread(boot_task->threads, boot_task->threads);
+}
+
+void schedule_next(void) {
+        struct task_block *task = _dequeue(ready_queue);
+        if (task == NULL)
+                return;
+        if (cur != NULL)
+                _enqueue(ready_queue, cur);
+        disable_ints();
+        switch_thread(cur->threads, task->threads);
+        enable_ints();
+        task->threads->state = THREAD_RUNNING;
+        cur->threads->state = THREAD_READY;
+        cur = task;
+}
+
+struct task_block* init_task(int nice, unsigned int ppid) {
+        struct task_block *ret = kmalloc(sizeof(struct task_block));
+        ret->pid = next_id++;
+        ret->parent_pid = ppid;
+        ret->nice = nice;
+        ret->num_threads = 1;
+        ret->threads = kmalloc(sizeof(struct thread_block));
+        ret->threads->tid = ret->pid;
+        ret->threads->tgid = ret->pid;
+        return ret;
+}
-- 
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