sched: add base for scheduling and task switching

kernel/sched.c

@@ -0,0 +1,95 @@
+/* See the end of this file for copyright and license terms. */
+
+#include <arch/interrupt.h>
+#include <arch/sched.h>
+
+#include <gay/clist.h>
+#include <gay/sched.h>
+#include <gay/types.h>
+
+struct task kernel_task = {
+	.parent = nil,
+	.pid = 0,
+	.state = TASK_READY,
+};
+
+struct task *current = &kernel_task;
+
+/*
+ * Two run queues, one active and one inactive.  The scheduler takes tasks out
+ * of the active queue, runs them if possible, and then puts them into the
+ * inactive queue if they aren't dead.  When the active queue gets empty, the
+ * queues are switched and everything starts over again.
+ * The `current` task is always in neither queue.
+ */
+static struct clist _run_queues[2];
+static struct clist *active_queue = &_run_queues[0];
+static struct clist *inactive_queue = &_run_queues[1];
+
+int sched_init(void)
+{
+	clist_init(active_queue);
+	clist_init(inactive_queue);
+	return 0;
+}
+
+void schedule(void)
+{
+	struct task *old = current;
+	struct task *new = nil;
+
+	clist_add(inactive_queue, &old->run_queue);
+
+/* Dijkstra would be so mad if he saw this */
+try_again:
+	if (clist_is_empty(active_queue)) {
+		struct clist *tmp = active_queue;
+		active_queue = inactive_queue;
+		inactive_queue = tmp;
+	}
+
+	struct task *cursor, *tmp;
+	clist_foreach_entry_safe(active_queue, cursor, tmp, run_queue) {
+		clist_del(&cursor->run_queue);
+		if (cursor->state == TASK_READY) {
+			new = cursor;
+			break;
+		} else if (cursor->state == TASK_BLOCKED) {
+			clist_add(inactive_queue, &cursor->run_queue);
+		}
+	}
+
+	/*
+	 * This should really make the CPU go idle rather than constantly
+	 * looping around until some event occurs, but i have no idea how to do
+	 * that (yet)
+	 */
+	if (new == nil)
+		goto try_again;
+
+	if (new != old)
+		switch_to(new, old);
+}
+
+void switch_to(struct task *new, struct task *old)
+{
+	disable_intr();
+	current = new;
+	arch_switch_to(&new->tcb, &old->tcb);
+	current = old;
+	enable_intr();
+}
+
+/*
+ * This file is part of GayBSD.
+ * Copyright (c) 2021 fef <owo@fef.moe>.
+ *
+ * GayBSD is nonviolent software: you may only use, redistribute, and/or
+ * modify it under the terms of the Cooperative Nonviolent Public License
+ * (CNPL) as found in the LICENSE file in the source code root directory
+ * or at <https://git.pixie.town/thufie/npl-builder>; either version 7
+ * of the license, or (at your option) any later version.
+ *
+ * GayBSD comes with ABSOLUTELY NO WARRANTY, to the extent
+ * permitted by applicable law.  See the CNPL for details.
+ */