sched: avoid memory allocation for kernel task

kernel/sched.c

 /* See the end of this file for copyright, license, and warranty information. */
 
+/**
+ * @file sched.c
+ * @brief Simple round-robin scheduler.
+ *
+ * Tasks are stored in a lookup table, `tasks`, which is indexed by pid.
+ * The global `current` variable points to the task that is currently running,
+ * which must only be accessed from scheduling context (i.e. from within a
+ * syscall or scheduling interrupt handler).
+ *
+ * When `schedule()` is called, it first processes the kevent queue in which irq
+ * handlers store broadcasts for changes in hardware state, such as a DMA buffer
+ * having been fully transmitted.  Tasks register an event listener for the
+ * event they are waiting for before entering I/O wait, and remove their waiting
+ * flag in the listener callback.
+ *
+ * After all events are processed, `schedule()` iterates over the task table
+ * starting from one task after the one that has been currently running, and
+ * chooses the first one it encounters that is suitable for being woken back up
+ * (i.e. is in state `TASK_QUEUE`).  Thus, the previously running task is only
+ * executed again if no other tasks are ready to be executed.  If no task is
+ * runnable, the idle task is selected.
+ *
+ * The last step is performing the in-kernel context switch to the next task
+ * to be run, which is done by `do_switch()`.  This routine stores the current
+ * register state in the old task's TCB and loads the registers from the new
+ * one.  Execution then continues where the task that is switched to previously
+ * called `do_switch()`, and eventually returns back to userspace by returning
+ * from the exception handler.
+ */
+
 #include <arch-generic/do_switch.h>
 #include <arch-generic/sched.h>
 #include <arch-generic/watchdog.h>
@@ -17,15 +47,16 @@
 extern uint32_t _sstack;
 extern uint32_t _estack;
 
-static struct task *tasktab[CONFIG_SCHED_MAXTASK];
+static struct task *tasks[CONFIG_SCHED_MAXTASK];
 struct task *volatile current;
 
+static struct task kernel_task;
 static struct task idle_task;
 
 static void task_destroy(struct kent *kent)
 {
 	struct task *task = container_of(kent, struct task, kent);
-	tasktab[task->pid] = NULL;
+	tasks[task->pid] = NULL;
 	free(task);
 }
 
@@ -33,26 +64,22 @@ int sched_init(void)
 {
 	int err;
 
-	struct task *ktask = malloc(sizeof(*ktask));
-	if (ktask == NULL)
-		return -ENOMEM;
-
-	ktask->kent.parent = kent_root;
-	ktask->kent.destroy = task_destroy;
-	err = kent_init(&ktask->kent);
+	kernel_task.kent.parent = kent_root;
+	kernel_task.kent.destroy = task_destroy;
+	err = kent_init(&kernel_task.kent);
 	if (err != 0)
 		goto out;
 
-	memset(&ktask->tcb, 0, sizeof(ktask->tcb));
-	ktask->bottom = &_estack;
-	ktask->pid = 0;
-	ktask->state = TASK_READY;
+	memset(&kernel_task.tcb, 0, sizeof(kernel_task.tcb));
+	kernel_task.bottom = &_estack;
+	kernel_task.pid = 0;
+	kernel_task.state = TASK_READY;
 
-	tasktab[0] = ktask;
-	current = ktask;
+	tasks[0] = &kernel_task;
+	current = &kernel_task;
 
-	for (unsigned int i = 1; i < ARRAY_SIZE(tasktab); i++)
-		tasktab[i] = NULL;
+	for (unsigned int i = 1; i < ARRAY_SIZE(tasks); i++)
+		tasks[i] = NULL;
 
 	err = arch_watchdog_init();
 	if (err != 0)
@@ -110,15 +137,16 @@ void schedule(void)
 
 	if (old->state == TASK_READY)
 		old->state = TASK_QUEUE;
-	for (unsigned int i = 0; i < ARRAY_SIZE(tasktab); i++) {
+
+	for (unsigned int i = 0; i < ARRAY_SIZE(tasks); i++) {
 		/*
 		 * increment nextpid before accessing the task table
 		 * because it is -1 if the idle task was running
 		 */
 		nextpid++;
-		nextpid %= ARRAY_SIZE(tasktab);
+		nextpid %= ARRAY_SIZE(tasks);
 
-		struct task *tmp = tasktab[nextpid];
+		struct task *tmp = tasks[nextpid];
 		if (tmp != NULL && can_run(tmp)) {
 			new = tmp;
 			break;
@@ -140,50 +168,14 @@ void schedule(void)
 
 void yield(enum task_state state)
 {
-	struct task *task = current;
-	task->state = state;
+	current->state = state;
 	schedule();
 }
 
-struct task *sched_fork(struct task *parent)
-{
-	pid_t pid;
-	struct task *child = malloc(sizeof(*child));
-
-	if (child == NULL)
-		goto err_alloc;
-
-	for (pid = 0; pid < CONFIG_SCHED_MAXTASK; pid++) {
-		if (tasktab[pid] == NULL)
-			break;
-	}
-	if (pid == CONFIG_SCHED_MAXTASK)
-		goto err_maxtask;
-
-	child->kent.parent = &parent->kent;
-	child->kent.destroy = task_destroy;
-	if (kent_init(&child->kent) != 0)
-		goto err_kent;
-
-	child->pid = pid;
-	return child;
-
-err_kent:
-err_maxtask:
-	free(child);
-err_alloc:
-	return NULL;
-}
-
-void msleep(unsigned long int ms)
-{
-	current->sleep = ms_to_ticks(ms);
-	yield(TASK_SLEEP);
-}
-
 long sys_sleep(unsigned long int millis)
 {
-	msleep(millis);
+	current->sleep = ms_to_ticks(millis);
+	yield(TASK_SLEEP);
 	/* TODO: return actual milliseconds */
 	return 0;
 }