Refactor scheduler to RR cursor-based O(1) design

Previously, the scheduler performed an O(N) scan of the global task list
(kcb->tasks) to locate the next TASK_READY task. This resulted in
non-deterministic selection latency and unstable round-robin rotation
under heavy load or frequent task state transitions.

This change introduces a strict O(1) scheduler based on per-priority
ready queues and round-robin (RR) cursors. Each priority level maintains
its own ready queue and cursor, enabling constant-time selection of the
next runnable task while preserving fairness within the same priority.

Author: vicLin8712

include/sys/task.h

@@ -124,8 +124,6 @@ typedef struct {
 extern kcb_t *kcb;
 
 /* System Configuration Constants */
-#define SCHED_IMAX \
-    500 /* Safety limit for scheduler iterations to prevent livelock */
 #define MIN_TASK_STACK_SIZE \
     256 /* Minimum stack size to prevent stack overflow */
 #define TASK_CACHE_SIZE \

kernel/task.c

@@ -471,16 +471,15 @@ void _sched_block_enqueue(tcb_t *blocked_task)
  *
  * Selects the next ready task using circular traversal of the master task list.
  *
- * Complexity: O(n) where n = number of tasks
- * - Best case: O(1) when next task in sequence is ready
- * - Worst case: O(n) when only one task is ready and it's the last checked
- * - Typical case: O(k) where k << n (number of non-ready tasks to skip)
+ * Complexity: O(1)
+ * - Always constant-time selection, regardless of total task count.
+ * - No need to traverse the task list.
  *
  * Performance characteristics:
- * - Excellent for small-to-medium task counts (< 50 tasks)
- * - Simple and reliable implementation
- * - Good cache locality due to sequential list traversal
- * - Priority-aware time slice allocation
+ * - Ideal for systems with frequent context switches or many tasks.
+ * - Excellent cache locality: only touches nodes in the active ready queue.
+ * - Priority-aware: highest non-empty ready queue is chosen via bitmap lookup.
+ * - Each priority level maintains its own rr_cursor to ensure fair rotation.
  */
 uint16_t sched_select_next_task(void)
 {
@@ -493,53 +492,39 @@ uint16_t sched_select_next_task(void)
     if (current_task->state == TASK_RUNNING)
         current_task->state = TASK_READY;
 
-    /* Round-robin search: find next ready task in the master task list */
-    list_node_t *start_node = kcb->task_current;
-    list_node_t *node = start_node;
-    int iterations = 0; /* Safety counter to prevent infinite loops */
-
-    do {
-        /* Move to next task (circular) */
-        node = list_cnext(kcb->tasks, node);
-        if (!node || !node->data)
-            continue;
+    /* Bitmap search, from bit0 (highest priority level) to bit7 (lowest
+     * priority level) */
+    uint8_t bitmap = kcb->ready_bitmap;
+    if (unlikely(bitmap == 0))
+        panic(ERR_NO_TASKS);
+    
+    uint8_t top_prio_level = 0;
+    while (top_prio_level < TASK_PRIORITY_LEVELS) {
+        if (bitmap & 1U)
+            break;
 
-        tcb_t *task = node->data;
+        bitmap >>= 1;
+        top_prio_level++;
+    }
 
-        /* Skip non-ready tasks */
-        if (task->state != TASK_READY)
-            continue;
+    list_node_t **cursor = &kcb->rr_cursors[top_prio_level];
+    list_t *rq = kcb->ready_queues[top_prio_level];
+    if (unlikely(!rq || !*cursor))
+        panic(ERR_NO_TASKS);
 
-        /* Found a ready task */
-        kcb->task_current = node;
-        task->state = TASK_RUNNING;
-        task->time_slice = get_priority_timeslice(task->prio_level);
+    /* Update next task with top priority cursor */
+    kcb->task_current = *cursor;
 
-        return task->id;
+    /* Advance top priority cursor to next task node */
+    *cursor = list_cnext(rq, *cursor);
 
-    } while (node != start_node && ++iterations < SCHED_IMAX);
+    /* Update new task properties */
+    tcb_t *new_task = kcb->task_current->data;
+    new_task->time_slice = get_priority_timeslice(new_task->prio_level);
+    new_task->state = TASK_RUNNING;
 
-    /* No ready tasks found in preemptive mode - all tasks are blocked.
-     * This is normal for periodic RT tasks waiting for their next period.
-     * We CANNOT return a BLOCKED task as that would cause it to run.
-     * Instead, find ANY task (even blocked) as a placeholder, then wait for
-     * interrupt.
-     */
-    if (kcb->preemptive) {
-        /* Select any task as placeholder (dispatcher won't actually switch to
-         * it if blocked) */
-        list_node_t *any_node = list_next(kcb->tasks->head);
-        while (any_node && any_node != kcb->tasks->tail) {
-            if (any_node->data) {
-                kcb->task_current = any_node;
-                tcb_t *any_task = any_node->data;
-                return any_task->id;
-            }
-            any_node = list_next(any_node);
-        }
-        /* No tasks at all - this is a real error */
-        panic(ERR_NO_TASKS);
-    }
+    if (kcb->task_current)
+        return new_task->id;
 
     /* In cooperative mode, having no ready tasks is an error */
     panic(ERR_NO_TASKS);