sched/hrtimer: Add guard timer.

This commit added the guard timer for hrtimer.
The guard timer uses a small memory footprint, offering two main advantages:
- Reduced branches checking for an empty hrtimer queue, simplifying code implementation and improving the performance.
- Additional health monitoring allows the system to enter a safe state in case of time acquisition errors, and supports custom error handling callback functions.

Signed-off-by: ouyangxiangzhen <ouyangxiangzhen@xiaomi.com>

sched/hrtimer/hrtimer.h

@@ -84,6 +84,7 @@ extern seqcount_t g_hrtimer_lock;
 /* Red-Black tree containing all active high-resolution timers */
 
 extern struct hrtimer_tree_s g_hrtimer_tree;
+extern struct hrtimer_s      g_hrtimer_guard;
 extern struct FAR hrtimer_s *g_hrtimer_head;
 #else
 /* List containing all active high-resolution timers */
@@ -262,8 +263,7 @@ static inline_function bool hrtimer_insert(FAR hrtimer_t *hrtimer)
   bool is_head = false;
   RB_INSERT(hrtimer_tree_s, &g_hrtimer_tree, hrtimer);
 
-  if (g_hrtimer_head == NULL ||
+  if (HRTIMER_TIME_BEFORE(hrtimer->expired, g_hrtimer_head->expired))
-      HRTIMER_TIME_BEFORE(hrtimer->expired, g_hrtimer_head->expired))
     {
       g_hrtimer_head = hrtimer;
       is_head        = true;

sched/hrtimer/hrtimer_cancel.c

@@ -97,11 +97,8 @@ int hrtimer_cancel(FAR hrtimer_t *hrtimer)
 
       if (hrtimer_remove(hrtimer))
         {
-          uint64_t next_expired;
-
           first = hrtimer_get_first();
-          next_expired = first != NULL ? first->expired : UINT64_MAX;
+          hrtimer_reprogram(first->expired);
-          hrtimer_reprogram(next_expired);
         }
     }
 

sched/hrtimer/hrtimer_initialize.c

@@ -58,15 +58,28 @@ seqcount_t g_hrtimer_lock = SEQLOCK_INITIALIZER;
  * both configurations.
  */
 
+struct hrtimer_s g_hrtimer_guard =
+{
 #ifdef CONFIG_HRTIMER_TREE
-/* Red-black tree for hrtimer storage (requires CONFIG_HRTIMER_TREE) */
+  { NULL },
-
-struct hrtimer_tree_s g_hrtimer_tree = RB_INITIALIZER(g_hrtimer_tree);
-struct FAR hrtimer_s *g_hrtimer_head;
 #else
-/* Linked list for hrtimer storage (fallback when tree is disabled) */
+  { &g_hrtimer_list, &g_hrtimer_list },
+#endif
+  NULL,
+  INT64_MAX
+};
 
-struct list_node g_hrtimer_list = LIST_INITIAL_VALUE(g_hrtimer_list);
+#ifdef CONFIG_HRTIMER_TREE
+struct hrtimer_tree_s g_hrtimer_tree =
+{
+  &g_hrtimer_guard
+};
+struct FAR hrtimer_s *g_hrtimer_head = &g_hrtimer_guard;
+#else
+struct list_node g_hrtimer_list =
+{
+  &g_hrtimer_guard.node, &g_hrtimer_guard.node
+};
 #endif
 
 /****************************************************************************

sched/hrtimer/hrtimer_process.c

@@ -84,16 +84,15 @@ void hrtimer_process(uint64_t now)
   /* Fetch the earliest active timer */
 
   hrtimer = hrtimer_get_first();
+  expired = hrtimer->expired;
 
   /* Check if the timer has expired */
 
-  while (hrtimer != NULL && HRTIMER_TIME_BEFORE_EQ(hrtimer->expired, now))
+  while (HRTIMER_TIME_BEFORE_EQ(expired, now))
     {
-      func    = hrtimer->func;
-      expired = hrtimer->expired;
-
       /* Remove the expired timer from the timer queue */
 
+      func = hrtimer->func;
       hrtimer_remove(hrtimer);
 
       hrtimer_mark_running(hrtimer, cpu);
@@ -130,17 +129,18 @@ void hrtimer_process(uint64_t now)
       /* Fetch the next earliest timer */
 
       hrtimer = hrtimer_get_first();
+      expired = hrtimer->expired;
     }
 
   hrtimer_unmark_running(cpu);
 
   /* Schedule the next timer expiration */
 
-  if (hrtimer != NULL)
+  if (expired != now)
     {
       /* Start timer for the next earliest expiration */
 
-      hrtimer_reprogram(hrtimer->expired);
+      hrtimer_reprogram(expired);
     }
 
   /* Release the lock and give up the ownership of the hrtimer queue. */