kernel: use skip list to implement timer list

Skip list is a "random" data structure that in high possibilities it
would get O(log(N)) time complexity in inserting while the old list get
O(N). Forthermore, when set RT_TIMER_SKIP_LIST_LEVEL to 1, it will just
the same as the old double linked list, both in time and space
complexity.

Benchmarks shows that when RT_TIMER_SKIP_LIST_LEVEL is 3, the average
time of random insertion of new timer is about 2 times faster than the
old timer when there are 100 timers and 3 times faster when there are
200 timers.

However, it restores the deprecated funcion rt_system_timer_init. BSPs
must invoke it upon system startup.

1 files changed, 2 insertions, 7 deletions

diff --git a/src/thread.c b/src/thread.c
index da77ace289..c2282f6ee6 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -61,8 +61,7 @@ static void rt_thread_exit(void)
     thread->stat = RT_THREAD_CLOSE;
 
     /* remove it from timer list */
-    rt_list_remove(&(thread->thread_timer.list));
-    rt_object_detach((rt_object_t)&(thread->thread_timer));
+    rt_timer_detach(&thread->thread_timer);
 
     if ((rt_object_is_systemobject((rt_object_t)thread) == RT_TRUE) &&
         thread->cleanup == RT_NULL)
@@ -613,11 +612,7 @@ rt_err_t rt_thread_resume(rt_thread_t thread)
     /* remove from suspend list */
     rt_list_remove(&(thread->tlist));
 
-    /* remove thread timer */
-    rt_list_remove(&(thread->thread_timer.list));
-
-    /* change timer state */
-    thread->thread_timer.parent.flag &= ~RT_TIMER_FLAG_ACTIVATED;
+    rt_timer_stop(&thread->thread_timer);
 
     /* enable interrupt */
     rt_hw_interrupt_enable(temp);