sched: rt-group: optimize dequeue_rt_stack

Now that the group hierarchy can have an arbitrary depth the O(n^2) nature
of RT task dequeues will really hurt. Optimize this by providing space to
store the tree path, so we can walk it the other way.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0a32059..887f5db 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1005,6 +1005,7 @@ struct sched_rt_entity {
        unsigned long timeout;
        int nr_cpus_allowed;
 
+       struct sched_rt_entity *back;
 #ifdef CONFIG_RT_GROUP_SCHED
        struct sched_rt_entity  *parent;
        /* rq on which this entity is (to be) queued: */

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 736fb8f..c2730a5 100644 (file)
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -479,26 +479,21 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
 /*
  * Because the prio of an upper entry depends on the lower
  * entries, we must remove entries top - down.
- *
- * XXX: O(1/2 h^2) because we can only walk up, not down the chain.
  */
 static void dequeue_rt_stack(struct task_struct *p)
 {
-       struct sched_rt_entity *rt_se, *top_se;
+       struct sched_rt_entity *rt_se, *back = NULL;
 
-       /*
-        * dequeue all, top - down.
-        */
-       do {
-               rt_se = &p->rt;
-               top_se = NULL;
-               for_each_sched_rt_entity(rt_se) {
-                       if (on_rt_rq(rt_se))
-                               top_se = rt_se;
-               }
-               if (top_se)
-                       dequeue_rt_entity(top_se);
-       } while (top_se);
+       rt_se = &p->rt;
+       for_each_sched_rt_entity(rt_se) {
+               rt_se->back = back;
+               back = rt_se;
+       }
+
+       for (rt_se = back; rt_se; rt_se = rt_se->back) {
+               if (on_rt_rq(rt_se))
+                       dequeue_rt_entity(rt_se);
+       }
 }
 
 /*