find below the fix that puts yielding tasks to the rightmost position in
the rbtree. I have not tested it extensively yet, but it appears to work
on x86. (i tested yield using interactive tasks and they get hurt now
under load - but this would be expected.)
Ingo
---------------------->
Subject: sched: make yield more agressive
From: Ingo Molnar <mingo@elte.hu>
make sys_sched_yield() more agressive, by moving the yielding task
to the last position in the rbtree.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
kernel/sched.c | 5 +----
kernel/sched_fair.c | 39 ++++++++++++++++++++++++++++++++++-----
2 files changed, 35 insertions(+), 9 deletions(-)
Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -4550,10 +4550,7 @@ asmlinkage long sys_sched_yield(void)
struct rq *rq = this_rq_lock();
schedstat_inc(rq, yld_cnt);
- if (unlikely(rq->nr_running == 1))
- schedstat_inc(rq, yld_act_empty);
- else
- current->sched_class->yield_task(rq, current);
+ current->sched_class->yield_task(rq, current);
/*
* Since we are going to call schedule() anyway, there's
Index: linux/kernel/sched_fair.c
===================================================================
--- linux.orig/kernel/sched_fair.c
+++ linux/kernel/sched_fair.c
@@ -902,14 +902,43 @@ static void dequeue_task_fair(struct rq
static void yield_task_fair(struct rq *rq, struct task_struct *p)
{
struct cfs_rq *cfs_rq = task_cfs_rq(p);
+ struct rb_node **link = &cfs_rq->tasks_timeline.rb_node;
+ struct sched_entity *rightmost, *se = &p->se;
+ struct rb_node *parent;
- __update_rq_clock(rq);
/*
- * Dequeue and enqueue the task to update its
- * position within the tree:
+ * Are we the only task in the tree?
*/
- dequeue_entity(cfs_rq, &p->se, 0);
- enqueue_entity(cfs_rq, &p->se, 0);
+ if (unlikely(cfs_rq->nr_running == 1))
+ return;
+ /*
+ * Find the rightmost entry in the rbtree:
+ */
+ do {
+ parent = *link;
+ link = &parent->rb_right;
+ } while (*link);
+
+ rightmost = rb_entry(parent, struct sched_entity, run_node);
+ /*
+ * Already in the rightmost position?
+ */
+ if (unlikely(rightmost == se))
+ return;
+
+ /*
+ * Minimally necessary key value to be last in the tree:
+ */
+ se->fair_key = rightmost->fair_key + 1;
+
+ if (cfs_rq->rb_leftmost == &se->run_node)
+ cfs_rq->rb_leftmost = rb_next(&se->run_node);
+ /*
+ * Relink the task to the rightmost position:
+ */
+ rb_erase(&se->run_node, &cfs_rq->tasks_timeline);
+ rb_link_node(&se->run_node, parent, link);
+ rb_insert_color(&se->run_node, &cfs_rq->tasks_timeline);
}
/*
-
