sched: add /proc/sys/kernel/sched_compat_yield

add /proc/sys/kernel/sched_compat_yield to make sys_sched_yield()
more agressive, by moving the yielding task to the last position
in the rbtree.

with sched_compat_yield=0:

   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
  2539 mingo     20   0  1576  252  204 R   50  0.0   0:02.03 loop_yield
  2541 mingo     20   0  1576  244  196 R   50  0.0   0:02.05 loop

with sched_compat_yield=1:

   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
  2584 mingo     20   0  1576  248  196 R   99  0.0   0:52.45 loop
  2582 mingo     20   0  1576  256  204 R    0  0.0   0:00.00 loop_yield

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

include/linux/sched.h

@@ -1406,6 +1406,7 @@ extern unsigned int sysctl_sched_wakeup_granularity;
 extern unsigned int sysctl_sched_batch_wakeup_granularity;
 extern unsigned int sysctl_sched_stat_granularity;
 extern unsigned int sysctl_sched_runtime_limit;
+extern unsigned int sysctl_sched_compat_yield;
 extern unsigned int sysctl_sched_child_runs_first;
 extern unsigned int sysctl_sched_features;
 

kernel/sched.c

@@ -4550,9 +4550,6 @@ 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);
 
 	/*

kernel/sched_fair.c

@@ -42,6 +42,14 @@ unsigned int sysctl_sched_latency __read_mostly = 20000000ULL;
  */
 unsigned int sysctl_sched_min_granularity __read_mostly = 2000000ULL;
 
+/*
+ * sys_sched_yield() compat mode
+ *
+ * This option switches the agressive yield implementation of the
+ * old scheduler back on.
+ */
+unsigned int __read_mostly sysctl_sched_compat_yield;
+
 /*
  * SCHED_BATCH wake-up granularity.
  * (default: 25 msec, units: nanoseconds)
@@ -897,12 +905,24 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int sleep)
 }
 
 /*
- * sched_yield() support is very simple - we dequeue and enqueue
+ * sched_yield() support is very simple - we dequeue and enqueue.
+ *
+ * If compat_yield is turned on then we requeue to the end of the tree.
  */
 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;
 
+	/*
+	 * Are we the only task in the tree?
+	 */
+	if (unlikely(cfs_rq->nr_running == 1))
+		return;
+
+	if (likely(!sysctl_sched_compat_yield)) {
 		__update_rq_clock(rq);
 		/*
 		 * Dequeue and enqueue the task to update its
@@ -910,6 +930,37 @@ static void yield_task_fair(struct rq *rq, struct task_struct *p)
 		 */
 		dequeue_entity(cfs_rq, &p->se, 0);
 		enqueue_entity(cfs_rq, &p->se, 0);
+
+		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);
 }
 
 /*

kernel/sysctl.c

@@ -303,6 +303,14 @@ static ctl_table kern_table[] = {
 		.proc_handler	= &proc_dointvec,
 	},
 #endif
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "sched_compat_yield",
+		.data		= &sysctl_sched_compat_yield,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 #ifdef CONFIG_PROVE_LOCKING
 	{
 		.ctl_name	= CTL_UNNUMBERED,