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>

1 files changed, 1 insertions, 4 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index deeb1f8e0c30..63e0971c8fbb 100644
--- a/kernel/sched.c
+++ b/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