sched_wakeup_new and sched_kthread_stop events cause great overload

[Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>]

We have done sysbench test for ftrace's performance and it looks sched_wakeup_new
and sched_kthread_stop events can cause great overload.

When we only enable sched_wakeup_new and sched_kthread_stop events, sysbench.threads
shows the overload is 10%, sysbench.mutex shows the overload is 7.5%.

The more weird thing is that we found the sched_kthread_stop event is never called
in this test, the test steps as follow:

echo > debugfs/tracing/set_event
echo 1 > debugfs/tracing/events/sched/sched_wakeup_new/enable
echo 1 > debugfs/tracing/events/sched/sched_kthread_stop/enable

com_opt="--num-threads=5000 --max-requests=50000"
echo > debugfs/tracing/trace
sysbench $com_opt --test=threads --thread-yields=1000 --thread-locks=10000 run >& log
[or sysbench $com_opt --test=mutex --mutex-num=100 --mutex-locks=50000 --mutex-loops=10000 run for mutex]
echo > debugfs/tracing/set_event

For sysbench.threads:
cat debugfs/tracing/trace | grep "sched_wakeup_new" | wc -l
5001
cat debugfs/tracing/trace | grep "sched_kthread_stop" | wc -l
0

For sysbench.mutex:
cat debugfs/tracing/trace | grep "sched_wakeup_new" | wc -l
5001
cat debugfs/tracing/trace | grep "sched_kthread_stop" | wc -l
0

And, if only enable sched_kthread_stop event, the sysbench.threads's
overload is 5.90%, the sysbench.mutex's overload is 3.36%.

It hardly explain why sched_kthread_stop is never called but cause great overload.