to #28739709

Export wait_latency in "cpuacct.wait_latency", which indicates the
time that tasks in a cpuacct cgroup wait on a cfs_rq to be scheduled.

This is like "perf sched", but it gives smaller overhead. So it can
be used as monitor constantly.

wait_latency is useful to debug application's high RT problem. It can
tell if it's caused by scheduling or not. If it is, loadavg can tell
if it's caused by bad scheduling bahaviour or system overloads.

System admins can also use wait_latency to define SLA. To ensure SLA
is guaranteed, there are various ways to decrease wait_latency.

This feature is disabled by default for performance concerns. It can
be switched on dynamically by "echo 0 > /proc/cpusli/sched_lat_enable"

Example:

  $ cat /sys/fs/cgroup/cpuacct/a/cpuacct.wait_latency
    0-1ms:  4139
    1-4ms:  317
    4-7ms:  568
    7-10ms:         0
    10-100ms:       42324
    100-500ms:      9131
    500-1000ms:     95
    1000-5000ms:    134
    5000-10000ms:   0
    >=10000ms:      0
    total(ms):      4256455
Signed-off-by: NYihao Wu <wuyihao@linux.alibaba.com>
Acked-by: NXunlei Pang <xlpang@linux.alibaba.com>
Reviewed-by: NShanpei Chen <shanpeic@linux.alibaba.com>
Acked-by: NMichael Wang <yun.wang@linux.alibaba.com>