psi: Reduce calls to sched_clock() in psi

We noticed that the cost of psi increases with the increase in the
levels of the cgroups. Particularly the cost of cpu_clock() sticks out
as the kernel calls it multiple times as it traverses up the cgroup
tree. This patch reduces the calls to cpu_clock().

Performed perf bench on Intel Broadwell with 3 levels of cgroup.

Before the patch:

$ perf bench sched all
 # Running sched/messaging benchmark...
 # 20 sender and receiver processes per group
 # 10 groups == 400 processes run

     Total time: 0.747 [sec]

 # Running sched/pipe benchmark...
 # Executed 1000000 pipe operations between two processes

     Total time: 3.516 [sec]

       3.516689 usecs/op
         284358 ops/sec

After the patch:

$ perf bench sched all
 # Running sched/messaging benchmark...
 # 20 sender and receiver processes per group
 # 10 groups == 400 processes run

     Total time: 0.640 [sec]

 # Running sched/pipe benchmark...
 # Executed 1000000 pipe operations between two processes

     Total time: 3.329 [sec]

       3.329820 usecs/op
         300316 ops/sec

Signed-off-by: Shakeel Butt <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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