sched/pelt: Add a new runnable average signal

Now that runnable_load_avg has been removed, we can replace it by a new
signal that will highlight the runnable pressure on a cfs_rq. This signal
track the waiting time of tasks on rq and can help to better define the
state of rqs.

At now, only util_avg is used to define the state of a rq:
  A rq with more that around 80% of utilization and more than 1 tasks is
  considered as overloaded.

But the util_avg signal of a rq can become temporaly low after that a task
migrated onto another rq which can bias the classification of the rq.

When tasks compete for the same rq, their runnable average signal will be
higher than util_avg as it will include the waiting time and we can use
this signal to better classify cfs_rqs.

The new runnable_avg will track the runnable time of a task which simply
adds the waiting time to the running time. The runnable _avg of cfs_rq
will be the /Sum of se's runnable_avg and the runnable_avg of group entity
will follow the one of the rq similarly to util_avg.

Signed-off-by: Vincent Guittot <[email protected]>
Signed-off-by: Mel Gorman <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: "Dietmar Eggemann <[email protected]>"
Acked-by: Peter Zijlstra <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Valentin Schneider <[email protected]>
Cc: Phil Auld <[email protected]>
Cc: Hillf Danton <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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