FROMLIST: sched/fair: util_est: fast ramp-up EWMA on utilization increases

The estimated utilization for a task:

   util_est = max(util_avg, est.enqueue, est.ewma)

is defined based on:
 - util_avg: the PELT defined utilization
 - est.enqueued: the util_avg at the end of the last activation
 - est.ewma:     a exponential moving average on the est.enqueued
                 samples

According to this definition, when a task suddenly change its bandwidth
requirements from small to big, the EWMA will need to collect multiple
samples before converging up to track the new big utilization.

This slow convergence towards bigger utilization values is not
aligned to the default scheduler behavior, which is to optimize for
performance. Moreover, the est.ewma component fails to compensate for
temporarely utilization drops which spans just few est.enqueued samples.

To let util_est do a better job in the scenario depicted above, change
its definition by making util_est directly follow upward motion and
only decay the est.ewma on downward.

Signed-off-by: Patrick Bellasi <patrick.bellasi@matbug.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
(am from https://lkml.org/lkml/2019/10/23/1071

)
Signed-off-by: Adam W. Willis <return.of.octobot@gmail.com>
Signed-off-by: Yaroslav Furman <yaro330@gmail.com>