sched: Make scale_rt invariant with frequency

sched: Make scale_rt invariant with frequency

The average running time of RT tasks is used to estimate the remaining compute
capacity for CFS tasks. This remaining capacity is the original capacity scaled
down by a factor (aka scale_rt_capacity). This estimation of available capacity
must also be invariant with frequency scaling.

A frequency scaling factor is applied on the running time of the RT tasks for
computing scale_rt_capacity.

In sched_rt_avg_update(), we now scale the RT execution time like below:

  rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT

Then, scale_rt_capacity can be summarized by:

  scale_rt_capacity = SCHED_CAPACITY_SCALE * available / total

with available = total - rq->rt_avg

This has been been optimized in current code by:

  scale_rt_capacity = available / (total >> SCHED_CAPACITY_SHIFT)

But we can also developed the equation like below:

  scale_rt_capacity = SCHED_CAPACITY_SCALE - ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / total)

and we can optimize the equation by removing SCHED_CAPACITY_SHIFT shift in
the computation of rq->rt_avg and scale_rt_capacity().

so rq->rt_avg += rt_delta * arch_scale_freq_capacity()
and
scale_rt_capacity = SCHED_CAPACITY_SCALE - (rq->rt_avg / total)

arch_scale_frequency_capacity() will be called in the hot path of the scheduler
which implies to have a short and efficient function.

As an example, arch_scale_frequency_capacity() should return a cached value that
is updated periodically outside of the hot path.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Morten.Rasmussen@arm.com
Cc: dietmar.eggemann@arm.com
Cc: efault@gmx.de
Cc: kamalesh@linux.vnet.ibm.com
Cc: linaro-kernel@lists.linaro.org
Cc: nicolas.pitre@linaro.org
Cc: preeti@linux.vnet.ibm.com
Cc: riel@redhat.com
Link: http://lkml.kernel.org/r/1425052454-25797-6-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>