[5/11] issue 5: Frequency and uarch invariant task load

[Morten Rasmussen]

Related to the issue of potential cpu capacity, task load is influenced
directly by the current P-state of the cpu it is running on. For
energy-aware task placement decisions the scheduler would need to
estimate the energy impact of scheduling a specific task on a specific
cpu. Depending on the resulting P-state it may be more energy efficient
to wake-up another cpu (see system 1 in mail 11 for energy efficiency
example).

The frequency and uarch impact can be rather significant. On modern
systems frequency scaling covers a range of 5-6x. On top of that uarch
differences may give another 1.5-3x for a total cpu capacity range
covering >10x.

Measurements on ARM TC2 for a simple periodic test workload (single
task, 16 ms period):

        cpu load        load_avg_contrib (10 sample avg.)
Freq    A7      A15     A7      A15
500     16.76%   9.94%  ~201    ~135
700     12.06%   6.95%  ~145     ~87
1000     8.19%   5.23%  ~103     ~65

The cpu load estimate used for load balancing is based on
load_avg_contrib which means that for this example the load estimate may
vary 3x depending on where tasks are scheduled and the frequency scaling
governors used.

Potential solution: Frequency invariance has been proposed before [1]
where the task load is scaled by the cur/max freq ratio. Another
possibility is to use hardware counters if such are available on the
platform.

[1] https://lkml.org/lkml/2013/4/16/289

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