cpufreq: schedutil: Make iowait boost more energy efficient

Currently the iowait_boost feature in schedutil makes the frequency
go to max on iowait wakeups.  This feature was added to handle a case
that Peter described where the throughput of operations involving
continuous I/O requests [1] is reduced due to running at a lower
frequency, however the lower throughput itself causes utilization to
be low and hence causing frequency to be low hence its "stuck".

Instead of going to max, its also possible to achieve the same effect
by ramping up to max if there are repeated in_iowait wakeups
happening. This patch is an attempt to do that. We start from a lower
frequency (policy->min) and double the boost for every consecutive
iowait update until we reach the maximum iowait boost frequency
(iowait_boost_max).

I ran a synthetic test (continuous O_DIRECT writes in a loop) on an
x86 machine with intel_pstate in passive mode using schedutil.  In
this test the iowait_boost value ramped from 800MHz to 4GHz in 60ms.
The patch achieves the desired improved throughput as the existing
behavior.

[1] https://patchwork.kernel.org/patch/9735885/



Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Joel Fernandes <joelaf@google.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>