cpufreq: interactive: Rearm governor timer at max freq

cpufreq: interactive: Rearm governor timer at max freq

Interactive governor doesn't rearm per-cpu timer if target_freq is
equal to policy->max. However, this does not have clear performance
benefits. Profiling doesn't show any difference in benchmarks, games
or other workloads, if timers are always rearmed.

At same time, there are a few issues caused by not rearming timer
at policy->max.

1) min_sample_time enforcement is inconsistent

For target frequency that is lower than policy->max, it will not
drop until min_sample_time has passed since last frequency evaluation
selected current frequency. However, for policy->max, it will
always drop immediately as long as CPU has been run for longer than
min_sample_time. This is because timer is not running and thus
floor_freq and floor_validate_time is not updated.

Example: assume min_sample_time is 59ms and timer_rate is 20ms.
Frequency X < Y. Let's say CPU would pick the following frequencies
before accounting for min_sample_time in each 20ms sampling window.
Y, Y, Y, Y, X, X, X, X, X
If Y is not policy->max, the final target_freq after considering
min_sample_time will be Y, Y, Y, Y, *Y, *Y, X, X, X
* marks the windows where frequency is prevented from dropping.
If Y is policy->max, the final target_freq will be
Y, Y, Y, Y, X, X, X, X, X

2) Rearm timer in IDLE_START does not work as intended

IDLE_START/END is sent in arch_cpu_idle_enter/exit(). However, next
wake up is decided in tick_nohz_idle_enter(), which traverses the
timer list before idle notification is sent out. Therefore, rearming
timer in idle notification won't take effect until CPU wakes up at
least once. In rare scenarios when a CPU goes to idle and sleeps for a
long time immediately after a heavy load stops, it may not wake up
to drop its frequency vote for a long time, defeating the purpose of
having a slack_timer.

3) Need to rearm timer for policy->max change

commit 535a553fc1c4b4c3627c73214ade6326615a7463
(cpufreq: interactive: restructure CPUFREQ_GOV_LIMITS) mentions the
problem of timer getting indefinitely pushed back due to frequency
changes in policy->min/max. However, it still cancels and rearms timer
if policy->max is increased, and same problem could still happen if
policy->max is frequently changing after the fix. The best solution is
to always rearm timer for each CPU even if it's running at
policy->max.

Rearming timers even if target_freq is policy->max solves these
problems cleanly. It also simplifies the design and code of interactive
governor.

Change-Id: I973853d2375ea6f697fa4cee04a89efe6b8bf735
Reviewed-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>