lowmemorykiller: adapt to vmpressure

There were issues reported, where page cache thrashing was
observed because of LMK not killing tasks when required,
resulting in sluggishness and higher app launch latency.
LMK does not kill a task for the following reasons.
1. The free and file pages are above the LMK thresholds
2. LMK tries to pick task with an adj level corresponding
to current thresholds, but fails to do so because of the
absence of tasks in that level.
But sometimes it is better to kill a lower adj task, than thrashing.
And there are cases where the number of file pages are huge, though
we dont thrash, the reclaim process becomes time consuming, since
LMK triggers will be delayed because of higher number of file
pages. Even in such cases, when reclaim path finds it difficult
to reclaim pages, it is better to trigger lmk to free up some memory
faster.

The basic idea here is to make LMK more aggressive dynamically
when such a thrashing scenario is detected.

To detect thrashing, this patch uses vmpressure events.
The values of vmpressure upon which an action has to be taken,
was derived empirically.

This patch also adds tracepoints to validate this feature,
almk_shrink and almk_vmpressure.

Two knobs are available for the user to tune adaptive lmk
behaviour.

/sys/module/lowmemorykiller/parameters/adaptive_lmk - Write
1 to enable the feature, 0 to disable. By default disabled.

/sys/module/lowmemorykiller/parameters/vmpressure_file_min -
This parameter controls the behaviour of LMK when vmpressure
is in the range of 90-94. Adaptive lmk triggers based on number file
pages wrt vmpressure_file_min, when vmpressure is in the range of
90-94. Usually this is a pseudo minfree value, higher than the
highest configured value in minfree array.

Change-Id: I1a08160c35d3e33bdfd1d2c789c288fc07d0f0d3
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>