mm: process reclaim: vmpressure based process reclaim

With this patch, anon pages of incative tasks can be reclaimed,
depending on memory pressure. Memory pressure is detected
using vmpressure events. 'N' best tasks in terms of anon
size is selected and pages proportional to their tasksize
is reclaimed. The total number of pages reclaimed at each
run of the swap work, can be tuned from userspace, the
default being SWAP_CLUSTER_MAX * 32.

The patch also adds tracepoints to debug and tune the
feature.

echo 1 > /sys/module/process_reclaim/parameters/enable_process_reclaim
to enable the feature.

echo <pages> > /sys/module/process_reclaim/parameters/per_swap_size,
to set the number of pages reclaimed in each scan.

/sys/module/process_reclaim/parameters/reclaim_avg_efficiency, provides
the average efficiency (scan to reclaim ratio) of the algorithm.

/sys/module/process_reclaim/parameters/swap_eff_win, to set the window
period (in unit of number of times reclaim is triggered) to detect
low efficiency runs.

/sys/module/process_reclaim/parameters/swap_opt_eff, to set the optimal
efficiency threshold for low efficiency detection.

Change-Id: I895986f10c997d1715761eaaadc4bbbee60db9d2
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>