mm: compaction: support triggering of proactive compaction by user

The proactive compaction[1] gets triggered for every 500msec and run
compaction on the node for COMPACTION_HPAGE_ORDER (usually order-9) pages
based on the value set to sysctl.compaction_proactiveness.  Triggering the
compaction for every 500msec in search of COMPACTION_HPAGE_ORDER pages is
not needed for all applications, especially on the embedded system
usecases which may have few MB's of RAM.  Enabling the proactive
compaction in its state will endup in running almost always on such
systems.

Other side, proactive compaction can still be very much useful for getting
a set of higher order pages in some controllable manner(controlled by
using the sysctl.compaction_proactiveness).  So, on systems where enabling
the proactive compaction always may proove not required, can trigger the
same from user space on write to its sysctl interface.  As an example, say
app launcher decide to launch the memory heavy application which can be
launched fast if it gets more higher order pages thus launcher can prepare
the system in advance by triggering the proactive compaction from
userspace.

This triggering of proactive compaction is done on a write to
sysctl.compaction_proactiveness by user.

[1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=facdaa917c4d5a376d09d25865f5a863f906234a

[[email protected]: tweak vm.rst, per Mike]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Charan Teja Reddy <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Rafael Aquini <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Iurii Zaikin <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Nitin Gupta <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Khalid Aziz <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Vinayak Menon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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