add extra free kbytes tunable

Add a userspace visible knob to tell the VM to keep an extra amount
of memory free, by increasing the gap between each zone's min and
low watermarks.

This is useful for realtime applications that call system
calls and have a bound on the number of allocations that happen
in any short time period.  In this application, extra_free_kbytes
would be left at an amount equal to or larger than than the
maximum number of allocations that happen in any burst.

It may also be useful to reduce the memory use of virtual
machines (temporarily?), in a way that does not cause memory
fragmentation like ballooning does.

[ccross]
Revived for use on old kernels where no other solution exists.
The tunable will be removed on kernels that do better at avoiding
direct reclaim.

Change-Id: I765a42be8e964bfd3e2886d1ca85a29d60c3bb3e
Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Colin Cross <ccross@android.com>
Git-commit: f8ade3666c8612c33929f499b3c8800d83f43d84
Git-repo: https://android.googlesource.com/kernel/common.git


[vinmenon@codeaurora.org: avoid user setting of watermark_scale_factor
to any value other than 0, make the default watermark_scale_factor 0,
changes because of the presence of watermark_scale_factor. Move the
declaration of extra_free_kbytes to include file and remove the
initialization to zero in the definition. This fixes the checkpatch
warnings. Make the inclusion of variable "one_thousand" in kernel/sysctl.c
dependent on CONFIG_PERF_EVENTS which is the only user left now (earlier
being watermark_scale_factor]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>