For some applications, we need to allocate almost all memory as
hugepages. However, on a running system, higher-order allocations can
fail if the memory is fragmented. Linux kernel currently does on-demand
compaction as we request more hugepages, but this style of compaction
incurs very high latency. Experiments with one-time full memory
compaction (followed by hugepage allocations) show that kernel is able
to restore a highly fragmented memory state to a fairly compacted memory
state within <1 sec for a 32G system. Such data suggests that a more
proactive compaction can help us allocate a large fraction of memory as
hugepages keeping allocation latencies low.

For a more proactive compaction, the approach taken here is to define
a new tunable called 'proactiveness' which dictates bounds for external
fragmentation wrt HUGETLB_PAGE_ORDER order which kcompactd tries to
maintain.

The tunable is exposed through sysctl:
  /proc/sys/vm/compaction_proactiveness

It takes value in range [0, 100], with a default of 20.