mm, pcp: decrease PCP high if free pages < high watermark

One target of PCP is to minimize pages in PCP if the system free pages is
too few.  To reach that target, when page reclaiming is active for the
zone (ZONE_RECLAIM_ACTIVE), we will stop increasing PCP high in allocating
path, decrease PCP high and free some pages in freeing path.  But this may
be too late because the background page reclaiming may introduce latency
for some workloads.  So, in this patch, during page allocation we will
detect whether the number of free pages of the zone is below high
watermark.  If so, we will stop increasing PCP high in allocating path,
decrease PCP high and free some pages in freeing path.  With this, we can
reduce the possibility of the premature background page reclaiming caused
by too large PCP.

The high watermark checking is done in allocating path to reduce the
overhead in hotter freeing path.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: "Huang, Ying" <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Sudeep Holla <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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