mm: vmscan: decide whether to compact the pgdat based on reclaim progress

In the past, kswapd makes a decision on whether to compact memory after
the pgdat was considered balanced.  This more or less worked but it is
late to make such a decision and does not fit well now that kswapd makes
a decision whether to exit the zone scanning loop depending on reclaim
progress.

This patch will compact a pgdat if at least the requested number of
pages were reclaimed from unbalanced zones for a given priority.  If any
zone is currently balanced, kswapd will not call compaction as it is
expected the necessary pages are already available.

Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Valdis Kletnieks <[email protected]>
Tested-by: Zlatko Calusic <[email protected]>
Cc: dormando <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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