memory tiering: adjust hot threshold automatically

The promotion hot threshold is workload and system configuration
dependent.  So in this patch, a method to adjust the hot threshold
automatically is implemented.  The basic idea is to control the number of
the candidate promotion pages to match the promotion rate limit.  If the
hint page fault latency of a page is less than the hot threshold, we will
try to promote the page, and the page is called the candidate promotion
page.

If the number of the candidate promotion pages in the statistics interval
is much more than the promotion rate limit, the hot threshold will be
decreased to reduce the number of the candidate promotion pages. 
Otherwise, the hot threshold will be increased to increase the number of
the candidate promotion pages.

To make the above method works, in each statistics interval, the total
number of the pages to check (on which the hint page faults occur) and the
hot/cold distribution need to be stable.  Because the page tables are
scanned linearly in NUMA balancing, but the hot/cold distribution isn't
uniform along the address usually, the statistics interval should be
larger than the NUMA balancing scan period.  So in the patch, the max scan
period is used as statistics interval and it works well in our tests.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: "Huang, Ying" <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Tested-by: Baolin Wang <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: osalvador <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Wei Xu <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Zhong Jiang <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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