mm: only force scan in reclaim when none of the LRUs are big enough.

mm: only force scan in reclaim when none of the LRUs are big enough.

Prior to this change, we would decide whether to force scan a LRU during
reclaim if that LRU itself was too small for the current priority.
However, this can lead to the file LRU getting force scanned even if
there are a lot of anonymous pages we can reclaim, leading to hot file
pages getting needlessly reclaimed.

To address this, we instead only force scan when none of the reclaimable
LRUs are big enough.

Gives huge improvements with zswap.  For example, when doing -j20 kernel
build in a 500MB container with zswap enabled, runtime (in seconds) is
greatly reduced:

x without this change
+ with this change
    N           Min           Max        Median           Avg        Stddev
x   5       700.997       790.076       763.928        754.05      39.59493
+   5       141.634       197.899       155.706         161.9     21.270224
Difference at 95.0% confidence
        -592.15 +/- 46.3521
        -78.5293% +/- 6.14709%
        (Student's t, pooled s = 31.7819)

Should also give some improvements in regular (non-zswap) swap cases.

Yes, hughd found significant speedup using regular swap, with several
memcgs under pressure; and it should also be effective in the non-memcg
case, whenever one or another zone LRU is forced too small.

Signed-off-by: Suleiman Souhlal <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Cc: Suleiman Souhlal <[email protected]>
Cc: Mel Gorman <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Acked-by: Rafael Aquini <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Yuanhan Liu <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Bob Liu <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Luigi Semenzato <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>