mm: compaction: skip fast freepages isolation if enough freepages are isolated

I've observed that fast isolation often isolates more pages than
cc->migratepages, and the excess freepages will be released back to the
buddy system.  So skip fast freepages isolation if enough freepages are
isolated to save some CPU cycles.

Link: https://lkml.kernel.org/r/f39c2c07f2dba2732fd9c0843572e5bef96f7f67.1685018752.git.baolin.wang@linux.alibaba.comSigned-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/compaction.c

@@ -1528,6 +1528,10 @@ static void fast_isolate_freepages(struct compact_control *cc)
 
 		spin_unlock_irqrestore(&cc->zone->lock, flags);
 
+		/* Skip fast search if enough freepages isolated */
+		if (cc->nr_freepages >= cc->nr_migratepages)
+			break;
+
 		/*
 		 * Smaller scan on next order so the total scan is related
 		 * to freelist_scan_limit.