Comment 12 for bug 1648015

Maybe there's something wrong with page cleaner refactoring in mysql 5.7:

Log info:
2016-12-08T07:33:32.381816Z 0 [Note] InnoDB: page_cleaner: 1000ms intended loop took 875364ms. The settings might not be optimal. (flushed=0, during the time.)
2016-12-08T07:33:32.382092Z 0 [Note] InnoDB: Waiting for purge to start
<freezes here>

Also while dropping databases, there ale lot of dirty pages so maybe page cleaner is not tuned to scan this amount of databases/tables

Change in 5.7:
http://dev.mysql.com/worklog/task/?id=6642

Similar Problem:
https://bugs.mysql.com/bug.php?id=76661