The purpose of the InnoDB page cleaner subsystem is to write out
modified pages from the buffer pool to data files. When the
innodb_max_dirty_pages_pct_lwm is not exceeded or
innodb_adaptive_flushing=ON decides not to write out anything,
the page cleaner should keep sleeping indefinitely until the state
of the system changes: a dirty page is added to the buffer pool such
that the page cleaner would no longer be idle.

buf_flush_page_cleaner(): Explicitly note when the page cleaner is idle.
When that happens, use mysql_cond_wait() instead of mysql_cond_timedwait().

buf_flush_insert_into_flush_list(): Wake up the page cleaner if needed.

innodb_max_dirty_pages_pct_update(),
innodb_max_dirty_pages_pct_lwm_update():
Wake up the page cleaner just in case.

Note: buf_flush_ahead(), buf_flush_wait_flushed() and shutdown are
already waking up the page cleaner thread.