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.