• D
    ext4: improve ext4lazyinit scalability · e22834f0
    Dmitry Monakhov 提交于
    ext4lazyinit is a global thread. This thread performs itable
    initalization under li_list_mtx mutex.
    
    It basically does the following:
    ext4_lazyinit_thread
      ->mutex_lock(&eli->li_list_mtx);
      ->ext4_run_li_request(elr)
        ->ext4_init_inode_table-> Do a lot of IO if the list is large
    
    And when new mount/umount arrive they have to block on ->li_list_mtx
    because  lazy_thread holds it during full walk procedure.
    ext4_fill_super
     ->ext4_register_li_request
       ->mutex_lock(&ext4_li_info->li_list_mtx);
       ->list_add(&elr->lr_request, &ext4_li_info >li_request_list);
    In my case mount takes 40minutes on server with 36 * 4Tb HDD.
    Common user may face this in case of very slow dev ( /dev/mmcblkXXX)
    Even more. If one of filesystems was frozen lazyinit_thread will simply
    block on sb_start_write() so other mount/umount will be stuck forever.
    
    This patch changes logic like follows:
    - grab ->s_umount read sem before processing new li_request.
      After that it is safe to drop li_list_mtx because all callers of
      li_remove_request are holding ->s_umount for write.
    - li_thread skips frozen SB's
    
    Locking order:
    Mh KOrder is asserted by umount path like follows: s_umount ->li_list_mtx so
    the only way to to grab ->s_mount inside li_thread is via down_read_trylock
    
    xfstests:ext4/023
    #PSBM-49658
    Signed-off-by: NDmitry Monakhov <dmonakhov@openvz.org>
    Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
    e22834f0
super.c 158.1 KB