• Wang Xiaoguang's avatar
    btrfs: improve delayed refs iterations · 1d57ee94
    Wang Xiaoguang authored
    This issue was found when I tried to delete a heavily reflinked file,
    when deleting such files, other transaction operation will not have a
    chance to make progress, for example, start_transaction() will blocked
    in wait_current_trans(root) for long time, sometimes it even triggers
    soft lockups, and the time taken to delete such heavily reflinked file
    is also very large, often hundreds of seconds. Using perf top, it reports
    that:
    
    PerfTop:    7416 irqs/sec  kernel:99.8%  exact:  0.0% [4000Hz cpu-clock],  (all, 4 CPUs)
    ---------------------------------------------------------------------------------------
        84.37%  [btrfs]             [k] __btrfs_run_delayed_refs.constprop.80
        11.02%  [kernel]            [k] delay_tsc
         0.79%  [kernel]            [k] _raw_spin_unlock_irq
         0.78%  [kernel]            [k] _raw_spin_unlock_irqrestore
         0.45%  [kernel]            [k] do_raw_spin_lock
         0.18%  [kernel]            [k] __slab_alloc
    It seems __btrfs_run_delayed_refs() took most cpu time, after some debug
    work, I found it's select_delayed_ref() causing this issue, for a delayed
    head, in our case, it'll be full of BTRFS_DROP_DELAYED_REF nodes, but
    select_delayed_ref() will firstly try to iterate node list to find
    BTRFS_ADD_DELAYED_REF nodes, obviously it's a disaster in this case, and
    waste much time.
    
    To fix this issue, we introduce a new ref_add_list in struct btrfs_delayed_ref_head,
    then in select_delayed_ref(), if this list is not empty, we can directly use
    nodes in this list. With this patch, it just took about 10~15 seconds to
    delte the same file. Now using perf top, it reports that:
    
    PerfTop:    2734 irqs/sec  kernel:99.5%  exact:  0.0% [4000Hz cpu-clock],  (all, 4 CPUs)
    ----------------------------------------------------------------------------------------
    
        20.74%  [kernel]          [k] _raw_spin_unlock_irqrestore
        16.33%  [kernel]          [k] __slab_alloc
         5.41%  [kernel]          [k] lock_acquired
         4.42%  [kernel]          [k] lock_acquire
         4.05%  [kernel]          [k] lock_release
         3.37%  [kernel]          [k] _raw_spin_unlock_irq
    
    For normal files, this patch also gives help, at least we do not need to
    iterate whole list to found BTRFS_ADD_DELAYED_REF nodes.
    Signed-off-by: default avatarWang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
    Reviewed-by: default avatarLiu Bo <bo.li.liu@oracle.com>
    Tested-by: default avatarHolger Hoffstätte <holger@applied-asynchrony.com>
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    1d57ee94
disk-io.c 126 KB