on 2/23/2024 9:42 PM, Jan Kara wrote:
> On Fri 09-02-24 01:20:18, Kemeng Shi wrote:
>> In kupdate writeback, only expired inode (have been dirty for longer than
>> dirty_expire_interval) is supposed to be written back. However, kupdate
>> writeback will writeback non-expired inode left in b_io or b_more_io from
>> last wb_writeback. As a result, writeback will keep being triggered
>> unexpected when we keep dirtying pages even dirty memory is under
>> threshold and inode is not expired. To be more specific:
>> Assume dirty background threshold is > 1G and dirty_expire_centisecs is
>>> 60s. When we running fio -size=1G -invalidate=0 -ioengine=libaio
>> --time_based -runtime=60... (keep dirtying), the writeback will keep
>> being triggered as following:
>> wb_workfn
>> wb_do_writeback
>> wb_check_background_flush
>> /*
>> * Wb dirty background threshold starts at 0 if device was idle and
>> * grows up when bandwidth of wb is updated. So a background
>> * writeback is triggered.
>> */
>> wb_over_bg_thresh
>> /*
>> * Dirtied inode will be written back and added to b_more_io list
>> * after slice used up (because we keep dirtying the inode).
>> */
>> wb_writeback
>>
>> Writeback is triggered per dirty_writeback_centisecs as following:
>> wb_workfn
>> wb_do_writeback
>> wb_check_old_data_flush
>> /*
>> * Write back inode left in b_io and b_more_io from last wb_writeback
>> * even the inode is non-expired and it will be added to b_more_io
>> * again as slice will be used up (because we keep dirtying the
>> * inode)
>> */
>> wb_writeback
>>
>> Fix this by moving non-expired inode in io list from last wb_writeback to
>> dirty list in kudpate writeback.
>>
>> Test as following:
>> /* make it more easier to observe the issue */
>> echo 300000 > /proc/sys/vm/dirty_expire_centisecs
>> echo 100 > /proc/sys/vm/dirty_writeback_centisecs
>> /* create a idle device */
>> mkfs.ext4 -F /dev/vdb
>> mount /dev/vdb /bdi1/
>> /* run buffer write with fio */
>> fio -name test -filename=/bdi1/file -size=800M -ioengine=libaio -bs=4K \
>> -iodepth=1 -rw=write -direct=0 --time_based -runtime=60 -invalidate=0
>>
>> Result before fix (run three tests):
>> 1360MB/s
>> 1329MB/s
>> 1455MB/s
>>
>> Result after fix (run three tests);
>> 790MB/s
>> 1820MB/s
>> 1804MB/s
>>
>> Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
>
> OK, I don't find this a particularly troubling problem but I agree it might
> be nice to fix. But filtering the lists in wb_writeback() like this seems
> kind of wrong - the queueing is managed in queue_io() and I'd prefer to
> keep it that way. What if we just modified requeue_inode() to not
> requeue_io() inodes in case we are doing kupdate style writeback and inode
> isn't expired?
Sure, this could solve the reported problem and is acceptable to me. Thanks
for the advise. I will try it in next version.
>
> Sure we will still possibly writeback unexpired inodes once before calling
> redirty_tail_locked() on them but that shouldn't really be noticeable?
>
> Honza
>> ---
>> fs/fs-writeback.c | 20 ++++++++++++++++++++
>> 1 file changed, 20 insertions(+)
>>
>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>> index 5ab1aaf805f7..a9a918972719 100644
>> --- a/fs/fs-writeback.c
>> +++ b/fs/fs-writeback.c
>> @@ -2046,6 +2046,23 @@ static long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages,
>> return nr_pages - work.nr_pages;
>> }
>>
>> +static void filter_expired_io(struct bdi_writeback *wb)
>> +{
>> + struct inode *inode, *tmp;
>> + unsigned long expired_jiffies = jiffies -
>> + msecs_to_jiffies(dirty_expire_interval * 10);
>> +
>> + spin_lock(&wb->list_lock);
>> + list_for_each_entry_safe(inode, tmp, &wb->b_io, i_io_list)
>> + if (inode_dirtied_after(inode, expired_jiffies))
>> + redirty_tail(inode, wb);
>> +
>> + list_for_each_entry_safe(inode, tmp, &wb->b_more_io, i_io_list)
>> + if (inode_dirtied_after(inode, expired_jiffies))
>> + redirty_tail(inode, wb);
>> + spin_unlock(&wb->list_lock);
>> +}
>> +
>> /*
>> * Explicit flushing or periodic writeback of "old" data.
>> *
>> @@ -2070,6 +2087,9 @@ static long wb_writeback(struct bdi_writeback *wb,
>> long progress;
>> struct blk_plug plug;
>>
>> + if (work->for_kupdate)
>> + filter_expired_io(wb);
>> +
>> blk_start_plug(&plug);
>> for (;;) {
>> /*
>> --
>> 2.30.0
>>
