Re: [PATCH v2 1/6] fs/writeback: avoid to writeback non-expired inode in kupdate writeback

[Jan Kara]

On Wed 28-02-24 17:19:53, 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 to dirty list instead of more io
> list for kupdate writeback in requeue_inode.
> 
> 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
> 
> Fio result before fix (run three tests):
> 1360MB/s
> 1329MB/s
> 1455MB/s
> 
> Fio result after fix (run three tests):
> 1737MB/s
> 1729MB/s
> 1789MB/s
> 
> Writeback for non-expired inode is gone as expeted. Observe this with trace
> writeback_start and writeback_written as following:
> echo 1 > /sys/kernel/debug/tracing/events/writeback/writeback_start/enab
> echo 1 > /sys/kernel/debug/tracing/events/writeback/writeback_written/enable
> cat /sys/kernel/tracing/trace_pipe
> 
> Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  fs/fs-writeback.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 5ab1aaf805f7..4e6166e07eaf 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1561,7 +1561,8 @@ static void inode_sleep_on_writeback(struct inode *inode)
>   * thread's back can have unexpected consequences.
>   */
>  static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
> -			  struct writeback_control *wbc)
> +			  struct writeback_control *wbc,
> +			  unsigned long dirtied_before)
>  {
>  	if (inode->i_state & I_FREEING)
>  		return;
> @@ -1594,7 +1595,8 @@ static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
>  		 * We didn't write back all the pages.  nfs_writepages()
>  		 * sometimes bales out without doing anything.
>  		 */
> -		if (wbc->nr_to_write <= 0) {
> +		if (wbc->nr_to_write <= 0 &&
> +		    !inode_dirtied_after(inode, dirtied_before)) {
>  			/* Slice used up. Queue for next turn. */
>  			requeue_io(inode, wb);
>  		} else {
> @@ -1862,6 +1864,11 @@ static long writeback_sb_inodes(struct super_block *sb,
>  	unsigned long start_time = jiffies;
>  	long write_chunk;
>  	long total_wrote = 0;  /* count both pages and inodes */
> +	unsigned long dirtied_before = jiffies;
> +
> +	if (work->for_kupdate)
> +		dirtied_before = jiffies -
> +			msecs_to_jiffies(dirty_expire_interval * 10);
>  
>  	while (!list_empty(&wb->b_io)) {
>  		struct inode *inode = wb_inode(wb->b_io.prev);
> @@ -1967,7 +1974,7 @@ static long writeback_sb_inodes(struct super_block *sb,
>  		spin_lock(&inode->i_lock);
>  		if (!(inode->i_state & I_DIRTY_ALL))
>  			total_wrote++;
> -		requeue_inode(inode, tmp_wb, &wbc);
> +		requeue_inode(inode, tmp_wb, &wbc, dirtied_before);
>  		inode_sync_complete(inode);
>  		spin_unlock(&inode->i_lock);
>  
> -- 
> 2.30.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR