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Date: Fri, 23 Feb 2024 14:42:12 +0100
From: Jan Kara <jack@suse.cz>
To: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/7] fs/writeback: avoid to writeback non-expired inode
 in kupdate writeback
Message-ID: <20240223134212.g6m7oluhkjlpur2r@quack3>
References: <20240208172024.23625-1-shikemeng@huaweicloud.com>
 <20240208172024.23625-2-shikemeng@huaweicloud.com>
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In-Reply-To: <20240208172024.23625-2-shikemeng@huaweicloud.com>

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@huaweicloud.com>

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 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
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR