Re: [PATCH 3/6] writeback: sync expired inodes first in background writeback

Jan Kara
2011-04-21T16:04:14Z

On Thu 21-04-11 12:10:11, Wu Fengguang wrote:
> > > Still, given wb_writeback() is the only caller of both
> > > __writeback_inodes_sb and writeback_inodes_wb(), I'm wondering if
> > > moving the queue_io calls up into wb_writeback() would clean up this
> > > logic somewhat. I think Jan mentioned doing something like this as
> > > well elsewhere in the thread...
> >
> > Unfortunately they call queue_io() inside the lock..
>
> OK, let's try moving up the lock too. Do you like this change? :)
>
> Thanks,
> Fengguang
> ---
> fs/fs-writeback.c | 22 ++++++----------------
> mm/backing-dev.c | 4 ++++
> 2 files changed, 10 insertions(+), 16 deletions(-)
>
> --- linux-next.orig/fs/fs-writeback.c 2011-04-21 12:04:02.000000000 +0800
> +++ linux-next/fs/fs-writeback.c 2011-04-21 12:05:54.000000000 +0800
> @@ -591,7 +591,6 @@ void writeback_inodes_wb(struct bdi_writ
>
> if (!wbc->wb_start)
> wbc->wb_start = jiffies; /* livelock avoidance */
> - spin_lock(&inode_wb_list_lock);
>
> if (list_empty(&wb->b_io))
> queue_io(wb, wbc);
> @@ -610,22 +609,9 @@ void writeback_inodes_wb(struct bdi_writ
> if (ret)
> break;
> }
> - spin_unlock(&inode_wb_list_lock);
> /* Leave any unwritten inodes on b_io */
> }
>
> -static void __writeback_inodes_sb(struct super_block *sb,
> - struct bdi_writeback *wb, struct writeback_control *wbc)
> -{
> - WARN_ON(!rwsem_is_locked(&sb->s_umount));
> -
> - spin_lock(&inode_wb_list_lock);
> - if (list_empty(&wb->b_io))
> - queue_io(wb, wbc);
> - writeback_sb_inodes(sb, wb, wbc, true);
> - spin_unlock(&inode_wb_list_lock);
> -}
> -
> static inline bool over_bground_thresh(void)
> {
> unsigned long background_thresh, dirty_thresh;
> @@ -652,7 +638,7 @@ static unsigned long writeback_chunk_siz
> * The intended call sequence for WB_SYNC_ALL writeback is:
> *
> * wb_writeback()
> - * __writeback_inodes_sb() <== called only once
> + * writeback_sb_inodes() <== called only once
> * write_cache_pages() <== called once for each inode
> * (quickly) tag currently dirty pages
> * (maybe slowly) sync all tagged pages
> @@ -742,10 +728,14 @@ static long wb_writeback(struct bdi_writ
>
> retry:
> trace_wbc_writeback_start(&wbc, wb->bdi);
> + spin_lock(&inode_wb_list_lock);
> + if (list_empty(&wb->b_io))
> + queue_io(wb, wbc);
> if (work->sb)
> - __writeback_inodes_sb(work->sb, wb, &wbc);
> + writeback_sb_inodes(work->sb, wb, &wbc, true);
> else
> writeback_inodes_wb(wb, &wbc);
> + spin_unlock(&inode_wb_list_lock);
> trace_wbc_writeback_written(&wbc, wb->bdi);
>
> bdi_update_write_bandwidth(wb->bdi, wbc.wb_start);
> --- linux-next.orig/mm/backing-dev.c 2011-04-21 12:06:02.000000000 +0800
> +++ linux-next/mm/backing-dev.c 2011-04-21 12:06:31.000000000 +0800
> @@ -268,7 +268,11 @@ static void bdi_flush_io(struct backing_
> .nr_to_write = 1024,
> };
>
> + spin_lock(&inode_wb_list_lock);
> + if (list_empty(&wb->b_io))
> + queue_io(wb, wbc);
> writeback_inodes_wb(&bdi->wb, &wbc);
> + spin_unlock(&inode_wb_list_lock);
> }
Three notes here:
1) You are missing the call to writeback_inodes_wb() in
balance_dirty_pages() (the patch should really work for vanilla kernels).
2) The intention of both bdi_flush_io() and balance_dirty_pages() is to
write .nr_to_write pages. So they should either do queue_io()
unconditionally (I kind of like that for simplicity) or they should requeue
once if they have not written enough - otherwise it could happen that they
are called just at the moment when b_io contains a single inode with a few
dirty pages and they end up doing almost nothing.
3) I guess your patch does not compile because queue_io() is static ;).

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