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

Jan Kara
2011-04-22T21:13:11Z

On Fri 22-04-11 10:24:59, Wu Fengguang wrote:
> > 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.
>
> It makes much more sense to keep the policy consistent. When the
> flusher and the throttled tasks are both actively manipulating the
> shared lists but in different ways, how are we going to analyze the
> resulted mixture behavior?
>
> Note that bdi_flush_io() and balance_dirty_pages() both have outer
> loops to retry writeout, so smallish b_io is not a problem at all.
Well, it changes how balance_dirty_pages() behaves in some corner cases
(I'm not that much concerned about bdi_flush_io() because that is a last
resort thing anyway). But I see your point in consistency as well.

> > 3) I guess your patch does not compile because queue_io() is static ;).
>
> Yeah, good spot~ :) Here is the updated patch. I feel like moving
> bdi_flush_io() to fs-writeback.c rather than exporting the low level
> queue_io() (and enable others to conveniently change the queue policy!).
>
> balance_dirty_pages() cannot be moved.. so I plan to submit it after
> any IO-less merges. It's a cleanup patch after all.
Can't we just have a wrapper in fs/fs-writeback.c that will do:
spin_lock(&bdi->wb.list_lock);
if (list_empty(&bdi->wb.b_io))
queue_io(&bdi->wb, &wbc);
writeback_inodes_wb(&bdi->wb, &wbc);
spin_unlock(&bdi->wb.list_lock);

And call it wherever we need? We can then also unexport
writeback_inodes_wb() which is not really a function someone would want to
call externally after your changes.

Honza
> ---
> Subject: writeback: move queue_io() up
> Date: Thu Apr 21 12:06:32 CST 2011
>
> Refactor code for more logical code layout.
> No behavior change.
>
> - kill __writeback_inodes_sb()
> - move bdi_flush_io() to fs-writeback.c
> - elevate queue_io() and locking up to wb_writeback() and bdi_flush_io()
>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> fs/fs-writeback.c | 33 ++++++++++++++++++---------------
> include/linux/writeback.h | 1 +
> mm/backing-dev.c | 12 ------------
> 3 files changed, 19 insertions(+), 27 deletions(-)
>
> --- linux-next.orig/fs/fs-writeback.c 2011-04-21 20:11:53.000000000 +0800
> +++ linux-next/fs/fs-writeback.c 2011-04-21 21:11:02.000000000 +0800
> @@ -577,10 +577,6 @@ void writeback_inodes_wb(struct bdi_writ
>
> if (!wbc->wb_start)
> wbc->wb_start = jiffies; /* livelock avoidance */
> - spin_lock(&wb->list_lock);
> -
> - if (list_empty(&wb->b_io))
> - queue_io(wb, wbc);
>
> while (!list_empty(&wb->b_io)) {
> struct inode *inode = wb_inode(wb->b_io.prev);
> @@ -596,20 +592,23 @@ void writeback_inodes_wb(struct bdi_writ
> if (ret)
> break;
> }
> - spin_unlock(&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)
> +void bdi_flush_io(struct backing_dev_info *bdi)
> {
> - WARN_ON(!rwsem_is_locked(&sb->s_umount));
> + struct writeback_control wbc = {
> + .sync_mode = WB_SYNC_NONE,
> + .older_than_this = NULL,
> + .range_cyclic = 1,
> + .nr_to_write = 1024,
> + };
>
> - spin_lock(&wb->list_lock);
> - if (list_empty(&wb->b_io))
> - queue_io(wb, wbc);
> - writeback_sb_inodes(sb, wb, wbc, true);
> - spin_unlock(&wb->list_lock);
> + spin_lock(&bdi->wb.list_lock);
> + if (list_empty(&bdi->wb.b_io))
> + queue_io(&bdi->wb, &wbc);
> + writeback_inodes_wb(&bdi->wb, &wbc);
> + spin_unlock(&bdi->wb.list_lock);
> }
>
> /*
> @@ -674,7 +673,7 @@ static long wb_writeback(struct bdi_writ
> * 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
> @@ -722,10 +721,14 @@ static long wb_writeback(struct bdi_writ
>
> retry:
> trace_wbc_writeback_start(&wbc, wb->bdi);
> + spin_lock(&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(&wb->list_lock);
> trace_wbc_writeback_written(&wbc, wb->bdi);
>
> work->nr_pages -= write_chunk - wbc.nr_to_write;
> --- linux-next.orig/mm/backing-dev.c 2011-04-21 20:11:52.000000000 +0800
> +++ linux-next/mm/backing-dev.c 2011-04-21 20:16:15.000000000 +0800
> @@ -260,18 +260,6 @@ int bdi_has_dirty_io(struct backing_dev_
> return wb_has_dirty_io(&bdi->wb);
> }
>
> -static void bdi_flush_io(struct backing_dev_info *bdi)
> -{
> - struct writeback_control wbc = {
> - .sync_mode = WB_SYNC_NONE,
> - .older_than_this = NULL,
> - .range_cyclic = 1,
> - .nr_to_write = 1024,
> - };
> -
> - writeback_inodes_wb(&bdi->wb, &wbc);
> -}
> -
> /*
> * kupdated() used to do this. We cannot do it from the bdi_forker_thread()
> * or we risk deadlocking on ->s_umount. The longer term solution would be
> --- linux-next.orig/include/linux/writeback.h 2011-04-21 20:20:20.000000000 +0800
> +++ linux-next/include/linux/writeback.h 2011-04-21 21:10:29.000000000 +0800
> @@ -56,6 +56,7 @@ struct writeback_control {
> */
> struct bdi_writeback;
> int inode_wait(void *);
> +void bdi_flush_io(struct backing_dev_info *bdi);
> void writeback_inodes_sb(struct super_block *);
> void writeback_inodes_sb_nr(struct super_block *, unsigned long nr);
> int writeback_inodes_sb_if_idle(struct super_block *);
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR