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

Wu Fengguang
2011-04-26T05:37:12Z

On Sat, Apr 23, 2011 at 05:12:55AM +0800, Jan Kara wrote:
> 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.

OK, this avoids the need to move bdi_flush_io(). Here is the updated
patch, do you see any more problems?

Thanks,
Fengguang
---
Subject: writeback: elevate queue_io() into wb_writeback()
Date: Thu Apr 21 12:06:32 CST 2011

Code refactor for more logical code layout.
No behavior change.

- remove the mis-named __writeback_inodes_sb()

- wb_writeback()/writeback_inodes_wb() will decide when to queue_io()
before calling __writeback_inodes_wb()

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/fs-writeback.c | 27 ++++++++++++---------------
1 file changed, 12 insertions(+), 15 deletions(-)

--- linux-next.orig/fs/fs-writeback.c 2011-04-26 13:20:17.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2011-04-26 13:30:19.000000000 +0800
@@ -570,17 +570,13 @@ static int writeback_sb_inodes(struct su
return 1;
}

-void writeback_inodes_wb(struct bdi_writeback *wb,
- struct writeback_control *wbc)
+static void __writeback_inodes_wb(struct bdi_writeback *wb,
+ struct writeback_control *wbc)
{
int ret = 0;

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);
@@ -596,19 +592,16 @@ 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 writeback_inodes_wb(struct bdi_writeback *wb,
+ struct writeback_control *wbc)
{
- WARN_ON(!rwsem_is_locked(&sb->s_umount));
-
spin_lock(&wb->list_lock);
if (list_empty(&wb->b_io))
queue_io(wb, wbc);
- writeback_sb_inodes(sb, wb, wbc, true);
+ __writeback_inodes_wb(wb, wbc);
spin_unlock(&wb->list_lock);
}

@@ -674,7 +667,7 @@ 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 +715,14 @@ 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);
+ __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;