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

[Wu Fengguang]

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@xxxxxxxxx>
---
 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;
--
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