Re: [PATCH 2/5] writeback: stop periodic/background work on seeing sync works

[Jan Kara <jack@suse.cz>]

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On Fri 30-07-10 12:03:06, Wu Fengguang wrote:
> On Fri, Jul 30, 2010 at 12:20:27AM +0800, Jan Kara wrote:
> > On Thu 29-07-10 19:51:44, Wu Fengguang wrote:
> > > The periodic/background writeback can run forever. So when any
> > > sync work is enqueued, increase bdi->sync_works to notify the
> > > active non-sync works to exit. Non-sync works queued after sync
> > > works won't be affected.
> >   Hmm, wouldn't it be simpler logic to just make for_kupdate and
> > for_background work always yield when there's some other work to do (as
> > they are livelockable from the definition of the target they have) and
> > make sure any other work isn't livelockable?
> 
> Good idea!
> 
> > The only downside is that
> > non-livelockable work cannot be "fair" in the sense that we cannot switch
> > inodes after writing MAX_WRITEBACK_PAGES.
> 
> Cannot switch indoes _before_ finish with the current
> MAX_WRITEBACK_PAGES batch? 
  Well, even after writing all those MAX_WRITEBACK_PAGES. Because what you
want to do in a non-livelockable work is: take inode, write it, never look at
it again for this work. Because if you later return to the inode, it can
have newer dirty pages and thus you cannot really avoid livelock. Of
course, this all assumes .nr_to_write isn't set to something small. That
avoids the livelock as well.

> >   I even had a patch for this but it's already outdated by now. But I
> > can refresh it if we decide this is the way to go.
> 
> I'm very interested in your old patch, would you post it? Let's see
> which one is easier to work with :)
  OK, attached is the patch. I've rebased it against 2.6.35.

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

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>From a6df0d4db148f983fe756df4791409db28dff459 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Mon, 2 Aug 2010 22:30:25 +0200
Subject: [PATCH] mm: Stop background writeback if there is other work queued for the thread

Background writeback and kupdate-style writeback are easily livelockable
(from a definition of their target). This is inconvenient because it can
make sync(1) stall forever waiting on its queued work to be finished.
Fix the problem by interrupting background and kupdate writeback if there
is some other work to do. We can return to them after completing all the
queued work.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/fs-writeback.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index d5be169..542471e 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -633,6 +633,14 @@ static long wb_writeback(struct bdi_writeback *wb,
 			break;
 
 		/*
+		 * Background writeout and kupdate-style writeback are
+		 * easily livelockable. Stop them if there is other work
+		 * to do so that e.g. sync can proceed.
+		 */
+		if ((work->for_background || work->for_kupdate) &&
+		    !list_empty(&wb->bdi->work_list))
+			break;
+		/*
 		 * For background writeout, stop when we are below the
 		 * background dirty threshold
 		 */
-- 
1.6.4.2