On Tue, Aug 03, 2010 at 04:51:52AM +0800, Jan Kara wrote: > 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 do have a poor man's solution that can handle this case. https://kerneltrap.org/mailarchive/linux-fsdevel/2009/10/7/6476473/thread It may do more extra works, but will stop livelock in theory. A related question is, what if some for_reclaim works get enqueued? Shall we postpone the sync work as well? The global sync is not likely to hit the dirty pages in a small memcg, or may take long time. It seems not a high priority task though. > > > I even had a patch for this but it's already outdated by now.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@xxxxxxx> > SUSE Labs, CR > From a6df0d4db148f983fe756df4791409db28dff459 Mon Sep 17 00:00:00 2001 > From: Jan Kara <jack@xxxxxxx> > 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@xxxxxxx> > --- > 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; > + /* I like it. It's much simpler. Reviewed-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
