On Tue 03-08-10 11:01:25, Wu Fengguang wrote: > 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. So I don't think sync work on it's own is a problem. There we can just give up any fairness and just go inode by inode. IMHO it's much simpler that way. The remaining types of work we have are "for_reclaim" and then ones triggered by filesystems to get rid of delayed allocated data. These cases can easily have well defined and low nr_to_write so they wouldn't be livelockable either. What do you think? > 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 see some incentive to do this but the simple thing with for_background and for_kupdate work is that they are essentially state-less and so they can be easily (and automatically) restarted. It would be really hard to implement something like this for sync and still avoid livelocks. > > > > 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. I think I'll try to get this merged via Jens' tree in this merge window. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
