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

[Jan Kara <jack@suse.cz>]

On Tue 03-08-10 20:59:24, Wu Fengguang wrote:
> On Tue, Aug 03, 2010 at 08:39:22PM +0800, Jan Kara wrote:
> > On Tue 03-08-10 12:55:20, Jan Kara wrote:
> > > 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?
> >   Fengguang, how about merging also the attached simple patch together with
> > my fix? With these two patches, I'm not able to trigger any sync livelock
> > while without one of them I hit them quite easily...
> 
> This looks OK. However note that redirty_tail() can modify
> dirtied_when unexpectedly. So the more we rely on wb_start, the more
> possibility an inode is (wrongly) skipped by sync. I have a bunch of
> patches to remove redirty_tail(). However they may not be good
> candidates for 2.6.36..
  Yes, I'm aware of this. But if I'm right, after your changes to the
logic in writeback_single_inode() Andrew has in his tree, we use
requeue_io() in case inode still has any dirty pages. Thus after these
patches we should be mostly fine. Shouldn't we?

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