Message-ID: <20090325143815.GB5859@localhost>
Date:	Wed, 25 Mar 2009 22:38:15 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	Ian Kent <raven@...maw.net>, Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH] writeback: reset inode dirty time when adding it back
	to empty s_dirty list

On Wed, Mar 25, 2009 at 10:28:33PM +0800, Jeff Layton wrote:
> On Wed, 25 Mar 2009 22:16:18 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > On Wed, Mar 25, 2009 at 10:00:49PM +0800, Jeff Layton wrote:
> > > On Wed, 25 Mar 2009 22:38:47 +0900
> > > Ian Kent <raven@...maw.net> wrote:
> > > 
> > > > Ian Kent wrote:
> > > > > Jeff Layton wrote:
> > > > >> On Wed, 25 Mar 2009 20:17:43 +0800
> > > > >> Wu Fengguang <fengguang.wu@...el.com> wrote:
> > > > >>
> > > > >>> On Wed, Mar 25, 2009 at 07:51:10PM +0800, Jeff Layton wrote:
> > > > >>>> On Wed, 25 Mar 2009 10:50:37 +0800
> > > > >>>> Wu Fengguang <fengguang.wu@...el.com> wrote:
> > > > >>>>
> > > > >>>>>> Given the right situation though (or maybe the right filesystem), it's
> > > > >>>>>> not too hard to imagine this problem occurring even in current mainline
> > > > >>>>>> code with an inode that's frequently being redirtied.
> > > > >>>>> My reasoning with recent kernel is: for kupdate, s_dirty enqueues only
> > > > >>>>> happen in __mark_inode_dirty() and redirty_tail().  Newly dirtied
> > > > >>>>> inodes will be parked in s_dirty for 30s. During which time the
> > > > >>>>> actively being-redirtied inodes, if their dirtied_when is an old stuck
> > > > >>>>> value, will be retried for writeback and then re-inserted into a
> > > > >>>>> non-empty s_dirty queue and have their dirtied_when refreshed.
> > > > >>>>>
> > > > >>>> Doesn't that assume that there are new inodes that are being dirtied?
> > > > >>>> If you only have the same inodes being redirtied and never any new
> > > > >>>> ones, the problem still occurs, right?
> > > > >>> Yes. But will a production server run months without making one single
> > > > >>> new dirtied inode? (Just out of curiosity. Not that I'm not willing to
> > > > >>> fix this possible issue.:)
> > > > >>>
> > > > >> Yes. It's not that the box will run that long without creating a
> > > > >> single new dirtied inode, but rather that it won't necessarily create
> > > > >> one on all of its mounts. It's often the case that someone has a
> > > > >> mountpoint for a dedicated purpose.
> > > > >>
> > > > >> Consider a host that has a mountpoint that contains logfiles that are
> > > > >> being heavily written. There's nothing that says that they must rotate
> > > > >> those logs over a particular period (assuming the fs has enough space,
> > > > >> etc). If the same ones are constantly being redirtied and no new
> > > > >> ones are created, then I think this problem can easily happen.
> > > > >>
> > > > >>>>>>> ...I see no obvious reasons against unconditionally resetting dirtied_when.
> > > > >>>>>>>
> > > > >>>>>>> (a) Delaying an inode's writeback for 30s maybe too long - its blocking
> > > > >>>>>>> condition may well go away within 1s. (b) And it would be very undesirable
> > > > >>>>>>> if one big file is repeatedly redirtied hence its writeback being
> > > > >>>>>>> delayed considerably.
> > > > >>>>>>>
> > > > >>>>>>> However, redirty_tail() currently only tries to speedup writeback-after-redirty
> > > > >>>>>>> in a _best effort_ way. It at best partially hides the above issues,
> > > > >>>>>>> if there are any. In particular, if (b) is possible, the bug should
> > > > >>>>>>> already show up at least in some situations.
> > > > >>>>>>>
> > > > >>>>>>> For XFS, immediately sync of redirtied inode is actually discouraged:
> > > > >>>>>>>
> > > > >>>>>>>         http://lkml.org/lkml/2008/1/16/491
> > > > >>>>>>>
> > > > >>>>>>>
> > > > >>>>>> Ok, those are good points that I need to think about.
> > > > >>>>>>
> > > > >>>>>> Thanks for the help so far. I'd welcome any suggestions you have on
> > > > >>>>>> how best to fix this.
> > > > >>>>> For NFS, is it desirable to retry a redirtied inode after 30s, or
> > > > >>>>> after a shorter 5s, or after 0.1~5s? Or the exact timing simply
> > > > >>>>> doesn't matter?
> > > > >>>>>
> > > > >>>> I don't really consider NFS to be a special case here. It just happens
> > > > >>>> to be where we saw the problem originally. Some of its characteristics
> > > > >>>> might make it easier to hit this, but I'm not certain of that.
> > > > >>> Now there are now two possible solutions:
> > > > >>> - unconditionally update dirtied_when in redirty_tail();
> > > > >>> - keep dirtied_when and redirty inodes to a new dedicated queue.
> > > > >>> The first one involves less code, the second one allows more flexible timing.
> > > > >>>
> > > > >>> NFS/XFS could be a good starting point for discussing the
> > > > >>> requirements, so that we can reach a suitable solution.
> > > > >>>
> > > > >> It sounds like it, yes. I saw that you posted some patches in January
> > > > >> (including your s_more_io_wait patch). I'll give those a closer look.
> > > > >> Adding the new s_more_io_wait queue is interesting and might sidestep
> > > > >> this problem nicely.
> > > > >>
> > > > > 
> > > > > Yes, I was looking at that bit of code but, so far, I think it won't be
> > > > > called for the case we are trying to describe.
> > > > 
> > > > I take that back.
> > > > As Jeff pointed out I haven't seen these patches and can't seem to find
> > > > them in my fsdevel list folder, Wu can you send me a copy please?
> > > > 
> > > 
> > > Actually, I think you were right. We still have this check in
> > > generic_sync_sb_inodes() even with Wu's January 2008 patches:
> > > 
> > > 	/* Was this inode dirtied after sync_sb_inodes was called? */
> > > 	if (time_after(inode->dirtied_when, start))
> > > 		break;
> > 
> > Yeah, ugly code. Jens' per-bdi flush daemons should eliminate it...
> > 
> 
> Ok, good to know. I need to look at those more closely I guess...
> 
> > > ...this check is the crux of the problem. We're assuming that the
> > > dirtied_when value will never appear to be in the future. If we change
> > > this check so that it's checking that dirtied_when is between "start"
> > > and "now", then this problem basically goes away.
> > 
> > Yeah that turns the problem into a temporary and tolerable one.
> > 
> 
> Yes.
> 
> > > We'll probably also need to change the test in move_expired_inodes
> > > too, unless Wu's changes go in.
> > 
> > So the most simple (and complete) solution is still this one ;-)
> > 
> 
> I suppose so. I guess that also takes care of the problem on XFS (and
> maybe other filesystems too?) of inodes getting flushed too frequently
> when they're redirtied.
> 
> The downside sounds like that it'll mean that big files that are being
> frequently redirtied might get less frequent writeout attempts. We can
> easily dirty pages faster than we can write them out (at least with
> most filesystems). Will that cause problem where we accumulate too many
> dirty pages for the inode? That also means that the I/O will be more
> "spiky"...
> 
> pdflush writes out some data
> inode goes back on s_dirty and dirtied_when gets restamped
> wait 30s...
> pdflush writes out more data
> etc...
> 
> That seems sub-optimal.

Yup, adding 30s delay on each redirty sounds too much. That's why
Andrew tried to keep dirtied_when untouched, and why I proposed the
s_more_io_wait queue.

So let's refresh the s_more_io_wait patchset?
I'll do it tomorrow...in a fresh day :-)

Thanks,
Fengguang
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