linux-kernel - Re: [PATCH] writeback: reset inode dirty time when adding it back to empty s

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Message-ID: <49CA33E7.6090309@themaw.net>
Date:	Wed, 25 Mar 2009 22:38:47 +0900
From:	Ian Kent <raven@...maw.net>
To:	Jeff Layton <jlayton@...hat.com>
CC:	Wu Fengguang <fengguang.wu@...el.com>,
	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

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?

Ian

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