Re: Highly reflinked and fragmented considered harmful?

[Dave Chinner <david@fromorbit.com>]

On Tue, May 10, 2022 at 12:19:18PM -0700, Darrick J. Wong wrote:
> On Tue, May 10, 2022 at 06:16:32PM +1000, Dave Chinner wrote:
> > On Tue, May 10, 2022 at 04:30:51PM +1000, Chris Dunlop wrote:
> > > On Mon, May 09, 2022 at 10:10:57PM -0700, Darrick J. Wong wrote:
> > > > On Tue, May 10, 2022 at 07:07:35AM +0300, Amir Goldstein wrote:
> > > > > On Mon, May 09, 2022 at 12:46:59PM +1000, Chris Dunlop wrote:
> > > > > > Is it to be expected that removing 29TB of highly reflinked and fragmented
> > > > > > data could take days, the entire time blocking other tasks like "rm" and
> > > > > > "df" on the same filesystem?
> > > ...
> > > > > From a product POV, I think what should have happened here is that
> > > > > freeing up the space would have taken 10 days in the background, but
> > > > > otherwise, filesystem should not have been blocking other processes
> > > > > for long periods of time.
> > 
> > Sure, that's obvious, and without looking at the code I know what
> > that is: statfs()
> > 
> > > > Indeed.  Chris, do you happen to have the sysrq-w output handy?  I'm
> > > > curious if the stall warning backtraces all had xfs_inodegc_flush() in
> > > > them, or were there other parts of the system stalling elsewhere too?
> > > > 50 billion updates is a lot, but there shouldn't be stall warnings.
> > > 
> > > Sure: https://file.io/25za5BNBlnU8  (6.8M)
> > > 
> > > Of the 3677 tasks in there, only 38 do NOT show xfs_inodegc_flush().
> > 
> > yup, 3677 tasks in statfs(). The majority are rm, df, and check_disk
> > processes, there's a couple of veeamagent processes stuck and
> > an (ostnamed) process, whatever that is...
> > 
> > No real surprises there, and it's not why the filesystem is taking
> > so long to remove all the reflink references.
> > 
> > There is just one background inodegc worker thread running, so
> > there's no excessive load being generated by inodegc, either. It's
> > no different to a single rm running xfs_inactive() directly on a
> > slightly older kernel and filling the journal:
> > 
> > May 06 09:49:01 d5 kernel: task:kworker/6:1     state:D stack:    0 pid:23258 ppid:     2 flags:0x00004000
> > May 06 09:49:01 d5 kernel: Workqueue: xfs-inodegc/dm-1 xfs_inodegc_worker [xfs]
> > May 06 09:49:01 d5 kernel: Call Trace:
> > May 06 09:49:01 d5 kernel:  <TASK>
> > May 06 09:49:01 d5 kernel:  __schedule+0x241/0x740
> > May 06 09:49:01 d5 kernel:  schedule+0x3a/0xa0
> > May 06 09:49:01 d5 kernel:  schedule_timeout+0x271/0x310
> > May 06 09:49:01 d5 kernel:  __down+0x6c/0xa0
> > May 06 09:49:01 d5 kernel:  down+0x3b/0x50
> > May 06 09:49:01 d5 kernel:  xfs_buf_lock+0x40/0xe0 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_buf_find.isra.32+0x3ee/0x730 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_buf_get_map+0x3c/0x430 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_buf_read_map+0x37/0x2c0 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_trans_read_buf_map+0x1cb/0x300 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_btree_read_buf_block.constprop.40+0x75/0xb0 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_btree_lookup_get_block+0x85/0x150 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_btree_overlapped_query_range+0x33c/0x3c0 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_btree_query_range+0xd5/0x100 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_rmap_query_range+0x71/0x80 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_rmap_lookup_le_range+0x88/0x180 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_rmap_unmap_shared+0x89/0x560 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_rmap_finish_one+0x201/0x260 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_rmap_update_finish_item+0x33/0x60 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_defer_finish_noroll+0x215/0x5a0 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_defer_finish+0x13/0x70 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_itruncate_extents_flags+0xc4/0x240 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_inactive_truncate+0x7f/0xc0 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_inactive+0x10c/0x130 [xfs]
> > May 06 09:49:01 d5 kernel:  xfs_inodegc_worker+0xb5/0x140 [xfs]
> > May 06 09:49:01 d5 kernel:  process_one_work+0x2a8/0x4c0
> > May 06 09:49:01 d5 kernel:  worker_thread+0x21b/0x3c0
> > May 06 09:49:01 d5 kernel:  kthread+0x121/0x140
> > 
> > There are 18 tasks in destroy_inode() blocked on a workqueue flush
> > - these are new unlinks that are getting throttled because that
> > per-cpu inodegc queue is full and work is ongoing. Not a huge
> > deal, maybe we should look to hand full queues to another CPU if
> > the neighbour CPU has an empty queue. That would reduce
> > unnecessary throttling, though it may mean more long running
> > inodegc processes in the background....
> > 
> > Really, though, I don't see anything deadlocked, just a system
> > backed up doing a really large amount of metadata modification.
> > Everything is sitting on throttles or waiting on IO and making
> > slow forwards progress as metadata writeback allows log space to be
> > freed.
> > 
> > I think we should just accept that statfs() can never really report
> > exactly accurate space usagei to userspace and get rid of the flush.
> 
> What about all the code that flushes gc work when we hit ENOSPC/EDQUOT?
> Do we let those threads stall too because the fs is out of resources and
> they can just wait?  Or would that also be better off with a flush
> timeout and userspace can just eat the ENOSPC/EDQUOT after 30 seconds?

1. Not an immediate problem we need to solve.
2. flush at enospc/edquot is best effort, so doesn't need to block
   waiting on inodegc. the enospc/edquot flush will get repeated
   soon enough, so that will take into account progress made by
   long running inodegc ops.
3. we leave pending work on the per-cpu queues under the
   flush/throttle thresholds indefinitely.
4. to be accurate, statfs() needs to flush #3.
5. While working on the rework of inode reclaimation, I converted the
   inodegc queues to use delayed works to ensure work starts on
   per-cpu queues within 10ms of queueing to avoid #3 causing
   problems.
6. I added a non-blocking async flush mechanism that works by
   modifying the queue timer to 0 to trigger immedate work
   scheduling for anything that hasn't been run.

#4 is the problem that Chris hit - we're trying to be perfectly
accurate when perfect accuracy is impossible and we're paying the
price for that.

Fix #3, and the need to block statfs() largely goes away. And as per
#2, we don't really need to block edquot/enospc flushes, either. We
jsut need to push the queues to make sure they are running and doing
work...

> > Work around the specific fstests dependencies on rm always
> > immediately making unlinked inodes free space in fstests rather than
> > in the kernel code.
> 
> <grumble> I *really* don't want to launch *yet another* full scale audit
> of XFS + supporting software behavior on tiny and/or full filesystems.

This doesn't need an audit. Just fix the "single unlinks don't get
processed until a flush occurs" problem, and most of the issues
fstests has will go away.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com