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@xxxxxxxxxxxxx
