[00/28] mm, xfs: non-blocking inode reclaim
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Message ID 20191031234618.15403-1-david@fromorbit.com
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  • mm, xfs: non-blocking inode reclaim
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Dave Chinner Oct. 31, 2019, 11:45 p.m. UTC
Hi folks,

This is an updated version of the non-blocking inode reclaim
patchset for XFS. Original description of the problem and patchset
is below the changelog.

Thoughts, comments and improvemnts welcome.




- rebased on 5.4-rc5 + linux-xfs/for-next
- fixed various typos and whitespace damage
- new patch to factor out inode lookup from xfs_ifree_cluster() to
  simplify the loop walking all the in-core inodes in the chunk
  being freed.
- fixed up issues with temporary xfs_ail_push_sync() scaffolding
- fixed up per-ag locking description in commit message of patch
  "xfs: use AIL pushing for inode reclaim IO"
- fixed missing per-ag reclaim cursor unlock
- made sync LSN calculation consistent between inode grab and
  reclaim for pinned inodes
- check against NULLCOMMITLSN rather than implicitly relying on
  XFS_LSN_CMP() to do the right thing.
- renamed xfs_reclaim_inodes -> xfs_reclaim_all_inodes().:w
- moved new LRU list init to xfs_fs_fill_super()
- fixed incorrect locking around xfs_ifunlock() in
- added lockdep_assert_held() to __xfs_iflock_nowait()
- use list_first_entry_or_null() in xfs_dispose_inodes()
- moved removal of ag reclaim walk into patch that changes reclaim
  to use the lru list walk.
- added patch to introduce down_write_non_owner() and friends
- added mrupdate_*_non_owner() for inode reclaim to use.
- fixed incorrect flag manipulation in xfs_iget_cache_hit()
- removed spurious extra xfs_ail_push_all() calls



- added current_reclaim_account_pages() wrapper for reclaim_state
- moved xfs_buf page free accounting to the page freeing code rather
  than the reclaim loop that drops the LRU buffer reference.
- increased log CIL flush limit by 2x
- moved xfs_buf GFP_NOFS reclaim changes to correct patch
- renamed sc->will_defer to sc->defer_work
- used min() in do_shrink_slab() appropriately
- converted xfs_trans_ail_update_bulk() to use new
  xfs_ail_update_finish() function (fixes missing wakeups w/
- fixed CIL size limit calculation units - was calculating min size
  in sectors rather than bytes. Fixes performance degradation w/
  patch series.
- dropped shadow buffer freeing - Brain pointed out it wasn't doing
  exactly what I thought it was, and the memory saving were from the
  massively undersized CIL limits, not early freeing of the shadow
  buffers. Needs rethinking.
- fixed stray NULLCOMMITLSN in comments
- pinned items don't track the commit LSN, just the CIL sequence
  number so we can't use that to push the AIL.
- removed stale tracing debug from AIL push code.
- fixes to memory reclaim shrinker accounting in 5.3-rc3 result in
  direct reclaim backoff working a whole lot better, such that it's
  no long necessary for the XFS inode shrinker to wait for IO to
  complete. Changed the LRU reclaim logic to simply push on the AIL
  if dirty inodes are hit, but never wait on them. Relevant commits

  0308f7cf19c9 ("mm/vmscan.c: calculate reclaimed slab caches in all reclaim paths")
  e5ca8071fe65 ("mm/vmscan.c: add a new member reclaim_state in struct shrink_control")

- Added a patch to convert xfs_reclaim_inodes() to use
  xfs_ail_push_all() which gets rid of the last user of
  xfs_ail_push_sync(). This allows it to be removed as "temporary
  infrastructure for the series" rather than having to be fixed up
  and made robust. The optimisations and factoring it drove are
  retained, as they are still a net improvement overall.
- fixed atomic_long vs atomic64 issues with shrinker deferral
- don't drop ag reclaim cursor locking any more, it gets removed
  when all the old reclaim code is removed.
- added patch to change inode reclaim vs unreferenced XFS indoe
  lookup done by inode write clustering and inode cluster freeing.
  This gets rid of the need to cycle the ILOCK before running
  call_rcu() to queue the inode to be freed when the current RCU
  grace period expires. In doing so, the last major blocking point
  in XFS inode reclaim is removed.

V1 (original RFC):


Original text:

We've had a problem with inode reclaim for a long time - XFS is
capable of caching tens of millions of inodes with ease and dirtying
hundreds of thousands of those cached inodes every second. It is
also capable of reclaiming more than half a million clean inodes per
second per reclaim thread.

The result of this is that when there is a significant change in
sustained memory pressure on a system ith a large inode cache,
memory reclaim rapdily frees all the clean inodes, but cannot make
progress on reclaiming dirty inodes because they are rate limited
by IO.

However, the shrinker infrastructure in the kernel has no way to
feed back rate limiting to the core memory reclaim algorithms.  In
fact there are no feedback mechanisms at all, and so when reclaim
has freed all the clean inodes and starts hitting dirty inodes, the
filesystem has no way of telling reclaim that the inode reclaim rate
has dropped from 500k/s to 500/s.

The result is that reclaim continues to try to free memory, and
because it makes no progress freeing inodes, it puts much more
pressure on the page LRUs and frees pages. When it runs out of
pages, it starts swapping, and when it runs out of swap or can't get
a page for swap-in it starts going on an OOM kill rampage.

That does nothing to "fix" the shortage of memory caused by the
slowness of dirty inode reclaim - if memory demand continues we just
keep hitting the OOM killer until either something critical is
killed or memory demand eases.

For a long time, XFS has avoided the insane spiral of shouty
OOM-killer rage death by cleaning inodes directly in the shrinker.
This has the effect of throttling memory reclaim to the rate at
which dirty inodes can be cleaned, and so when we get into the state
when memory reclaim is dependent on inode reclaim making progress
we don't ever allow LRU reclaim to run so far ahead of inode reclaim
that it winds up reclaim priority and runs out of LRU pages to
reclaim and/or swap.

This has a downside, though. When there is a large amount of clean
page cache and a small amount of inode cache that is dirty (e.g.
lots of file data pressure and/or application memory demand) the
inode reclaim shrinkers can run out of clean inodes to reclaim and
start blocking on inode writeback. This can result in long reclaim
latencies even though there is lots of memory that can be
immediately reclaimed from the page cache.

There are other issues, too. We have to block kswapd, too, because
it will continue running until watermarks are satisfied, and that
is largely the vector for shouty swappy death if it doesn't back
off before priority windup from lack of progress occurs. Blocking
kswapd then affects direct reclaim function, which often backs off
expecting kswapd to make progress in the mean time. But if kswapd
is not making progress, direct reclaim ends up in priority windup
from lack of progress, too. This is especially prevalent in
workloads that have a high percentage of GFP_NOFS allocations (e.g.
filesystem modification workloads).

The shrinkers have another problem w/ GFP_NOFS reclaim: the work
that is deferred because the shrinker cannot make progress gets
lumped on the first reclaim context that can do that work. That
means a direct reclaimer might get lumped with scanning millions of
objects during low priority scanning when it should only be scanning
a couple of thousand objects. This can result in highly
unpredictable and extremely long direct reclaim delays.

This is most definitely sub-optimal, but it's better than random
and/or premature OOM killer invocation under trivial workloads and
lots of reclaimable memory still being available.

This patch set aims to fix all these problems. The memory reclaim
and shrinker changes involve:

- a substantial rework of how the shrinker defers work, moving all
  the deferred work to kswapd to remove all the unpredictability
  from direct reclaim.  Direct reclaim will only do the work the
  direct reclaim context determines is necesary.

- deferred work is capped, and the amount of deferred work kswapd
  will do in each scan is increased linearly w.r.t. increasing
  reclaim priority. Hence when we are desparate for memory, kswapd
  will be running all the deferred work as quickly as possible.

- The amount of deferred work and the amount of scanning that is
  done by the shrinkers is now tracked in the struct reclaim_state.
  This allows shrink_node() to see how much work is being done in
  comparison to both the LRU scanning and how much is being deferred
  to kswapd. This allows direct reclaim to back off when too much
  work is being deferred and hence allow kswapd to make progress on
  the deferred work while it waits.

- A "need backoff" flag has been added to the struct reclaim_state.
  This allows individual shrinkers to indicate to kswapd that they
  need some time to finish work before being scanned again. This is
  basically for the same case as kswapd backs off from LRU scanning.

  i.e. the LRU scanning has run into the tail of the LRU and is
  finding dirty objects that require IO to complete before reclaim
  can make further progress. This is exactly the same problem we
  have with inode reclaim in XFS, and it is this mechanism that
  enables us to move to IO-less inode reclaim.

The XFS changes are all over the place, and address both the reclaim
blocking problems and all the other related issues I found while
working on this patchest. These involve:

- fixing IO priority inversion problems between metadata
  writeback (inodes!) and log IO caused by the block layer write
  throttling (more on this later).

- some slab caches weren't marked as reclaimable, so were
  incorrectly accounted. Also account for the pages xfs_buf reclaim

- reduced the delayed logging dirty item aggregation size (the CIL).
  This defines the minimum amount of memory XFS can operate in when
  there is heavy modifications in progress.

- reduced the memory footprint of the CIL when repeated
  modifications to objects occur.

- Added a mechanism to push the AIL to a specific LSN (metadata
  modification epoch) and wait for it. This forms the basis for
  direct inode reclaim deferring IO and waiting for some progress
  without issuing IO iteslf.

- reworked inode reclaim to use a list_lru to track inodes in
  reclaim rather than a radix tree tag in the inode cache. We
  iterated the radix tree for reclaim because it resulted in optimal
  IO patterns from multiple concurrent reclaimers, but we dont' have
  to care about that any more because all IO comes from the AIL now.

  This gives us try LRU reclaim, and it allows us to effectively
  determine when we've run out of clean inodes to easily reclaim and
  provide that feedback to the higher levels via the "need backoff"

- direct reclaim is non-blocking while scanning, but at the end of a
  scan it will still block waiting for IO, but only for /some/
  progress to be made and not specific individual IOs.

- kswapd based reclaim is fully non-blocking.

The result is that there is now enough feedback from the shrinkers
into the main memory reclaim loop for it to back off in the
situations where back-off is required to avoid OOM killer
invocation, despite XFS now largely doing non-blocking reclaim.

Testing involves at 16p/16GB machine running a fsmark workload that
creates sustained heavy dirty inode cache pressure, then
progressively locking 2GB of memory at time to squeeze the workload
into less and less memory. A vanilla kernel runs well up to 12GB
squeezed, but at 14GB squeezed performance goes to hell. With just
the hacky "don't block kswapd by removing SYNC_WAIT" patch that
people seem to like, OOM kills start when squeezed to 12GB. With
that extended to direct reclaim, OOM kills start with squeezed to
just 8GB. With the full patchset, it runs similar to a vanilla
kernel up to 12GB squeezed, and vastly out-performs the vanilla
kernel with 14GB squeezed. Performance only drops ~20% with a 14GB
squeeze, whereas the vanilla kernel sees up to a 90% drop in

I also run testing with simoop, a simulated workload that Chris
Mason put together to demonstrate the long tail latency and
allocation stall problems the blocking in inode reclaim was causing.
The vanilla kernel averaged ~5 stalls/s over a test period of 10
hours, this patch series resulted in:

alloc stall rate = 0.00/sec (avg: 0.04) (p50: 0.04) (p95: 0.16) (p99: 0.32)

stalls almost going away entirely.

So the signs are there that this is a workable solution to the
problems caused by blocking inode reclaim without re-introducing the
the Death-by-OOM-killer issues the blocking avoids.

Please note that I haven't full gone non-blocking on direct reclaim
for a couple of reasons:

1. congestion_wait() and wait_iff_congested() are completely broken.
The blkmq change-over ripped out all the block layer congestion
reporting in 5.0 and didn't replace it with anything, so unless you
are operating on an NFS client, Ceph, FUSE or a DVD, congestion
checks and backoff aren't actually doing what they are supposed to.
i.e. wait_iff_congested() never blocks, and congestion_wait() always
sleeps for it's full timeout.

IOWs, the whole bdi-based IO congestion feedback mechanism no longer
functions as intended, and so I'm betting a lot of the memory
reclaim heuristics no longer function as they were intended to...

2. The block layer write throttle is full of priority inversions.
Apart from the log IO one I fixed in this series, I noticed that
swap in/out has a major problem. I lost count of the number of OOM
kills that occurred from the swap in path when there were several
processes blocked in wbt_wait() in the block layer in the swap out
path. i.e. if swap out had been making progress, swap in would not
have oom killed. Hence I found it still necessary to throttle direct
reclaim back in the shrinker as there wasn't a realiable way to get
the core reclaim code to throttle effectively.

FWIW, from the swap in/out perspective, this whole inversion problem
is made worse by #1: the congestion_wait/wait_iff_congested
interfaces being broken. Direct reclaim uses wait_iff_congested() to
back off if kswapd has indicated that the node is congested
(PGDAT_CONGESTED) and reclaim is struggling to make progress.
However, this backoff never actually happens now and hence direct
reclaim barrels into the swap code as hard as it can and blocks in
wbt_wait() waiting behind other swap IO instead of backing off and
waiting for some IO to complete and then retrying it's allocation....

So maybe if we fix the bdi congestion interfaces so they work again
we can get rid of the waiting in direct reclaim, but right now I
don't see any other choice....

Dave Chinner (28):
  xfs: Lower CIL flush limit for large logs
  xfs: Throttle commits on delayed background CIL push
  xfs: don't allow log IO to be throttled
  xfs: Improve metadata buffer reclaim accountability
  xfs: correctly acount for reclaimable slabs
  xfs: factor common AIL item deletion code
  xfs: tail updates only need to occur when LSN changes
  xfs: factor inode lookup from xfs_ifree_cluster
  mm: directed shrinker work deferral
  shrinkers: use defer_work for GFP_NOFS sensitive shrinkers
  mm: factor shrinker work calculations
  shrinker: defer work only to kswapd
  shrinker: clean up variable types and tracepoints
  mm: reclaim_state records pages reclaimed, not slabs
  mm: back off direct reclaim on excessive shrinker deferral
  mm: kswapd backoff for shrinkers
  xfs: synchronous AIL pushing
  xfs: don't block kswapd in inode reclaim
  xfs: reduce kswapd blocking on inode locking.
  xfs: kill background reclaim work
  xfs: use AIL pushing for inode reclaim IO
  xfs: remove mode from xfs_reclaim_inodes()
  xfs: track reclaimable inodes using a LRU list
  xfs: reclaim inodes from the LRU
  xfs: remove unusued old inode reclaim code
  xfs: use xfs_ail_push_all in xfs_reclaim_inodes
  rwsem: introduce down/up_write_non_owner
  xfs: rework unreferenced inode lookups

 drivers/staging/android/ashmem.c |   8 +-
 fs/gfs2/glock.c                  |   5 +-
 fs/gfs2/quota.c                  |   6 +-
 fs/inode.c                       |   3 +-
 fs/nfs/dir.c                     |   6 +-
 fs/super.c                       |   6 +-
 fs/xfs/mrlock.h                  |  27 ++
 fs/xfs/xfs_buf.c                 |   4 +-
 fs/xfs/xfs_icache.c              | 632 +++++++++----------------------
 fs/xfs/xfs_icache.h              |  26 +-
 fs/xfs/xfs_inode.c               | 219 +++++------
 fs/xfs/xfs_inode.h               |  18 +
 fs/xfs/xfs_inode_item.c          |  28 +-
 fs/xfs/xfs_log.c                 |  10 +-
 fs/xfs/xfs_log_cil.c             |  37 +-
 fs/xfs/xfs_log_priv.h            |  53 ++-
 fs/xfs/xfs_mount.c               |  10 +-
 fs/xfs/xfs_mount.h               |   6 +-
 fs/xfs/xfs_qm.c                  |  11 +-
 fs/xfs/xfs_super.c               |  93 +++--
 fs/xfs/xfs_trace.h               |   1 +
 fs/xfs/xfs_trans_ail.c           |  88 +++--
 fs/xfs/xfs_trans_priv.h          |   6 +-
 include/linux/rwsem.h            |   6 +
 include/linux/shrinker.h         |   9 +-
 include/linux/swap.h             |  23 +-
 include/trace/events/vmscan.h    |  69 ++--
 kernel/locking/rwsem.c           |  23 ++
 mm/slab.c                        |   3 +-
 mm/slob.c                        |   4 +-
 mm/slub.c                        |   3 +-
 mm/vmscan.c                      | 231 +++++++----
 net/sunrpc/auth.c                |   5 +-
 33 files changed, 863 insertions(+), 816 deletions(-)