[PATCH v4 0/5] ceph: reduce mdsc->mutex contention in the cephfs kclient

This series reduces mdsc->mutex hold times from hundreds of
microseconds to tens of microseconds on the hot request-submit and
reply-handling paths.

The approach is incremental:
  1. Convert oldest_tid to atomic64_t so that __prepare_send_request()
     and __send_request() no longer need the mutex.
  2. Replace the request_tree rbtree with an xarray for O(1) lookups
     and internally-locked iteration.
  3. Add a dedicated wait_list_lock spinlock so wait-list operations
     no longer depend on the global mutex.
  4. Move mdsc->mutex acquisition inside __do_request(), then release
     it during the send phase (message construction and path walking),
     leaving only the brief setup/teardown under the lock.
  5. Narrow the mutex scope in replay_unsafe_requests() similarly.

Tested with concurrent readdir + stat on a 5000-file directory
(32 threads).  bpftrace measurements show:

                 before         after
  __do_request   354-2327 us    10-68 us    (34x)
  handle_reply   51-416 us     10-32 us    (13x)
  submit_request 89-211 us     10-41 us    (5x)

Benchmark results from test_i_caps (aggregate cache test):

  | Test                | Before   | After    | Improvement |
  |---------------------|----------|----------|-------------|
  | stat storm ST       | 692k/s   | 1,420k/s | +105% (2.1x)|
  | stat storm MT       | 722k/s   | 1,373k/s | +90%  (1.9x)|
  | open/close          | 403k/s   | 524k/s   | +30%        |
  | stat hot (cache)    | 353k/s   | 540k/s   | +53%        |
  | readdir             | 877/s    | 832/s    | ~0%         |
  | Total time          | 195s     | 121s     | -38%  (1.6x)|

The 2x stat throughput gain comes from __do_request() no longer
holding mdsc->mutex during the __send_request() phase, so dentry
path walking and message encoding in create_request_message()
run outside the lock.

No functional changes intended.

Signed-off-by: Xiubo Li <[email protected]>
---
Changes in v4:
- Fix ref leak in __register_request() xa_store error path
- Fix r_attempts data race: move r_attempts++ from
  __prepare_send_request() (lockless) into __do_request() and
  replay_unsafe_requests() under mdsc->mutex.  Adjust the retry
  overflow check accordingly.
- Fix kick_requests() list corruption: detach r_wait from the local
  kick_list before calling __do_request().
- Fix collect-then-replay list-node races in both kick_requests()
  and replay_unsafe_requests(): introduce r_aux_item, a dedicated
  list_head for temporary local list iteration, so that concurrent
  __unregister_request() cannot corrupt the iterator.
- Link to v3: https://patch.msgid.link/20260811-ceph-mdsc-mutex-optimization-v3-0-d031114419f4@clyso.com

Changes in v3:
- Restrict CEPH_FS to 64BIT to prevent xarray index truncation of
  u64 transaction IDs on 32-bit platforms, instead of the previous
  #if BITS_PER_LONG guard.  There are no 32-bit users.
  The survey: https://lore.kernel.org/ceph-devel/CAOJNxR+XiUxR1GNUgd8T18x2KH8kstWtpxwL-mwwE0i_qospJA@mail.gmail.com/T/#t
- Fix two plain reads of oldest_tid in the writer paths to use
  READ_ONCE() for consistency with the lockless read side.
- Replace the original replay_unsafe_requests() change with a
  proper collect-then-replay pattern: unsafe list entries and
  matching old xarray entries are gathered under the mutex with a
  reference taken, then replayed outside it.  Taking a reference
  ensures a concurrent reply handler cannot free an entry out
  from under the local-list iterator.
- Rewrite all commit messages as descriptive prose, focusing on
  the problem and the approach rather than enumerating modified
  functions.
- Link to v2: https://patch.msgid.link/20260715-ceph-mdsc-mutex-optimization-v2-0-90e81b726724@clyso.com

Changes in v2:
- Add xa_store() error handling and bail out on failure in the submit path
- Guard xarray conversion with BITS_PER_LONG==64, fall back to rbtree on 32-bit
- Fix missing mutex_unlock on early-return path in __do_request()
- Move mutex_unlock before __wake_requests() and kick_requests() calls to
  avoid recursive lock acquisition
- Pin requests with ceph_mdsc_get_request() across lockless __send_request()
  in replay_unsafe_requests() to prevent use-after-free
- Keep mdsc->mutex held on 32-bit for rb_first()/rb_next() iteration in
  replay_unsafe_requests()
- Drop stale "called under mdsc->mutex" comment on __wake_requests()
- Add benchmark results from test_i_caps (2.1x stat throughput improvement)
- Link to v1: https://patch.msgid.link/20260713-ceph-mdsc-mutex-optimization-v1-0-9ae5ac135c34@clyso.com

To: Ilya Dryomov <[email protected]>
To: Alex Markuze <[email protected]>
To: Viacheslav Dubeyko <[email protected]>
Cc: [email protected]
Cc: [email protected]

---
Xiubo Li (5):
      ceph: use READ_ONCE/WRITE_ONCE for oldest_tid
      ceph: replace the request_tree rbtree with an xarray keyed by r_tid
      ceph: add wait_list_lock for wait-list serialization
      ceph: move mdsc->mutex into __do_request()
      ceph: narrow mdsc->mutex scope in replay_unsafe_requests

 fs/ceph/Kconfig      |   1 +
 fs/ceph/debugfs.c    |   6 +-
 fs/ceph/mds_client.c | 302 +++++++++++++++++++++++++++++++--------------------
 fs/ceph/mds_client.h |   7 +-
 4 files changed, 195 insertions(+), 121 deletions(-)
---
base-commit: 30c0913a2b44a66dc17283a88489b3f3e12f4327
change-id: 20260713-ceph-mdsc-mutex-optimization-7e74ab6bbc8b

Best regards,
--  
Xiubo Li <[email protected]>