[PATCH v5 0/4] io_uring: remove ring quiesce in io_uring_register

Usama Arif
2022-02-03T23:34:44Z

For opcodes relating to registering/unregistering eventfds, this is done by
creating a new RCU data structure (io_ev_fd) as part of io_ring_ctx that
holds the eventfd_ctx, with reads to the structure protected by
rcu_read_lock and writes (register/unregister calls) protected by a mutex.

With the above approach ring quiesce can be avoided which is much more
expensive then using RCU lock. On the system tested, io_uring_reigster with
IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared to 15ms
before with ring quiesce.

The second patch creates the RCU protected data structure and removes ring
quiesce for IORING_REGISTER_EVENTFD and IORING_UNREGISTER_EVENTFD.

The third patch builds on top of the second patch and removes ring quiesce
for IORING_REGISTER_EVENTFD_ASYNC.

The fourth patch completely removes ring quiesce from io_uring_register,
as IORING_REGISTER_ENABLE_RINGS and IORING_REGISTER_RESTRICTIONS dont need
them.

---
v4->v5:
- Remove ring quiesce completely from io_uring_register (Pavel Begunkov)
- Replaced rcu_barrier with unregistering flag (Jens Axboe)
- Created a faster check for ctx->io_ev_fd in io_eventfd_signal and cleaned up
io_eventfd_unregister (Jens Axboe)

v3->v4:
- Switch back to call_rcu and use rcu_barrier incase io_eventfd_register fails
to make sure all rcu callbacks have finished.

v2->v3:
- Switched to using synchronize_rcu from call_rcu in io_eventfd_unregister.

v1->v2:
- Added patch to remove eventfd from tracepoint (Patch 1) (Jens Axboe)
- Made the code of io_should_trigger_evfd as part of io_eventfd_signal (Jens Axboe)

Usama Arif (4):
io_uring: remove trace for eventfd
io_uring: avoid ring quiesce while registering/unregistering eventfd
io_uring: avoid ring quiesce for IORING_REGISTER_EVENTFD_ASYNC
io_uring: remove ring quiesce for io_uring_register

fs/io_uring.c | 202 ++++++++++++++++----------------
include/trace/events/io_uring.h | 13 +-
2 files changed, 107 insertions(+), 108 deletions(-)

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2.25.1