linux-kernel - Re: [PATCH v5 0/4] io_uring: remove ring quiesce in io_uring

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Date:   Fri, 4 Feb 2022 00:02:08 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Usama Arif <usama.arif@...edance.com>, io-uring@...r.kernel.org,
        axboe@...nel.dk, linux-kernel@...r.kernel.org
Cc:     fam.zheng@...edance.com
Subject: Re: [PATCH v5 0/4] io_uring: remove ring quiesce in io_uring_register

On 2/3/22 23:34, Usama Arif wrote:
> 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.

Let me leave it just for history: I strongly dislike it considering
there is no one who uses or going to use it. Even more, I can't find a
single user of io_uring_unregister_eventfd() in liburing tests, so most
probably the paths are not tested at all.



> ---
> 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(-)
> 

-- 
Pavel Begunkov