Re: [PATCHSET v3 0/3] io_uring CQ ring backpressure

[Pavel Begunkov <asml.silence@gmail.com>]

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On 07/11/2019 19:00, Jens Axboe wrote:
> Currently we drop completion events, if the CQ ring is full. That's fine
> for requests with bounded completion times, but it may make it harder to
> use io_uring with networked IO where request completion times are
> generally unbounded. Or with POLL, for example, which is also unbounded.
> 
> This patch adds IORING_SETUP_CQ_NODROP, which changes the behavior a bit
> for CQ ring overflows. First of all, it doesn't overflow the ring, it
> simply stores backlog of completions that we weren't able to put into
> the CQ ring. To prevent the backlog from growing indefinitely, if the
> backlog is non-empty, we apply back pressure on IO submissions. Any
> attempt to submit new IO with a non-empty backlog will get an -EBUSY
> return from the kernel.
> 
> I think that makes for a pretty sane API in terms of how the application
> can handle it. With CQ_NODROP enabled, we'll never drop a completion
> event, but we'll also not allow submissions with a completion backlog.
> 
Looks good to me
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>


> Changes since v2:
> 
> - Add io_double_put_req() helper for the cases where we need to drop both
>   the submit and complete reference. We didn't need this before as we
>   could just free the request unconditionally, but we don't know if that's
>   the case anymore if add/fill grabs a reference to it.
> - Fix linked request dropping.
> 
> Changes since v1:
> 
> - Drop the cqe_drop structure and allocation, simply use the io_kiocb
>   for the overflow backlog
> - Rebase on top of Pavel's series which made this cleaner
> - Add prep patch for the fill/add CQ handler changes
> 
>  fs/io_uring.c                 | 209 +++++++++++++++++++++++-----------
>  include/uapi/linux/io_uring.h |   1 +
>  2 files changed, 143 insertions(+), 67 deletions(-)
> 

-- 
Pavel Begunkov


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