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@xxxxxxxxx> > 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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