Re: [PATCH V8 4/7] io_uring: support SQE group

[Jens Axboe <axboe@kernel.dk>]

On 10/25/24 6:22 AM, Ming Lei wrote:
> SQE group is defined as one chain of SQEs starting with the first SQE that
> has IOSQE_SQE_GROUP set, and ending with the first subsequent SQE that
> doesn't have it set, and it is similar with chain of linked SQEs.
> 
> Not like linked SQEs, each sqe is issued after the previous one is
> completed. All SQEs in one group can be submitted in parallel. To simplify
> the implementation from beginning, all members are queued after the leader
> is completed, however, this way may be changed and leader and members may
> be issued concurrently in future.
> 
> The 1st SQE is group leader, and the other SQEs are group member. The whole
> group share single IOSQE_IO_LINK and IOSQE_IO_DRAIN from group leader, and
> the two flags can't be set for group members. For the sake of
> simplicity, IORING_OP_LINK_TIMEOUT is disallowed for SQE group now.
> 
> When the group is in one link chain, this group isn't submitted until the
> previous SQE or group is completed. And the following SQE or group can't
> be started if this group isn't completed. Failure from any group member will
> fail the group leader, then the link chain can be terminated.
> 
> When IOSQE_IO_DRAIN is set for group leader, all requests in this group and
> previous requests submitted are drained. Given IOSQE_IO_DRAIN can be set for
> group leader only, we respect IO_DRAIN by always completing group leader as
> the last one in the group. Meantime it is natural to post leader's CQE
> as the last one from application viewpoint.
> 
> Working together with IOSQE_IO_LINK, SQE group provides flexible way to
> support N:M dependency, such as:
> 
> - group A is chained with group B together
> - group A has N SQEs
> - group B has M SQEs
> 
> then M SQEs in group B depend on N SQEs in group A.
> 
> N:M dependency can support some interesting use cases in efficient way:
> 
> 1) read from multiple files, then write the read data into single file
> 
> 2) read from single file, and write the read data into multiple files
> 
> 3) write same data into multiple files, and read data from multiple files and
> compare if correct data is written
> 
> Also IOSQE_SQE_GROUP takes the last bit in sqe->flags, but we still can
> extend sqe->flags with io_uring context flag, such as use __pad3 for
> non-uring_cmd OPs and part of uring_cmd_flags for uring_cmd OP.

Did you run the liburing tests with this? I rebased it on top of the
flags2 patch I just sent out, and it fails defer-taskrun and crashes
link_drain. Don't know if others fail too. I'll try the original one
too, but nothing between those two should make a difference. It passes
just fine with just the flags2 patch, so I'm a bit suspicious this patch
is the issue.

-- 
Jens Axboe