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
