On 10/31/24 3:24 PM, Jens Axboe wrote:
> 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.

False alarm, it was my messup adding the group flag. Works just fine.
I'm attaching the version I tested, on top of that flags2 patch.

Since we're on the topic - my original bundle patch used a bundle OP to
define an sqe grouping, which didn't need to use an sqe flag. Any
particular reason why you went with a flag for this one?

I do think it comes out nicer with a flag for certain things, like being
able to link groups. Maybe that's the primary reason.

-- 
Jens Axboe