Re: [PATCH 5/9] io_uring: support SQE group

[Ming Lei <ming.lei@redhat.com>]

On Mon, Apr 22, 2024 at 12:27:28PM -0600, Jens Axboe wrote:
> On 4/7/24 7:03 PM, Ming Lei wrote:
> > SQE group is defined as one chain of SQEs starting with the first sqe that
> > has IOSQE_EXT_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.
> > 
> > The 1st SQE is group leader, and the other SQEs are group member. The group
> > leader is always freed after all members are completed. Group members
> > aren't submitted until the group leader is completed, and there isn't any
> > dependency among group members, and IOSQE_IO_LINK can't be set for group
> > members, same with IOSQE_IO_DRAIN.
> > 
> > Typically the group leader provides or makes resource, and the other members
> > consume the resource, such as scenario of multiple backup, the 1st SQE is to
> > read data from source file into fixed buffer, the other SQEs write data from
> > the same buffer into other destination files. SQE group provides very
> > efficient way to complete this task: 1) fs write SQEs and fs read SQE can be
> > submitted in single syscall, no need to submit fs read SQE first, and wait
> > until read SQE is completed, 2) no need to link all write SQEs together, then
> > write SQEs can be submitted to files concurrently. Meantime application is
> > simplified a lot in this way.
> > 
> > Another use case is to for supporting generic device zero copy:
> > 
> > - the lead SQE is for providing device buffer, which is owned by device or
> >   kernel, can't be cross userspace, otherwise easy to cause leak for devil
> >   application or panic
> > 
> > - member SQEs reads or writes concurrently against the buffer provided by lead
> >   SQE
> 
> In concept, this looks very similar to "sqe bundles" that I played with
> in the past:
> 
> https://git.kernel.dk/cgit/linux/log/?h=io_uring-bundle

Indeed, so looks it is something which io_uring needs.

> 
> Didn't look too closely yet at the implementation, but in spirit it's
> about the same in that the first entry is processed first, and there's
> no ordering implied between the test of the members of the bundle /
> group.

Yeah.

> 
> I do think that's a flexible thing to support, particularly if:
> 
> 1) We can do it more efficiently than links, which are pretty horrible.

Agree, link is hard to use in async/.await of modern language per my
experience.

Also sqe group won't break link, and the group is thought as a whole
wrt. linking.

> 2) It enables new worthwhile use cases
> 3) It's done cleanly 
> 4) It's easily understandable and easy to document, so that users will
>    actually understand what this is and what use cases it enable. Part
>    of that is actually naming, it should be readily apparent what a
>    group is, what the lead is, and what the members are. Using your
>    terminology here, definitely worth spending some time on that to get
>    it just right and self evident.

All are nice suggestions, and I will follow above and make them in V2.


Thanks,
Ming