Re: [PATCH V4 4/8] io_uring: support SQE group

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

On Mon, Jul 29, 2024 at 02:58:58PM +0100, Pavel Begunkov wrote:
> On 7/25/24 11:33, Ming Lei wrote:
> > On Wed, Jul 24, 2024 at 02:41:38PM +0100, Pavel Begunkov wrote:
> > > On 7/23/24 15:34, Ming Lei wrote:
> ...
> > > > But grp_refs is dropped after io-wq request reference drops to
> > > > zero, then both io-wq and nor-io-wq code path can be unified
> > > > wrt. dealing with grp_refs, meantime it needn't to be updated
> > > > in extra(io-wq) context.
> > > 
> > > Let's try to describe how it can work. First, I'm only describing
> > > the dep mode for simplicity. And for the argument's sake we can say
> > > that all CQEs are posted via io_submit_flush_completions.
> > > 
> > > io_req_complete_post() {
> > > 	if (flags & GROUP) {
> > > 		req->io_task_work.func = io_req_task_complete;
> > > 		io_req_task_work_add(req);
> > > 		return;
> > > 	}
> > > 	...
> > > }
> > 
> > OK.
> > 
> > io_wq_free_work() still need to change to not deal with
> > next link & ignoring skip_cqe, because group handling(
> 
> No, it doesn't need to know about all that.
> 
> > cqe posting, link advance) is completely moved into
> > io_submit_flush_completions().
> 
> It has never been guaranteed that io_req_complete_post()
> will be the one completing the request,
> io_submit_flush_completions() can always happen.
> 
> 
> struct io_wq_work *io_wq_free_work(struct io_wq_work *work)
> {
> 	...
> 	if (req_ref_put_and_test(req)) {
> 		nxt = io_req_find_next(req);
> 		io_free_req();
> 	}
> }
> 
> We queue linked requests only when all refs are dropped, and
> the group handling in my snippet is done before we drop the
> owner's reference.
> 
> IOW, you won't hit io_free_req() in io_wq_free_work() for a
> leader unless all members in its group got completed and
> the leader already went through the code dropping those shared
> ublk buffers.

If io_free_req() won't be called for leader, leader won't be added
to ->compl_reqs, and it has to be generated when all members are
completed in __io_submit_flush_completions().

For !io_wq, we can align to this way by not completing leader in
io_req_complete_defer().

The above implementation looks simpler, and more readable.

> 
> 
> > > You can do it this way, nobody would ever care, and it shouldn't
> > > affect performance. Otherwise everything down below can probably
> > > be extended to io_req_complete_post().
> > > 
> > > To avoid confusion in terminology, what I call a member below doesn't
> > > include a leader. IOW, a group leader request is not a member.
> > > 
> > > At the init we have:
> > > grp_refs = nr_members; /* doesn't include the leader */
> > > 
> > > Let's also say that the group leader can and always goes
> > > through io_submit_flush_completions() twice, just how it's
> > > with your patches.
> > > 
> > > 1) The first time we see the leader in io_submit_flush_completions()
> > > is when it's done with resource preparation. For example, it was
> > > doing some IO into a buffer, and now is ready to give that buffer
> > > with data to group members. At this point it should queue up all group
> > > members. And we also drop 1 grp_ref. There will also be no
> > > io_issue_sqe() for it anymore.
> > 
> > Ok, then it is just the case with dependency.
> > 
> > > 
> > > 2) Members are executed and completed, in io_submit_flush_completions()
> > > they drop 1 grp_leader->grp_ref reference each.
> > > 
> > > 3) When all members complete, leader's grp_ref becomes 0. Here
> > > the leader is queued for io_submit_flush_completions() a second time,
> > > at which point it drops ublk buffers and such and gets destroyed.
> > > 
> > > You can post a CQE in 1) and then set CQE_SKIP. Can also be fitted
> > > into 3). A pseudo code for when we post it in step 1)
> > 
> > This way should work, but it confuses application because
> > the leader is completed before all members:
> > 
> > - leader usually provide resources in group wide
> > - member consumes this resource
> > - leader is supposed to be completed after all consumer(member) are
> > done.
> > 
> > Given it is UAPI, we have to be careful with CQE posting order.
> 
> That's exactly what I was telling about the uapi previously,
> I don't believe we want to inverse the natural CQE order.
> 
> Regardless, the order can be inversed like this:
> 
> __io_flush_completions() {
> 	...
> 	if (req->flags & (SKIP_CQE|GROUP)) {
> 		if (req->flags & SKIP_CQE)
> 			continue;
> 		// post a CQE only when we see it for a 2nd time in io_flush_completion();
> 		if (is_leader() && !(req->flags & ALREADY_SEEN))
> 			continue;

I am afraid that ALREADY_SEEN can't work, since leader can only be added
to ->compl_reqs once.


Thanks, 
Ming