Re: [RFC 7/7] io_uring,fs: introduce IORING_OP_GET_BUF

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

On Wed, May 03, 2023 at 03:54:02PM +0100, Pavel Begunkov wrote:
> On 5/2/23 15:57, Ming Lei wrote:
> > On Sun, Apr 30, 2023 at 10:35:29AM +0100, Pavel Begunkov wrote:
> > > There are several problems with splice requests, aka IORING_OP_SPLICE:
> > > 1) They are always executed by a worker thread, which is a slow path,
> > >     as we don't have any reliable way to execute it NOWAIT.
> > > 2) It can't easily poll for data, as there are 2 files it operates on.
> > >     It would either need to track what file to poll or poll both of them,
> > >     in both cases it'll be a mess and add lot of overhead.
> > > 3) It has to have pipes in the middle, which adds overhead and is not
> > >     great from the uapi design perspective when it goes for io_uring
> > >     requests.
> > > 4) We want to operate with spliced data as with a normal buffer, i.e.
> > >     write / send / etc. data as normally while it's zerocopy.
> > > 
> > > It can partially be solved, but the root cause is a suboptimal for
> > > io_uring design of IORING_OP_SPLICE. Introduce a new request type
> > > called IORING_OP_GET_BUF, inspired by splice(2) as well as other
> > > proposals like fused requests. The main idea is to use io_uring's
> > > registered buffers as the middle man instead of pipes. Once a buffer
> > > is fetched / spliced from a file using a new fops callback
> > > ->iou_get_buf, it's installed as a registered buffers and can be used
> > > by all operations supporting the feature.
> > > 
> > > Once the userspace releases the buffer, io_uring will wait for all
> > > requests using the buffer to complete and then use a file provided
> > > callback ->release() to return the buffer back. It operates on the
> > 
> > In the commit of "io_uring: add an example for buf-get op", I don't see
> > any code to release the buffer, can you explain it in details about how
> > to release the buffer in userspace? And add it in your example?
> 
> Sure, we need to add buf updates via request.

I guess it can't be buf update, here we need to release buf. At least
ublk needs to release the buffer explicitly after all consumers are done
with the buffer registered by IORING_OP_GET_BUF, when there may not be
any new buffer to be provided, and the buffer is per-IO for each io
request from /dev/ublkbN.

> 
> Particularly, in this RFC, the removal from the table was happening
> in io_install_buffer() by one of the test-only patches, the "remove
> previous entry on update" style as it's with files. Then it's
> released with the last ref put, either on removal with a request
> like:
> 
> io_free_batch_list()
>      io_req_put_rsrc_locked()
>          ...
> 
> > Here I guess the ->release() is called in the following code path:
> > 
> > io_buffer_unmap
> >      io_rsrc_buf_put
> >          io_rsrc_put_work
> >              io_rsrc_node_ref_zero
> >                  io_put_rsrc_node
> > 
> > If it is true, what is counter-pair code for io_put_rsrc_node()?
> > So far, only see io_req_set_rsrc_node() is called from
> > io_file_get_fixed(), is it needed for consumer OP of the buffer?
> > 
> > Also io_buffer_unmap() is called after io_rsrc_node's reference drops
> > to zero, which means ->release() isn't called after all its consumer(s)
> > are done given io_rsrc_node is shared by in-flight requests. If it is
> > true, this way will increase buffer lifetime a lot.
> 
> That's true. It's not a new downside, so might make more sense
> to do counting per rsrc (file, buffer), which is not so bad for
> now, but would be a bit concerning if we grow the number of rsrc
> types.

It may not be one deal for current fixed file user, which isn't usually
updated in fast path.

But IORING_OP_GET_BUF is supposed to be for fast path, this delay
release is really one problem. Cause if there is any new buffer provided &
consumed, old buffer can't be released any more. And this way can't
be used in ublk zero copy, let me share the ublk model a bit:

1) one batch ublk blk io requests(/dev/ublkbN) are coming, and batch size is
often QD of workload on /dev/ublkbN, and batch size could be 1 or more.

2) ublk driver notifies the blk io requests via io_uring command
completion

3) ublk server starts to handle this batch of io requests, by calling
IORING_OP_GET_BUF on /dev/ublkcN & normal OPs(rw, net recv/send, ...)
for each request in this batch

4) new batch of ublk blk io requests can come when handling the previous
batch of io requests, then the old buffer release is delayed until new
batch of ublk io requests are completed.

> 
> > ublk zero copy needs to call ->release() immediately after all
> > consumers are done, because the ublk disk request won't be completed
> > until the buffer is released(the buffer actually belongs to ublk block request).
> > 
> > Also the usage in liburing example needs two extra syscall(io_uring_enter) for
> > handling one IO, not take into account the "release OP". IMO, this way makes
> 
> Something is amiss here. It's 3 requests, which means 3 syscalls
> if you send requests separately (each step can be batch more
> requests), or 1 syscall if you link them together. There is an
> example using links for 2 requests in the test case.

See the final comment about link support.

> 
> > application more complicated, also might perform worse:
> > 
> > 1) for ublk zero copy, the original IO just needs one OP, but now it
> > takes three OPs, so application has to take coroutine for applying
> 
> Perhaps, you mean two requests for fused, IORING_OP_FUSED_CMD + IO
> request, vs three for IORING_OP_GET_BUF. There might be some sort of
> auto-remove on use, making it two requests, but that seems a bit ugly.

The most important part is that IORING_OP_GET_BUF adds two extra wait:

1) one io_uring_enter() is required after submitting IORING_OP_GET_BUF

2) another io_uring_enter() is needed after submitting buffer consumer
OPs, before calling buffer release OP(not added yet in your patchset)

The two waits not only causes application more complicated, but also
hurts performance, cause IOs/syscall is reduced. People loves io_uring
because it is really async io model, such as, all kinds of IO can be
submitted in single context, and wait in single io_uring_enter().

> 
> > 3 stages batch submission(GET_BUF, IO, release buffer) since IO_LINK can't
> > or not suggested to be used. In case of low QD, batch size is reduced much,
> > and performance may hurt because IOs/syscall is 1/3 of fused command.
> 
> I'm not a big fan of links for their inflexibility, but it can be
> used. The point is rather it's better not to be the only way to
> use the feature as we may need to stop in the middle, return

LINK always support to stop in the middle, right?

> control to the userspace and let it handle errors, do data processing
> and so on. The latter may need a partial memcpy() into the userspace,
> e.g. copy a handful bytes of headers to decide what to do with the
> rest of data.

At least this patchset doesn't work with IO_LINK, please see my previous
reply because the current rw/net zc retrieves fixed buffer in ->prep().

Yeah, the problem can be addressed by moving the buffer retrieving into
->issue().

But my concern is more about easy use and performance:

1) with io_link, the extra two waits(after IORING_OP_GET_BUF and before IORING_OP_GET_BUF)
required can be saved, then application doesn't need coroutine or
similar trick for avoiding the extra wait handling.

2) but if three OPs uses the buffer registered by IORING_OP_GET_BUF, the three
OPs have to be linked one by one, then all three have to be be submitted one
after the previous one is completed, performance is hurt a lot.

> 
> I deem fused cmds to be a variant of linking, so it's rather with
> it you link 2 requests vs optionally linking 3 with this patchset.

IMO, one extra 64byte touching may slow things a little, but I think it
won't be big deal, what really matters is that easy use of the interface
and performance.

Fused command solves both: allow to submit the consumer OPs concurrently;
avoid multiple wait point and make application easier to implement

For example:

1) fused command based application: (every IO command takes 1 syscall usually)

	for each reapped CQE:
		- if CQE is for io command from /dev/ublkc:
			- handle it by allocating/queuing three SQEs:
				- one SQE is for primary command,
				- the 2nd SQE is for READ to file 1
				- the 3rd SQE is for READ to file 2
		- if CQE is for primary command or normal OP:
			- just check if the whole fused command is completed
				- if yes, notify that the current io command is
				completed (the current io request from /dev/ublkbN will
				be completed in ublk driver, and this io command is
				queued for incoming request)
				- if any err, handle the err: retry or terminate,
				  similar with handling for 'yes'
				- otherwise, just return
		io_uring_enter();			//single wait point

2) IORING_OP_GET_BUF based application(every IO command needs two syscall)

	for each reapped CQE:
		- if CQE is for io command from /dev/ublkc:
			- handle it by:
				queuing IORING_OP_GET_BUF
				recoding this io needs to be hanlded

	io_uring_enter()	//wait until all IORING_OP_GET_BUF are done

	for each IO recorded in above, queue two new RW OPs(one for file1, one for file2)

	io_uring_enter()	//wait until the above two OPs are done

	for each reapped CQE:
		release the buffer registered by IORING_OP_GET_BUF

Or coroutine based approach: (every io command needs two syscalls)

	for each reaaped CQE:
		- if CQE is for command from /dev/ublkc:
				- queue IORING_OP_GET_BUF
				- corouine wait: until IORING_OP_GET_BUF is done
				- queue two RW OPs with the registered buffer
				- coroutine wait: until all above OP is done
				- release buffer registered by IORING_OP_GET_BUF
		- otherwise:
			- call related co routine wakeup handler for advancing in
			above co routine wait point
		io_uring_enter();

coroutine based approach may look clean and easier to implement, but
it depends on language support, and still needs extra care, also stackless
coroutine isn't easy to use.

Thanks,
Ming