Re: [PATCH V3 00/16] io_uring/ublk: add IORING_OP_FUSED_CMD

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

On Sat, Mar 18, 2023 at 04:23:35PM +0000, Pavel Begunkov wrote:
> On 3/16/23 03:13, Xiaoguang Wang wrote:
> > > Add IORING_OP_FUSED_CMD, it is one special URING_CMD, which has to
> > > be SQE128. The 1st SQE(master) is one 64byte URING_CMD, and the 2nd
> > > 64byte SQE(slave) is another normal 64byte OP. For any OP which needs
> > > to support slave OP, io_issue_defs[op].fused_slave needs to be set as 1,
> > > and its ->issue() can retrieve/import buffer from master request's
> > > fused_cmd_kbuf. The slave OP is actually submitted from kernel, part of
> > > this idea is from Xiaoguang's ublk ebpf patchset, but this patchset
> > > submits slave OP just like normal OP issued from userspace, that said,
> > > SQE order is kept, and batching handling is done too.
> > Thanks for this great work, seems that we're now in the right direction
> > to support ublk zero copy, I believe this feature will improve io throughput
> > greatly and reduce ublk's cpu resource usage.
> > 
> > I have gone through your 2th patch, and have some little concerns here:
> > Say we have one ublk loop target device, but it has 4 backend files,
> > every file will carry 25% of device capacity and it's implemented in stripped
> > way, then for every io request, current implementation will need issed 4
> > fused_cmd, right? 4 slave sqes are necessary, but it would be better to
> > have just one master sqe, so I wonder whether we can have another
> > method. The key point is to let io_uring support register various kernel
> > memory objects, which come from kernel, such as ITER_BVEC or
> > ITER_KVEC. so how about below actions:
> > 1. add a new infrastructure in io_uring, which will support to register
> > various kernel memory objects in it, this new infrastructure could be
> > maintained in a xarray structure, every memory objects in it will have
> > a unique id. This registration could be done in a ublk uring cmd, io_uring
> > offers registration interface.
> > 2. then any sqe can use these memory objects freely, so long as it
> > passes above unique id in sqe properly.
> > Above are just rough ideas, just for your reference.
> 
> It precisely hints on what I proposed a bit earlier, that makes
> me not alone thinking that it's a good idea to have a design allowing
> 1) multiple ops using a buffer and

Firstly fused command does cover this case, io_fused_cmd_provide_kbuf()
is very cheap, which just passes buffer reference.

Secondly, your original suggestion is to wire the per-io buffer with
context fixed buffer, which basically has to add two OPs:

1) one for registering buffer

2) another one for un-registering buffer

So one usual such IO may have to takes 3+ SQEs, which won't be efficient for
single or even double submission cases since the cost of touching global
context fixed buffer can't be ignored.

> 2) to limiting it to one single
> submission because the userspace might want to preprocess a part
> of the data, multiplex it or on the opposite divide.

Unfortunately ublk has to support multiple submissions, and there can
be lots of such use cases, logical volume manager(mirror, stripped),
distributed network storage, ...


Thanks, 
Ming