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

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On 3/21/23 09:17, Ziyang Zhang wrote:
On 2023/3/19 00:23, 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 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. I was mostly
coming from non ublk cases, and one example would be such zc recv,
parsing the app level headers and redirecting the rest of the data

I haven't got a chance to work on it but will return to it in
a week. The discussion was here:

Hi Pavel and all,

I think it is a good idea to register some kernel objects(such as bvec)
in io_uring and return a cookie(such as buf_idx) for READ/WRITE/SEND/RECV sqes.
There are some ways to register user's buffer such as IORING_OP_PROVIDE_BUFFERS
and IORING_REGISTER_PBUF_RING but there is not a way to register kernel buffer(bvec).

I do not think reusing splice is a good idea because splice should run in io-wq.

The reason why I disabled inline splice execution is because do_splice()
and below the stack doesn't support nowait well enough, which is not a
problem when we hook directly under the ->splice_read() callback and
operate only with one file at a time at the io_uring level.

If we have a big sq depth there may be lots of io-wqs. Then lots of context switch
may lower the IO performance especially for small IO size.

Here are some rough ideas:
(1) design a new OPCODE such as IORING_REGISTER_KOBJ to register kernel objects in
     io_uring or
(2) reuse uring-cmd. We can send uring-cmd to drivers(opcode may be CMD_REGISTER_KBUF)
     and let drivers call io_uring_provide_kbuf() to register kbuf. io_uring_provide_kbuf()
     is a new function provided by io_uring for drivers.
(3) let the driver call io_uring_provide_kbuf() directly. For ublk, this function is called
     before io_uring_cmd_done().

Pavel Begunkov

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