[RFC] a new way to achieve asynchronous IO

[Hao Xu <hao.xu@linux.dev>]

Hi,
I've some thought on the way of doing async IO. The current model is:
(given we are using SQPOLL mode)

the sqthread does:
(a) Issue a request with nowait/nonblock flag.
(b) If it would block, reutrn -EAGAIN
(c) The io_uring layer captures this -EAGAIN and wake up/create
a io-worker to execute the request synchronously.
(d) Try to issue other requests in the above steps again.

This implementation has two downsides:
(1) we have to find all the block point in the IO stack manually and
change them into "nowait/nonblock friendly".
(2) when we raise another io-worker to do the request, we submit the
request from the very beginning. This isn't a little bit inefficient.


While I think we can actually do it in a reverse way:
(given we are using SQPOLL mode)

the sqthread1 does:
(a) Issue a request in the synchronous way
(b) If it is blocked/scheduled soon, raise another sqthread2
(c) sqthread2 tries to issue other requests in the same way.

This solves problem (1), and may solve (2).
For (1), we just do the sqthread waken-up at the beginning of schedule()
just like what the io-worker and system-worker do. No need to find all
the block point.
For (2), we continue the blocked request from where it is blocked when
resource is satisfied.

What we need to take care is making sure there is only one task
submitting the requests.

To achieve this, we can maintain a pool of sqthread just like the iowq.

I've done a very simple/ugly POC to demonstrate this:

https://github.com/HowHsu/linux/commit/183be142493b5a816b58bd95ae4f0926227b587b

I also wrote a simple test to test it, which submits two sqes, one
read(pipe), one nop request. The first one will be block since no data
in the pipe. Then a new sqthread was created/waken up to submit the
second one and then some data is written to the pipe(by a unrelated
user thread), soon the first sqthread is waken up and continues the
request.

If the idea sounds no fatal issue I'll change the POC to real patches.
Any comments are welcome!