Re: [RFC] a new way to achieve asynchronous IO

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

On 6/23/22 22:08, Jens Axboe wrote:
> On 6/23/22 7:31 AM, Hao Xu wrote:
>> On 6/20/22 21:41, Jens Axboe wrote:
>>> On 6/20/22 6:01 AM, Hao Xu wrote:
>>>> 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!
>>>
>>> One thing I've always wanted to try out is kind of similar to this, but
>>> a superset of it. Basically io-wq isn't an explicit offload mechanism,
>>> it just happens automatically if the issue blocks. This applies to both
>>> SQPOLL and non-SQPOLL.
>>>
>>> This takes a page out of the old syslet/threadlet that Ingo Molnar did
>>> way back in the day [1], but it never really went anywhere. But the
>>> pass-on-block primitive would apply very nice to io_uring.
>>
>> I've read a part of the syslet/threadlet patchset, seems it has
>> something that I need, my first idea about the new iowq offload is
>> just like syslet----if blocked, trigger a new worker, deliver the
>> context to it, and then update the current context so that we return
>> to the place of sqe submission. But I just didn't know how to do it.

[1]

> 
> Exactly, what you mentioned was very close to what I had considered in
> the past, and what the syslet/threadlet attempted to do. Except it flips
> it upside down a bit, which I do think is probably the saner way to do
> it rather than have the original block and fork a new one.

Hi Jens,
I found that what the syslet does is also block the original and let the
new one return to the userspace.
If we want to achieve something like [1], we have to consider one
problem:
Code at block point usually looks like:

     put task to a wait table
     schedule
     move task out from the wait table

and code at wakeup point usually looks like:

       look through the wait table
       wake up a/all proper tasks

the problem is if we deliver the block context to a worker, how can we
wake it up since the task in wait table is the original one?

What do you think of this?

> 
>> By the way, may I ask why the syslet/threadlet is not merged to the
>> mainline. The mail thread is very long, haven't gotten a chance to
>> read all of it.
> 
> Not quite sure, it's been a long time. IMHO it's a good idea looking for
> the right interface, which we now have. So the time may be ripe to do
> something like this, finally.
>>
>> For the approach I posted, I found it is actually SQPOLL-nonrelated.
>> The original conext just wake up a worker in the pool to do the
>> submission, and if one blocks, another one wakes up to do the
>> submission. It is definitely easier to implement than something like
>> syslet(context delivery) since the new worker naturally goes to the
>> place of submission thus no context delivery needed. but a downside is
>> every time we call io_uring_enter to submit a batch of sqes, there is a
>> wakeup at the beginning.
>>
>> I'll try if I can implement a context delivery version.

Based on what I said above, I'm still implementing the old version..
I think the code is useful and can be leveraged to the context delivery
version if the latter is viable someday.

Regards,
Hao

> 
> Sounds good, thanks.
>