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

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

On 6/27/22 15:11, Hao Xu wrote:
> 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.
>>
>> 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.
>>
>>> 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.
> 
> I've been blocked by an issue:
> if we deliver context from task a to b, we may have no ways to wake it
> up... because when the resource which blocks a is released by another
> task like c, c wakes up a, not b.
> If we want to make it work, we have to deliver the struct task_struct
> as well. That means the original task uses a new task_struct and the
> new task uses the old one. And in the meanwhile we have to maintain
> the pid, parent task .etc stuff.(since we swap the task_struct, the
> pid and other stuff also changed).
> Any thoughts?
> 

Just ignore this, seems I misunderstood something..