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.
