Re: [ISSUE] The time cost of IOSQE_IO_LINK

Pavel Begunkov
2020-02-17T10:30:06Z

On 2/17/2020 1:23 AM, Jens Axboe wrote:
> On 2/16/20 12:06 PM, Pavel Begunkov wrote:
>> On 15/02/2020 09:01, Jens Axboe wrote:
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index fb94b8bac638..530dcd91fa53 100644
>>> @@ -4630,6 +4753,14 @@ static void __io_queue_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>> */
>>> if (ret == -EAGAIN && (!(req->flags & REQ_F_NOWAIT) ||
>>> (req->flags & REQ_F_MUST_PUNT))) {
>>> +
>>> + if (io_arm_poll_handler(req, &retry_count)) {
>>> + if (retry_count == 1)
>>> + goto issue;
>>
>> Better to sqe=NULL before retrying, so it won't re-read sqe and try to
>> init the req twice.
>
> Good point, that should get cleared after issue.
>
>> Also, the second sync-issue may -EAGAIN again, and as I remember,
>> read/write/etc will try to copy iovec into req->io. But iovec is
>> already in req->io, so it will self memcpy(). Not a good thing.
>
> I'll look into those details, that has indeed reared its head before.
>
>>> + else if (!retry_count)
>>> + goto done_req;
>>> + INIT_IO_WORK(&req->work, io_wq_submit_work);
>>
>> It's not nice to reset it as this:
>> - prep() could set some work.flags
>> - custom work.func is more performant (adds extra switch)
>> - some may rely on specified work.func to be called. e.g. close(), even though
>> it doesn't participate in the scheme
>
> It's totally a hack as-is for the "can't do it, go async". I did clean

And I don't understand lifetimes yet... probably would need a couple of
questions later.

> this up a bit (if you check the git version, it's changed quite a bit),

That's what I've been looking at

> but it's still a mess in terms of that and ->work vs union ownership.
> The commit message also has a note about that.
>
> So more work needed in that area for sure.

Right, I just checked a couple of things for you

--
Pavel Begunkov