Re: [PATCH 5.13 v2] io_uring: maintain drain requests' logic

[Hao Xu <haoxu@linux.alibaba.com>]

在 2021/4/8 下午6:16, Hao Xu 写道:
> 在 2021/4/7 下午11:49, Jens Axboe 写道:
>> On 4/7/21 5:23 AM, Hao Xu wrote:
>>> more tests comming, send this out first for comments.
>>>
>>> Hao Xu (3):
>>>    io_uring: add IOSQE_MULTI_CQES/REQ_F_MULTI_CQES for multishot 
>>> requests
>>>    io_uring: maintain drain logic for multishot requests
>>>    io_uring: use REQ_F_MULTI_CQES for multipoll IORING_OP_ADD
>>>
>>>   fs/io_uring.c                 | 34 +++++++++++++++++++++++++++++-----
>>>   include/uapi/linux/io_uring.h |  8 +++-----
>>>   2 files changed, 32 insertions(+), 10 deletions(-)
>>
>> Let's do the simple cq_extra first. I don't see a huge need to add an
>> IOSQE flag for this, probably best to just keep this on a per opcode
>> basis for now, which also then limits the code path to just touching
>> poll for now, as nothing else supports multishot CQEs at this point.
>>
> gotcha.
> a small issue here:
>   sqe-->sqe(link)-->sqe(link)-->sqe(link, multishot)-->sqe(drain)
> 
> in the above case, assume the first 3 single-shot reqs have completed.
> then I think the drian request won't be issued now unless the multishot 
> request in the linkchain has been issued. The trick is: a multishot req
> in a linkchain consumes cached_sq_head when io_get_sqe(), which means it
> is counted in seq, but we will deduct the sqe when it is issued if we
> want to do the job per opcode not in the main code path.
> before the multishot req issued:
>       all_sqes(4) - multishot_sqes(0) == all_cqes(3) - multishot_cqes(0)
> after the multishot req issued:
>       all_sqes(4) - multishot_sqes(1) == all_cqes(3) - multishot_cqes(0)

Sorry, my statement is wrong. It's not "won't be issued now unless the
multishot request in the linkchain has been issued". Actually I now
think the drain req won't be issued unless the multishot request in the
linkchain has completed. Because we may first check req_need_defer()
then issue(req->link), so:
    sqe0-->sqe1(link)-->sqe2(link)-->sqe3(link, multishot)-->sqe4(drain)

   sqe2 is completed:
     call req_need_defer:
     all_sqes(4) - multishot_sqes(0) == all_cqes(3) - multishot_cqes(0)
   sqe3 is issued:
     all_sqes(4) - multishot_sqes(1) == all_cqes(3) - multishot_cqes(0)
   sqe3 is completed:
     call req_need_defer:
     all_sqes(4) - multishot_sqes(1) == all_cqes(3) - multishot_cqes(0)

sqe4 shouldn't wait sqe3.