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

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

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)