Re: [PATCH 3/4] io_uring: tweak iopoll return for REQ_F_CQE_SKIP

[Jens Axboe <axboe@kernel.dk>]

On 12/4/21 3:48 PM, Pavel Begunkov wrote:
> On 12/4/21 22:21, Jens Axboe wrote:
>> On 12/4/21 1:49 PM, Pavel Begunkov wrote:
>>> Currently, IOPOLL returns the number of completed requests, but with
>>> REQ_F_CQE_SKIP there are not the same thing anymore. That may be
>>> confusing as non-iopoll wait cares only about CQEs, so make io_do_iopoll
>>> return the number of posted CQEs.
>>>
>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>> ---
>>>   fs/io_uring.c | 6 +++---
>>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 64add8260abb..ea7a0daa0b3b 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -2538,10 +2538,10 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
>>>   		/* order with io_complete_rw_iopoll(), e.g. ->result updates */
>>>   		if (!smp_load_acquire(&req->iopoll_completed))
>>>   			break;
>>> +		if (unlikely(req->flags & REQ_F_CQE_SKIP))
>>> +			continue;
>>>   
>>> -		if (!(req->flags & REQ_F_CQE_SKIP))
>>> -			__io_fill_cqe(ctx, req->user_data, req->result,
>>> -				      io_put_kbuf(req));
>>> +		__io_fill_cqe(ctx, req->user_data, req->result, io_put_kbuf(req));
>>>   		nr_events++;
>>>   	}
>>>   
>>
>> Not sure I follow the logic behind this change. Places like
>> io_iopoll_try_reap_events() just need a "did we find anything" return,
>> which is independent on whether or not we actually posted CQEs or not.
>> Other callers either don't care what the return value is or if it's < 0
>> or not (which this change won't affect).
>>
>> I feel like I'm missing something here, or that the commit message
>> better needs to explain why this change is done.
> 
> I was wrong on how I described it, but it means that the problem is in
> a different place.
> 
> int io_do_iopoll() {
> 	return nr_events;
> }
> 
> int io_iopoll_check() {
> 	do {
> 		nr_events += io_do_iopoll();
> 	while (nr_events < min && ...);
> }
> 
> And "events" there better to be CQEs, otherwise the semantics
> of @min + CQE_SKIP is not very clear and mismatches non-IOPOLL.

Can you do a v2 of this patch? Rest of them look good to me.

-- 
Jens Axboe