Re: [PATCH 11/11] io_uring: get rid of intermediate aux cqe caches

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/15/24 16:27, Jens Axboe wrote:
> On 3/15/24 10:25 AM, Jens Axboe wrote:
>> On 3/15/24 10:23 AM, Pavel Begunkov wrote:
>>> On 3/15/24 16:20, Jens Axboe wrote:
>>>> On 3/15/24 9:30 AM, Pavel Begunkov wrote:
>>>>> io_post_aux_cqe(), which is used for multishot requests, delays
>>>>> completions by putting CQEs into a temporary array for the purpose
>>>>> completion lock/flush batching.
>>>>>
>>>>> DEFER_TASKRUN doesn't need any locking, so for it we can put completions
>>>>> directly into the CQ and defer post completion handling with a flag.
>>>>> That leaves !DEFER_TASKRUN, which is not that interesting / hot for
>>>>> multishot requests, so have conditional locking with deferred flush
>>>>> for them.
>>>>
>>>> This breaks the read-mshot test case, looking into what is going on
>>>> there.
>>>
>>> I forgot to mention, yes it does, the test makes odd assumptions about
>>> overflows, IIRC it expects that the kernel allows one and only one aux
>>> CQE to be overflown. Let me double check
>>
>> Yeah this is very possible, the overflow checking could be broken in
>> there. I'll poke at it and report back.
> 
> It does, this should fix it:
> 
> 
> diff --git a/test/read-mshot.c b/test/read-mshot.c
> index 8fcb79857bf0..501ca69a98dc 100644
> --- a/test/read-mshot.c
> +++ b/test/read-mshot.c
> @@ -236,7 +236,7 @@ static int test(int first_good, int async, int overflow)
>   		}
>   		if (!(cqe->flags & IORING_CQE_F_MORE)) {
>   			/* we expect this on overflow */
> -			if (overflow && (i - 1 == NR_OVERFLOW))
> +			if (overflow && i >= NR_OVERFLOW)

Which is not ideal either, e.g. I wouldn't mind if the kernel stops
one entry before CQ is full, so that the request can complete w/o
overflowing. Not supposing the change because it's a marginal
case, but we shouldn't limit ourselves.

-- 
Pavel Begunkov