Re: Odd timeout behavior

[Jens Axboe <axboe@kernel.dk>]

On 4/13/20 2:21 AM, Pavel Begunkov wrote:
> On 4/12/2020 6:14 PM, Hrvoje Zeba wrote:
>> On Sun, Apr 12, 2020 at 5:15 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>>>
>>> On 4/12/2020 5:07 AM, Jens Axboe wrote:
>>>> On 4/11/20 5:00 PM, Hrvoje Zeba wrote:
>>>>> Hi,
>>>>>
>>>>> I've been looking at timeouts and found a case I can't wrap my head around.
>>>>>
>>>>> Basically, If you submit OPs in a certain order, timeout fires before
>>>>> time elapses where I wouldn't expect it to. The order is as follows:
>>>>>
>>>>> poll(listen_socket, POLLIN) <- this never fires
>>>>> nop(async)
>>>>> timeout(1s, count=X)
>>>>>
>>>>> If you set X to anything but 0xffffffff/(unsigned)-1, the timeout does
>>>>> not fire (at least not immediately). This is expected apart from maybe
>>>>> setting X=1 which would potentially allow the timeout to fire if nop
>>>>> executes after the timeout is setup.
>>>>>
>>>>> If you set it to 0xffffffff, it will always fire (at least on my
>>>>> machine). Test program I'm using is attached.
>>>>>
>>>>> The funny thing is that, if you remove the poll, timeout will not fire.
>>>>>
>>>>> I'm using Linus' tree (v5.6-12604-gab6f762f0f53).
>>>>>
>>>>> Could anybody shine a bit of light here?
>>>>
>>>> Thinking about this, I think the mistake here is using the SQ side for
>>>> the timeouts. Let's say you queue up N requests that are waiting, like
>>>> the poll. Then you arm a timeout, it'll now be at N + count before it
>>>> fires. We really should be using the CQ side for the timeouts.
>>>
>>> As I get it, the problem is that timeout(off=0xffffffff, 1s) fires
>>> __immediately__ (i.e. not waiting 1s).
>>
>> Correct.
>>
>>> And still, the described behaviour is out of the definition. It's sounds
>>> like int overflow. Ok, I'll debug it, rest assured. I already see a
>>> couple of flaws anyway.
>>
>> For this particular case,
>>
>> req->sequence = ctx->cached_sq_head + count - 1;
>>
>> ends up being 1 which triggers in __req_need_defer() for nop sq.
> 
> Right, that's it. The timeout's seq counter wraps around and triggers on
> previously submitted but still inflight requests.
> 
> Jens, could you remind, do we limit number of inflight requests? We
> discussed it before, but can't find the thread. If we don't, vile stuff
> can happen with sequences.

We don't.

-- 
Jens Axboe