Re: Odd timeout behavior

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

On 14/04/2020 03:44, Jens Axboe wrote:
> On 4/13/20 1:09 PM, Pavel Begunkov wrote:
>> On 13/04/2020 17:16, Jens Axboe wrote:
>>> 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.
>>
>> I was too quick to judge, there won't be anything too bad, and only if we throw
>> 2^32 requests (~1TB).
>>
>> For the issue at hand, how about limiting timeouts' sqe->off by 2^31? This will
>> solve the issue for now, and I can't imagine anyone waiting for over one billion
>> requests to pass.
> 
> I'm fine with that, but how do we handle someone asking for > INT_MAX?

>INT_MAX is allowed, but I want to return -EINVAL instead.
If you mean UINT_MAX, then sqe->off is u32, so can't happen.


-- 
Pavel Begunkov