Re: Odd timeout behavior

[Jens Axboe <axboe@kernel.dk>]

On 4/14/20 9:46 AM, Pavel Begunkov wrote:
> 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.

No, I mean count > INT_MAX, what you're suggesting we just don't support.
If there are apps right now using that, how do we handle it?

-- 
Jens Axboe