Re: Odd timeout behavior

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

On 14/04/2020 19:04, Jens Axboe wrote:
> 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.

Got it. That's what my question was about.

> If there are apps right now using that, how do we handle it?

1. if # of inflight requests is limited (empirically/naturally or not), then we
can extend req->seq to >= 33 bits (leaving sqe->off u32), and it'll just work.

e.g. if req->seq is u64, then we need 2^64 - 2^32 inflight requests to hit the
issue. And I don't expect anybody creating requests worth of 1ZB (2^30 TB).

2. or to think about something

I'll send 1., and then out of curiosity give a thought to 2.

-- 
Pavel Begunkov