Re: Odd timeout behavior

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

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.

-- 
Pavel Begunkov