Re: Odd timeout behavior

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

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). Currently, it should work more
like "fire after N events *submitted after the timeout* completed", so
SQ vs CQ is another topic, but IMHO is not related.

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.

BTW, I don't see why this offset feature is there in the first place. It
can be easily done in the userspace on CQ reaping. It won't help
multi-threaded apps as well.

-- 
Pavel Begunkov