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
