On 4/12/20 3:15 AM, Pavel Begunkov 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). 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. Yeah agree it's two separate issues, the -1U must be a simple overflow. So probably not that tricky to fix. Reason I bring up the other part is that Hrvoje's test case had other cases as well, and the SQ vs CQ trigger is worth looking into. For example, if we do: enqueue N polls enqueue timeout, count == 2, t = 10s enqueue 2 nops I'd logically expect the timeout to trigger when nop #2 is completed. But it won't be, because we still have N polls waiting. What the count == 2 is really saying (right now) is "trigger timeout when CQ passes SQ by 2", which seems a bit odd. -- Jens Axboe
