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@xxxxxxxxx> 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? -- Jens Axboe
