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. -- Jens Axboe
