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. -- Pavel Begunkov
