Re: Odd timeout behavior

[Hrvoje Zeba <zeba.hrvoje@gmail.com>]

On Sun, Apr 12, 2020 at 5:15 AM Pavel Begunkov <asml.silence@gmail.com> 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.

-- 
I doubt, therefore I might be.