Re: "Cannot allocate memory" on ring creation (not RLIMIT_MEMLOCK)

[Dmitry Kadashev <dkadashev@gmail.com>]

On Mon, Dec 21, 2020 at 5:35 PM Dmitry Kadashev <dkadashev@gmail.com> wrote:
>
> On Sun, Dec 20, 2020 at 7:59 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
> >
> > On 20/12/2020 00:25, Jens Axboe wrote:
> > > On 12/19/20 4:42 PM, Pavel Begunkov wrote:
> > >> On 19/12/2020 23:13, Jens Axboe wrote:
> > >>> On 12/19/20 2:54 PM, Jens Axboe wrote:
> > >>>> On 12/19/20 1:51 PM, Josef wrote:
> > >>>>>> And even more so, it's IOSQE_ASYNC on the IORING_OP_READ on an eventfd
> > >>>>>> file descriptor. You probably don't want/mean to do that as it's
> > >>>>>> pollable, I guess it's done because you just set it on all reads for the
> > >>>>>> test?
> > >>>>>
> > >>>>> yes exactly, eventfd fd is blocking, so it actually makes no sense to
> > >>>>> use IOSQE_ASYNC
> > >>>>
> > >>>> Right, and it's pollable too.
> > >>>>
> > >>>>> I just tested eventfd without the IOSQE_ASYNC flag, it seems to work
> > >>>>> in my tests, thanks a lot :)
> > >>>>>
> > >>>>>> In any case, it should of course work. This is the leftover trace when
> > >>>>>> we should be exiting, but an io-wq worker is still trying to get data
> > >>>>>> from the eventfd:
> > >>>>>
> > >>>>> interesting, btw what kind of tool do you use for kernel debugging?
> > >>>>
> > >>>> Just poking at it and thinking about it, no hidden magic I'm afraid...
> > >>>
> > >>> Josef, can you try with this added? Looks bigger than it is, most of it
> > >>> is just moving one function below another.
> > >>
> > >> Hmm, which kernel revision are you poking? Seems it doesn't match
> > >> io_uring-5.10, and for 5.11 io_uring_cancel_files() is never called with
> > >> NULL files.
> > >>
> > >> if (!files)
> > >>      __io_uring_cancel_task_requests(ctx, task);
> > >> else
> > >>      io_uring_cancel_files(ctx, task, files);
> > >
> > > Yeah, I think I messed up. If files == NULL, then the task is going away.
> > > So we should cancel all requests that match 'task', not just ones that
> > > match task && files.
> > >
> > > Not sure I have much more time to look into this before next week, but
> > > something like that.
> > >
> > > The problem case is the async worker being queued, long before the task
> > > is killed and the contexts go away. But from exit_files(), we're only
> > > concerned with canceling if we have inflight. Doesn't look right to me.
> >
> > In theory all that should be killed in io_ring_ctx_wait_and_kill(),
> > of course that's if the ring itself is closed.
> >
> > Guys, do you share rings between processes? Explicitly like sending
> > io_uring fd over a socket, or implicitly e.g. sharing fd tables
> > (threads), or cloning with copying fd tables (and so taking a ref
> > to a ring).
>
> We do not share rings between processes. Our rings are accessible from different
> threads (under locks), but nothing fancy.
>
> > In other words, if you kill all your io_uring applications, does it
> > go back to normal?
>
> I'm pretty sure it does not, the only fix is to reboot the box. But I'll find an
> affected box and double check just in case.

So, I've just tried stopping everything that uses io-uring. No io_wq* processes
remained:

$ ps ax | grep wq
    9 ?        I<     0:00 [mm_percpu_wq]
  243 ?        I<     0:00 [tpm_dev_wq]
  246 ?        I<     0:00 [devfreq_wq]
27922 pts/4    S+     0:00 grep --colour=auto wq
$

But not a single ring (with size 1024) can be created afterwards anyway.

Apparently the problem netty hit and this one are different?

-- 
Dmitry Kadashev