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

[Pavel Begunkov <asml.silence@gmail.com>]

On 21/12/2020 11:00, Dmitry Kadashev wrote:
> 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?

Yep, looks like it

-- 
Pavel Begunkov