On 20.12.2022 11:33, Stefano Garzarella wrote:
> On Tue, Dec 20, 2022 at 07:14:27AM +0000, Arseniy Krasnov wrote:
>> On 19.12.2022 18:41, Stefano Garzarella wrote:
>>
>> Hello!
>>
>>> Hi Arseniy,
>>>
>>> On Sat, Dec 17, 2022 at 8:42 PM Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx> wrote:
>>>>
>>>> Hello,
>>>>
>>>> seems I found strange thing(may be a bug) where sender('tx' later) and
>>>> receiver('rx' later) could stuck forever. Potential fix is in the first
>>>> patch, second patch contains reproducer, based on vsock test suite.
>>>> Reproducer is simple: tx just sends data to rx by 'write() syscall, rx
>>>> dequeues it using 'read()' syscall and uses 'poll()' for waiting. I run
>>>> server in host and client in guest.
>>>>
>>>> rx side params:
>>>> 1) SO_VM_SOCKETS_BUFFER_SIZE is 256Kb(e.g. default).
>>>> 2) SO_RCVLOWAT is 128Kb.
>>>>
>>>> What happens in the reproducer step by step:
>>>>
>>>
>>> I put the values of the variables involved to facilitate understanding:
>>>
>>> RX: buf_alloc = 256 KB; fwd_cnt = 0; last_fwd_cnt = 0;
>>> free_space = buf_alloc - (fwd_cnt - last_fwd_cnt) = 256 KB
>>>
>>> The credit update is sent if
>>> free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE [64 KB]
>>>
>>>> 1) tx tries to send 256Kb + 1 byte (in a single 'write()')
>>>> 2) tx sends 256Kb, data reaches rx (rx_bytes == 256Kb)
>>>> 3) tx waits for space in 'write()' to send last 1 byte
>>>> 4) rx does poll(), (rx_bytes >= rcvlowat) 256Kb >= 128Kb, POLLIN is set
>>>> 5) rx reads 64Kb, credit update is not sent due to *
>>>
>>> RX: buf_alloc = 256 KB; fwd_cnt = 64 KB; last_fwd_cnt = 0;
>>> free_space = 192 KB
>>>
>>>> 6) rx does poll(), (rx_bytes >= rcvlowat) 192Kb >= 128Kb, POLLIN is set
>>>> 7) rx reads 64Kb, credit update is not sent due to *
>>>
>>> RX: buf_alloc = 256 KB; fwd_cnt = 128 KB; last_fwd_cnt = 0;
>>> free_space = 128 KB
>>>
>>>> 8) rx does poll(), (rx_bytes >= rcvlowat) 128Kb >= 128Kb, POLLIN is set
>>>> 9) rx reads 64Kb, credit update is not sent due to *
>>>
>>> Right, (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE) is still false.
>>>
>>> RX: buf_alloc = 256 KB; fwd_cnt = 196 KB; last_fwd_cnt = 0;
>>> free_space = 64 KB
>>>
>>>> 10) rx does poll(), (rx_bytes < rcvlowat) 64Kb < 128Kb, rx waits in poll()
>>>
>>> I agree that the TX is stuck because we are not sending the credit
>>> update, but also if RX sends the credit update at step 9, RX won't be
>>> woken up at step 10, right?
>>
>> Yes, RX will sleep, but TX will wake up and as we inform TX how much
>> free space we have, now there are two cases for TX:
>> 1) send "small" rest of data(e.g. without blocking again), leave 'write()'
>> and continue execution. RX still waits in 'poll()'. Later TX will
>> send enough data to wake up RX.
>> 2) send "big" rest of data - if rest is too big to leave 'write()' and TX
>> will wait again for the free space - it will be able to send enough data
>> to wake up RX as we compared 'rx_bytes' with rcvlowat value in RX.
>
> Right, so I'd update the test to behave like this.
Sorry, You mean vsock_test? To cover TX waiting for free space at RX, thus checking
this kernel patch logic?
> And I'd explain better the problem we are going to fix in the commit message.
Ok
>
>>>
>>>>
>>>> * is optimization in 'virtio_transport_stream_do_dequeue()' which
>>>> sends OP_CREDIT_UPDATE only when we have not too much space -
>>>> less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE.
>>>>
>>>> Now tx side waits for space inside write() and rx waits in poll() for
>>>> 'rx_bytes' to reach SO_RCVLOWAT value. Both sides will wait forever. I
>>>> think, possible fix is to send credit update not only when we have too
>>>> small space, but also when number of bytes in receive queue is smaller
>>>> than SO_RCVLOWAT thus not enough to wake up sleeping reader. I'm not
>>>> sure about correctness of this idea, but anyway - I think that problem
>>>> above exists. What do You think?
>>>
>>> I'm not sure, I have to think more about it, but if RX reads less than
>>> SO_RCVLOWAT, I expect it's normal to get to a case of stuck.
>>>
>>> In this case we are only unstucking TX, but even if it sends that single
>>> byte, RX is still stuck and not consuming it, so it was useless to wake
>>> up TX if RX won't consume it anyway, right?
>>
>> 1) I think it is not useless, because we inform(not just wake up) TX that
>> there is free space at RX side - as i mentioned above.
>> 2) Anyway i think that this situation is a little bit strange: TX thinks that
>> there is no free space at RX and waits for it, but there is free space at RX!
>> At the same time, RX waits in poll() forever - it is ready to get new portion
>> of data to return POLLIN, but TX "thinks" exactly opposite thing - RX is full
>> of data. Of course, if there will be just stalls in TX data handling - it will
>> be ok - just performance degradation, but TX stucks forever.
>
> We did it to avoid a lot of credit update messages.
Yes, i see
> Anyway I think here the main point is why RX is setting SO_RCVLOWAT to 128 KB and then reads only half of it?
>
> So I think if the users set SO_RCVLOWAT to a value and then RX reads less then it, is expected to get stuck.
That a really interesting question, I've found nothing about this case in Google(not sure for 100%) or POSIX. But,
i can modify reproducer: it sets SO_RCVLOWAT to 128Kb BEFORE entering its last poll where it will stuck. In this
case behaviour looks more legal: it uses default SO_RCVLOWAT of 1, read 64Kb each time. Finally it sets SO_RCVLOWAT
to 128Kb(and imagine that it prepares 128Kb 'read()' buffer) and enters poll() - we will get same effect: TX will wait
for space, RX waits in 'poll()'.
>
> Anyway, since the change will not impact the default behaviour (SO_RCVLOWAT = 1) we can merge this patch, but IMHO we need to explain the case better and improve the test.
I see, of course I'm not sure about this change, just want to ask someone who knows this code better
>
>>
>>>
>>> If RX woke up (e.g. SO_RCVLOWAT = 64KB) and read the remaining 64KB,
>>> then it would still send the credit update even without this patch and
>>> TX will send the 1 byte.
>>
>> But how RX will wake up in this case? E.g. it calls poll() without timeout,
>> connection is established, RX ignores signal
>
> RX will wake up because SO_RCVLOWAT is 64KB and there are 64 KB in the buffer. Then RX will read it and send the credit update to TX because
> free_space is 0.
IIUC, i'm talking about 10 steps above, e.g. RX will never wake up, because TX is waiting for space.
>
> Thanks,
> Stefano
>
Thanks, Arseniy
