Re: [PATCH net-next v7 3/4] virtio/vsock: fix logic which reduces credit update messages

[Arseniy Krasnov]

On 07.12.2023 01:08, Michael S. Tsirkin wrote:
> On Thu, Dec 07, 2023 at 12:52:51AM +0300, Arseniy Krasnov wrote:
>>
>>
>> On 07.12.2023 00:53, Michael S. Tsirkin wrote:
>>> On Thu, Dec 07, 2023 at 12:18:48AM +0300, Arseniy Krasnov wrote:
>>>> Add one more condition for sending credit update during dequeue from
>>>> stream socket: when number of bytes in the rx queue is smaller than
>>>> SO_RCVLOWAT value of the socket. This is actual for non-default value
>>>> of SO_RCVLOWAT (e.g. not 1) - idea is to "kick" peer to continue data
>>>> transmission, because we need at least SO_RCVLOWAT bytes in our rx
>>>> queue to wake up user for reading data (in corner case it is also
>>>> possible to stuck both tx and rx sides, this is why 'Fixes' is used).
>>>> Also handle case when 'fwd_cnt' wraps, while 'last_fwd_cnt' is still
>>>> not.
>>>>
>>>> Fixes: b89d882dc9fc ("vsock/virtio: reduce credit update messages")
>>>> Signed-off-by: Arseniy Krasnov <avkrasnov@xxxxxxxxxxxxxxxxx>
>>>> ---
>>>>  Changelog:
>>>>  v6 -> v7:
>>>>   * Handle wrap of 'fwd_cnt'.
>>>>   * Do to send credit update when 'fwd_cnt' == 'last_fwd_cnt'.
>>>>
>>>>  net/vmw_vsock/virtio_transport_common.c | 18 +++++++++++++++---
>>>>  1 file changed, 15 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>>> index e137d740804e..39f8660d825d 100644
>>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>>> @@ -558,6 +558,8 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>>>  	struct virtio_vsock_sock *vvs = vsk->trans;
>>>>  	size_t bytes, total = 0;
>>>>  	struct sk_buff *skb;
>>>> +	u32 fwd_cnt_delta;
>>>> +	bool low_rx_bytes;
>>>>  	int err = -EFAULT;
>>>>  	u32 free_space;
>>>>  
>>>> @@ -601,7 +603,15 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>>>  		}
>>>>  	}
>>>>  
>>>> -	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
>>>> +	/* Handle wrap of 'fwd_cnt'. */
>>>> +	if (vvs->fwd_cnt < vvs->last_fwd_cnt)
>>>> +		fwd_cnt_delta = vvs->fwd_cnt + (U32_MAX - vvs->last_fwd_cnt);
>>>
>>> Are you sure there's no off by one here? for example if fwd_cnt is 0
>>> and last_fwd_cnt is 0xfffffffff then apparently delta is 0.
>>
>> Seems yes, I need +1 here
> 
> And then you will get a nop, because assigning U32_MAX + 1 to u32
> gives you 0. Adding () does nothing to change the result,
> + and - are commutative.

Ahh, unsigned here, yes.

@Stefano, what did You mean about wrapping here?

I think Michael is right, for example

vvs->fwd_cnt wraps and now == 5
vvs->last_fwd_cnt == 0xffffffff

now delta before this patch will be 6 - correct value

May be I didn't get your idea, so implement it very naive?

Thanks, Arseniy

> 
> 
>>>
>>>
>>>> +	else
>>>> +		fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;
>>>
>>> I actually don't see what is wrong with just
>>> 	fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt
>>> 32 bit unsigned math will I think handle wrap around correctly.
>>>
>>> And given buf_alloc is also u32 - I don't see where the bug is in
>>> the original code.
>>
>> I think problem is when fwd_cnt wraps, while last_fwd_cnt is not. In this
>> case fwd_cnt_delta will be too big, so we won't send credit update which
>> leads to stall for sender
>>
>> Thanks, Arseniy
> 
> Care coming up with an example?
> 
> 
>>>
>>>
>>>> +
>>>> +	free_space = vvs->buf_alloc - fwd_cnt_delta;
>>>> +	low_rx_bytes = (vvs->rx_bytes <
>>>> +			sock_rcvlowat(sk_vsock(vsk), 0, INT_MAX));
>>>>  
>>>>  	spin_unlock_bh(&vvs->rx_lock);
>>>>  
>>>> @@ -611,9 +621,11 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>>>  	 * too high causes extra messages. Too low causes transmitter
>>>>  	 * stalls. As stalls are in theory more expensive than extra
>>>>  	 * messages, we set the limit to a high value. TODO: experiment
>>>> -	 * with different values.
>>>> +	 * with different values. Also send credit update message when
>>>> +	 * number of bytes in rx queue is not enough to wake up reader.
>>>>  	 */
>>>> -	if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE)
>>>> +	if (fwd_cnt_delta &&
>>>> +	    (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE || low_rx_bytes))
>>>>  		virtio_transport_send_credit_update(vsk);
>>>>  
>>>>  	return total;
>>>> -- 
>>>> 2.25.1
>>>
>