Re: [PATCH net-next v8 3/4] virtio/vsock: fix logic which reduces credit update messages

[Arseniy Krasnov]

On 12.12.2023 19:11, Michael S. Tsirkin wrote:
> On Tue, Dec 12, 2023 at 06:50:39PM +0300, Arseniy Krasnov wrote:
>>
>>
>> On 12.12.2023 18:54, Michael S. Tsirkin wrote:
>>> On Tue, Dec 12, 2023 at 12:16:57AM +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).
>>>
>>> I don't get what does "to stuck both tx and rx sides" mean.
>>
>> I meant situation when tx waits for the free space, while rx doesn't send
>> credit update, just waiting for more data. Sorry for my English :)
>>
>>> Besides being agrammatical, is there a way to do this without
>>> playing with SO_RCVLOWAT?
>>
>> No, this may happen only with non-default SO_RCVLOWAT values (e.g. != 1)
>>
>> Thanks, Arseniy 
> 
> I am split on whether we need the Fixes tag. I guess if the other side
> is vhost with SO_RCVLOWAT then it might be stuck and it might apply
> without SO_RCVLOWAT on the local kernel?

IIUC your question, then this problem is actual for any transports: g2h, h2g and
loopback.

Thanks, Arseniy

> 
> 
>>>
>>>>
>>>> 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'.
>>>>  v7 -> v8:
>>>>   * Remove unneeded/wrong handling of wrap for 'fwd_cnt'.
>>>>
>>>>  net/vmw_vsock/virtio_transport_common.c | 13 ++++++++++---
>>>>  1 file changed, 10 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>>> index e137d740804e..8572f94bba88 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 +603,10 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>>>  		}
>>>>  	}
>>>>  
>>>> -	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
>>>> +	fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;
>>>> +	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 +616,11 @@ 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
>>>
>