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
>
>
>> + 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
>
>
>> +
>> + 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
>
