Re: [PATCH net-next v6 3/4] virtio/vsock: fix logic which reduces credit update messages

[Arseniy Krasnov]

On 05.12.2023 17:21, Stefano Garzarella wrote:
> On Tue, Dec 05, 2023 at 03:07:47PM +0300, Arseniy Krasnov wrote:
>>
>>
>> On 05.12.2023 13:54, Stefano Garzarella wrote:
>>> On Tue, Dec 05, 2023 at 09:48:05AM +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).
>>>>
>>>> Fixes: b89d882dc9fc ("vsock/virtio: reduce credit update messages")
>>>> Signed-off-by: Arseniy Krasnov <avkrasnov@xxxxxxxxxxxxxxxxx>
>>>> ---
>>>> net/vmw_vsock/virtio_transport_common.c | 9 +++++++--
>>>> 1 file changed, 7 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>>> index e137d740804e..461c89882142 100644
>>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>>> @@ -558,6 +558,7 @@ 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;
>>>> +    bool low_rx_bytes;
>>>>     int err = -EFAULT;
>>>>     u32 free_space;
>>>>
>>>> @@ -602,6 +603,8 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>>>     }
>>>>
>>>>     free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
>>>> +    low_rx_bytes = (vvs->rx_bytes <
>>>> +            sock_rcvlowat(sk_vsock(vsk), 0, INT_MAX));
>>>
>>> As in the previous patch, should we avoid the update it if `fwd_cnt` and `last_fwd_cnt` are the same?
>>>
>>> Now I'm thinking if it is better to add that check directly in virtio_transport_send_credit_update().
>>
>> Good point, but I think, that it is better to keep this check here, because access to 'fwd_cnt' and 'last_fwd_cnt'
>> requires taking rx_lock - so I guess it is better to avoid taking this lock every time in 'virtio_transport_send_credit_update()'.
> 
> Yeah, I agree.
> 
>> So may be we can do something like:
>>
>>
>> fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;
>> free_space = vvs->buf_alloc - fwd_cnt_delta;
> 
> Pre-existing issue, but should we handle the wrap (e.g. fwd_cnt wrapped, but last_fwd_cnt not yet?). Maybe in that case we can foce the status
> update.

Agree, I'll add this logic!

> 
>>
>> and then, after lock is released:
>>
>> if (fwd_cnt_delta && (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE ||
>>    low_rx_bytes))
>>        virtio_transport_send_credit_update(vsk);
>>
>> WDYT?
> 
> Yep, I agree.
> 
>>
>> Also, I guess that next idea to update this optimization(in next patchset), is to make
>> threshold depends on vvs->buf_alloc. Because if someone changes minimum buffer size to
>> for example 32KB, and then sets buffer size to 32KB, then free_space will be always
>> non-zero, thus optimization is off now and credit update is sent on every read.
> 
> But does it make sense to allow a buffer smaller than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE?
> 
> Maybe we should fail in virtio_transport_notify_buffer_size() or use it as minimum.

Yes, currently there is no limitation in this transport callback - only for maximum.

Thanks, Arseniy

> 
> Stefano
>