Re: [RFC v2] virtio-vsock: add description for datagram type

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, May 14, 2021 at 11:55:29AM -0700, Jiang Wang . wrote:
>On Fri, May 14, 2021 at 8:17 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
>> On Thu, May 13, 2021 at 04:26:03PM -0700, Jiang Wang . wrote:

[...]

>> >I see. I will add some limit to dgram packets. Also, when the 
>> >virtqueues
>> >are shared between stream and dgram, both of them need to grab a lock
>> >before using the virtqueue, so one will not completely block another one.
>>
>> I'm not worried about the concurrent access that we definitely need to
>> handle with a lock, but more about the uncontrolled packet sending that
>> dgram might have, flooding the queues and preventing others from
>> communicating.
>
>That is a valid concern. Let me explain how I would handle that if we
>don't add two new virtqueues. For dgram, I also add a dgram_send_pkt_list,
>which is similar to send_pkt_list for stream (and seqpacket). But there
>is one difference. The dgram_send_pkt_list has a maximum size setting,
>and keep tracking how many pkts are in the list. The track number
>(dgram_send_pkt_list_size) is  increased when a packet is added
>to the list and is decreased when a packet
>is removed from the list and added to the virtqueue. In
>virtio_transport_send_pkt, if the current
>dgram_send_pkt_list_size is equal
>to the maximum ( let's say 128), then it will not add to the
>dgram_send_pkt_list and return an error to the application.

For stream socket, we have the send_pkt_list and the send worker because 
the virtqueue can be full and the transmitter needs to wait available 
slots, because we can't discard packets.

For dgram I think we don't need this, so we can avoid the 
dgram_send_pkt_list and directly enqueue packets in the virtqueue.

If there are no slots available, we can simply drop the packet.
In this way we don't have to put limits other than the available space 
in the virtqueue.

>
>In this way, the number of pending dgram pkts to be sent is limited.
>Then both stream and dgram sockets will compete to hold a lock
>for the tx virtqueue. Depending on the linux scheduler, this competition
>will be somewhat fair. As a result, dgram will not block stream completely.
>It will compete and share the virtqueue with stream, but stream
>can still send some pkts.
>
>Basically, the virtqueue becomes a first-come first-serve resource for
>the stream and dgram. When both stream and dgram applications
>have lots of data to send, dgram_send_pkt_list and send_pkt_list
>will still be a limited size, and each will have some chance to send out
>the data via virtqueue.  Does this address your concern?
>
>
>> So having 2 dedicated queues could avoid a credit mechanism at all for
>> connection-less sockets, and simply the receiver discards packets that
>> it can't handle.
>
>With 2 dedicated queues, we still need some kind of accounting for
>the dgram. Like the dgram_send_pkt_size I mentioned above.  Otherwise,
>it will cause OOM. It is not a real credit mechanism, but code is similar
>with or without 2 dedicated queues in my current prototype.

I think in both cases we don't need an accounting, but we can drop 
packets if the virtqueue is full.

It's still not clear to me where OOM may occur. Can you elaborate on 
this?

>
>For receiver discarding packets part, could you explain more? I think
>receiver discarding pkts code also is same with or without 2 dedicated
>queues.

Yep.

> Both will use can_enqueue = virtio_transport_inc_rx_pkt(vvs, pkt);
>to check if a packet should be discarded or not.

I don't think we can use virtio_transport_inc_rx_pkt() for dgram.
This is based on the credit mechanism (e.g. buf_alloc) that works when 
you have a connection oriented socket.

With dgram you can have multiple transmitter for a single socket, so how 
we can exchange that information?

If we reuse the VMCI implementation with skbuff, the sk_receive_skb() 
already checks if the sk_rcvbuf is full and discards the packet in this 
case, so we don't need an internal rx_queue.

Maybe someday we should convert stream to skbuff too, it would make our 
lives easier.

Thanks,
Stefano

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