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

["Jiang Wang ." <jiang.wang@bytedance.com>]

On Mon, May 17, 2021 at 4:02 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
>
> 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.

I considered this approach before. One question not clear to me is
whether we should call virtqueue_kick for every dgram packet. If I
am not wrong, each virtqueue_kick will lead to a vm_exit to the host.
If we call virtqueue_kick() for each dgram packet, the performance
might be bad when there are lots of packets. One idea is to set
a threshold and a timer to call virtqueue_kick(). When there are
lots of packets, we wait until a threshold. When there are few packets,
the timer will make sure those packets will be delivered not too late.

Any other ideas for virtqueue_kick? If the threshold plus timer is fine,
I can go this direction too.

> >
> >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?

OOM depending on the implementation details. If we keep
dgram_send_pkt_list, and put no limitation,  it may increase indefinitely
and cause OOM. As long as we have some limitations or drop packets
quickly, then we should be fine.

> >
> >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?
>
In my view, we don't need to exchange that information. Buf_alloc for
dgram is local to a receiving socket. The sending sockets do not know
about that. For receiving socket, it just checks if there is still buffer
available to decide to drop a packet or not. If multiple transmitter
send to a single socket, they will share the same buf_alloc, and packets
will be dropped randomly for those transmitters.

> 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.

OK.

> Maybe someday we should convert stream to skbuff too, it would make our
> lives easier.
>
Yeah.  I remember the original virtio vsock patch did use skb, but somehow
it  did not use skb anymore when merging to the upstream, not sure why.

Thanks,

Jiang

> Thanks,
> Stefano
>
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