Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

>> Traffic shaping can introduce msec timescale latencies.
>>
>> The delay may actually be a useful signal. If the guest does not
>> orphan skbs early, TSQ will throttle the socket causing host
>> queue build up.
>>
>> But, if completions are queued in-order, unrelated flows may be
>> throttled as well. Allowing out of order completions would resolve
>> this HoL blocking.
>
> We can allow out of order, no guests that follow virtio spec
> will break. But this won't help in all cases
> - a single slow flow can occupy the whole ring, you will not
>   be able to make any new buffers available for the fast flow
> - what host considers a single flow can be multiple flows for guest
>
> There are many other examples.

These examples are due to exhaustion of the fixed ubuf_info pool,
right? We could use dynamic allocation or a resizable pool if these
issues are serious enough.

>> > Neither
>> > do I see why would using tx interrupts within guest be a work around -
>> > AFAIK windows driver uses tx interrupts.
>>
>> It does not address completion latency itself. What I meant was
>> that in an interrupt-driver model, additional starvation issues,
>> such as the potential deadlock raised at the start of this thread,
>> or the timer delay observed before packets were orphaned in
>> virtio-net in commit b0c39dbdc204, are mitigated.
>>
>> Specifically, it breaks the potential deadlock where sockets are
>> blocked waiting for completions (to free up budget in sndbuf, tsq, ..),
>> yet completion handling is blocked waiting for a new packet to
>> trigger free_old_xmit_skbs from start_xmit.
>
> This talk of potential deadlock confuses me - I think you mean we would
> deadlock if we did not orphan skbs in !use_napi - is that right?  If you
> mean that you can drop skb orphan and this won't lead to a deadlock if
> free skbs upon a tx interrupt, I agree, for sure.

Yes, that is what I meant.

>> >> That is the only thing keeping us from removing the HoL blocking in vhost-net zerocopy.
>> >
>> > We don't enable network watchdog on virtio but we could and maybe
>> > should.
>>
>> Can you elaborate?
>
> The issue is that holding onto buffers for very long times makes guests
> think they are stuck. This is funamentally because from guest point of
> view this is a NIC, so it is supposed to transmit things out in
> a timely manner. If host backs the virtual NIC by something that is not
> a NIC, with traffic shaping etc introducing unbounded latencies,
> guest will be confused.

That assumes that guests are fragile in this regard. A linux guest
does not make such assumptions. There are NICs with hardware
rate limiting, so I'm not sure how much of a leap host os rate
limiting is.