From: "Michael S. Tsirkin" <mst@redhat.com>
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Koichiro Den <den@klaipeden.com>,
Jason Wang <jasowang@redhat.com>,
virtualization@lists.linux-foundation.org,
Network Development <netdev@vger.kernel.org>
Subject: Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi
Date: Thu, 24 Aug 2017 23:50:53 +0300 [thread overview]
Message-ID: <20170824234551-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <CAF=yD-+9Ah8pC9i2w3Ad3WnhQit7Yo479pMrToty7priL6BFLw@mail.gmail.com>
On Thu, Aug 24, 2017 at 04:20:39PM -0400, Willem de Bruijn wrote:
> >> 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?
No - the ring size itself.
> We could use dynamic allocation or a resizable pool if these
> issues are serious enough.
We need some kind of limit on how many requests a guest can queue in the
host, or it's an obvious DoS attack vector. We used the ring size for
that.
> >> > 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.
Yes it does. Examples above:
> > - 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
it's easier to see if you enable the watchdog timer for virtio. Linux
supports that.
> There are NICs with hardware
> rate limiting, so I'm not sure how much of a leap host os rate
> limiting is.
I don't know what happens if these NICs hold onto packets for seconds.
--
MST
next prev parent reply other threads:[~2017-08-24 20:50 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-19 6:38 [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi Koichiro Den
2017-08-20 20:49 ` Willem de Bruijn
2017-08-21 12:40 ` Koichiro Den
2017-08-22 12:11 ` Willem de Bruijn
2017-08-22 14:04 ` Koichiro Den
2017-08-22 17:19 ` Willem de Bruijn
2017-08-23 14:26 ` Koichiro Den
2017-08-21 12:33 ` Jason Wang
2017-08-21 12:58 ` Koichiro Den
2017-08-21 15:41 ` Willem de Bruijn
2017-08-22 2:50 ` Jason Wang
2017-08-22 3:10 ` Willem de Bruijn
2017-08-22 11:47 ` Jason Wang
2017-08-22 13:42 ` Koichiro Den
2017-08-22 17:16 ` Willem de Bruijn
2017-08-23 14:24 ` Koichiro Den
2017-08-22 17:55 ` Michael S. Tsirkin
2017-08-22 18:01 ` David Miller
2017-08-22 18:28 ` Eric Dumazet
2017-08-22 18:39 ` Michael S. Tsirkin
2017-08-23 14:28 ` Koichiro Den
2017-08-23 14:47 ` Koichiro Den
2017-08-23 15:20 ` Willem de Bruijn
2017-08-23 22:57 ` Michael S. Tsirkin
2017-08-24 3:28 ` Willem de Bruijn
2017-08-24 4:34 ` Michael S. Tsirkin
2017-08-24 13:50 ` Michael S. Tsirkin
2017-08-24 20:20 ` Willem de Bruijn
2017-08-24 20:50 ` Michael S. Tsirkin [this message]
2017-08-25 22:44 ` Willem de Bruijn
2017-08-25 23:32 ` Michael S. Tsirkin
2017-08-26 1:03 ` Willem de Bruijn
2017-08-29 19:35 ` Willem de Bruijn
2017-08-29 19:42 ` Michael S. Tsirkin
2017-08-29 19:53 ` Willem de Bruijn
2017-08-29 20:40 ` Michael S. Tsirkin
2017-08-29 22:55 ` Willem de Bruijn
2017-08-30 1:45 ` Jason Wang
2017-08-30 3:11 ` Willem de Bruijn
2017-09-01 3:08 ` Jason Wang
2017-08-31 14:30 ` Willem de Bruijn
2017-09-01 3:25 ` Jason Wang
2017-09-01 16:15 ` Willem de Bruijn
2017-09-01 16:17 ` Willem de Bruijn
2017-09-04 3:03 ` Jason Wang
2017-09-05 14:09 ` Willem de Bruijn
2017-09-06 3:27 ` Michael S. Tsirkin
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