Re: [PATCH RFC v2 net-next 0/5] net: Qdisc backpressure infrastructure

[Eric Dumazet]

On Mon, Aug 29, 2022 at 9:47 AM Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>
> On Mon, Aug 22, 2022 at 09:22:39AM -0700, Eric Dumazet wrote:
> > On Mon, Aug 22, 2022 at 2:10 AM Peilin Ye <yepeilin.cs@xxxxxxxxx> wrote:
> > >
> > > From: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>
> > >
> > > Hi all,
> > >
> > > Currently sockets (especially UDP ones) can drop a lot of packets at TC
> > > egress when rate limited by shaper Qdiscs like HTB.  This patchset series
> > > tries to solve this by introducing a Qdisc backpressure mechanism.
> > >
> > > RFC v1 [1] used a throttle & unthrottle approach, which introduced several
> > > issues, including a thundering herd problem and a socket reference count
> > > issue [2].  This RFC v2 uses a different approach to avoid those issues:
> > >
> > >   1. When a shaper Qdisc drops a packet that belongs to a local socket due
> > >      to TC egress congestion, we make part of the socket's sndbuf
> > >      temporarily unavailable, so it sends slower.
> > >
> > >   2. Later, when TC egress becomes idle again, we gradually recover the
> > >      socket's sndbuf back to normal.  Patch 2 implements this step using a
> > >      timer for UDP sockets.
> > >
> > > The thundering herd problem is avoided, since we no longer wake up all
> > > throttled sockets at the same time in qdisc_watchdog().  The socket
> > > reference count issue is also avoided, since we no longer maintain socket
> > > list on Qdisc.
> > >
> > > Performance is better than RFC v1.  There is one concern about fairness
> > > between flows for TBF Qdisc, which could be solved by using a SFQ inner
> > > Qdisc.
> > >
> > > Please see the individual patches for details and numbers.  Any comments,
> > > suggestions would be much appreciated.  Thanks!
> > >
> > > [1] https://lore.kernel.org/netdev/cover.1651800598.git.peilin.ye@xxxxxxxxxxxxx/
> > > [2] https://lore.kernel.org/netdev/20220506133111.1d4bebf3@hermes.local/
> > >
> > > Peilin Ye (5):
> > >   net: Introduce Qdisc backpressure infrastructure
> > >   net/udp: Implement Qdisc backpressure algorithm
> > >   net/sched: sch_tbf: Use Qdisc backpressure infrastructure
> > >   net/sched: sch_htb: Use Qdisc backpressure infrastructure
> > >   net/sched: sch_cbq: Use Qdisc backpressure infrastructure
> > >
> >
> > I think the whole idea is wrong.
> >
>
> Be more specific?
>
> > Packet schedulers can be remote (offloaded, or on another box)
>
> This is not the case we are dealing with (yet).
>
> >
> > The idea of going back to socket level from a packet scheduler should
> > really be a last resort.
>
> I think it should be the first resort, as we should backpressure to the
> source, rather than anything in the middle.
>
> >
> > Issue of having UDP sockets being able to flood a network is tough, I
> > am not sure the core networking stack
> > should pretend it can solve the issue.
>
> It seems you misunderstand it here, we are not dealing with UDP on the
> network, just on an end host. The backpressure we are dealing with is
> from Qdisc to socket on _TX side_ and on one single host.
>
> >
> > Note that FQ based packet schedulers can also help already.
>
> It only helps TCP pacing.

FQ : Fair Queue.

It definitely helps without the pacing part...

>
> Thanks.