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

[Eric Dumazet]

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.

Packet schedulers can be remote (offloaded, or on another box)

The idea of going back to socket level from a packet scheduler should
really be a last resort.

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.

Note that FQ based packet schedulers can also help already.