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SubjectRe: [PATCH RFC v2 net-next 0/5] net: Qdisc backpressure infrastructure
On Mon, 22 Aug 2022 02:10:17 -0700 Peilin Ye wrote:
> 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@bytedance.com/
> [2] https://lore.kernel.org/netdev/20220506133111.1d4bebf3@hermes.local/

Similarly to Eric's comments on v1 I'm not seeing the clear motivation
here. Modern high speed UDP users will have a CC in user space, back
off and set transmission time on the packets. Could you describe your
_actual_ use case / application in more detail?

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