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@xxxxxxxxxxxxx/ > [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?
