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

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Date:   Mon, 22 Aug 2022 09:17:37 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Peilin Ye <yepeilin.cs@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Peilin Ye <peilin.ye@...edance.com>, netdev@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Cong Wang <cong.wang@...edance.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Dave Taht <dave.taht@...il.com>
Subject: Re: [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?