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Date:   Thu, 10 Sep 2020 20:28:44 -0700
From:   Martin KaFai Lau <>
To:     Neal Cardwell <>
CC:     Alexei Starovoitov <>, <>,
        Neal Cardwell <>
Subject: Re: [PATCH bpf-next v3 0/5] tcp: increase flexibility of EBPF
 congestion control initialization

On Thu, Sep 10, 2020 at 03:35:31PM -0400, Neal Cardwell wrote:
> From: Neal Cardwell <>
> This patch series reorganizes TCP congestion control initialization so that if
> EBPF code called by tcp_init_transfer() sets the congestion control algorithm
> by calling setsockopt(TCP_CONGESTION) then the TCP stack initializes the
> congestion control module immediately, instead of having tcp_init_transfer()
> later initialize the congestion control module.
> This increases flexibility for the EBPF code that runs at connection
> establishment time, and simplifies the code.
> This has the following benefits:
> (1) This allows CC module customizations made by the EBPF called in
>     tcp_init_transfer() to persist, and not be wiped out by a later
>     call to tcp_init_congestion_control() in tcp_init_transfer().
> (2) Does not flip the order of EBPF and CC init, to avoid causing bugs
>     for existing code upstream that depends on the current order.
> (3) Does not cause 2 initializations for for CC in the case where the
>     EBPF called in tcp_init_transfer() wants to set the CC to a new CC
>     algorithm.
> (4) Allows follow-on simplifications to the code in net/core/filter.c
>     and net/ipv4/tcp_cong.c, which currently both have some complexity
>     to special-case CC initialization to avoid double CC
>     initialization if EBPF sets the CC.
> changes in v2:
> o rebase onto bpf-next
> o add another follow-on simplification suggested by Martin KaFai Lau:
>    "tcp: simplify tcp_set_congestion_control() load=false case"
> changes in v3:
> o no change in commits
> o resent patch series from, since mail from
>   stopped being accepted at mid-way through processing
>   the v2 patch series (between patches 2 and 3), confusing patchwork about
>   which patches belonged to the v2 patch series
Acked-by: Martin KaFai Lau <>

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