netdev - Re: [PATCH bpf-next v2 0/5] tcp: increase flexibility of EBPF congestion control initialization

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Message-ID: <20200910181654.abyts7h5s2vznqhe@kafai-mbp>
Date:   Thu, 10 Sep 2020 11:16:54 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Neal Cardwell <ncardwell@...gle.com>
CC:     Alexei Starovoitov <ast@...nel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 0/5] tcp: increase flexibility of EBPF
 congestion control initialization

On Thu, Sep 10, 2020 at 10:04:23AM -0400, Neal Cardwell wrote:
> 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"
Acked-by: Martin KaFai Lau <kafai@...com>