netdev - Re: [PATCH net-next] tcp: tracking packets with CE marks in BW rate sample

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Date:   Fri, 24 Sep 2021 16:30:28 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Luke Hsiao <luke.w.hsiao@...il.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Lawrence Brakmo <brakmo@...com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Luke Hsiao <lukehsiao@...gle.com>
Subject: Re: [PATCH net-next] tcp: tracking packets with CE marks in BW rate sample

On Fri, Sep 24, 2021 at 1:20 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 23 Sep 2021 21:17:07 +0000 Luke Hsiao wrote:
> > From: Yuchung Cheng <ycheng@...gle.com>
> >
> > In order to track CE marks per rate sample (one round trip), TCP needs a
> > per-skb header field to record the tp->delivered_ce count when the skb
> > was sent. To make space, we replace the "last_in_flight" field which is
> > used exclusively for NV congestion control. The stat needed by NV can be
> > alternatively approximated by existing stats tcp_sock delivered and
> > mss_cache.
> >
> > This patch counts the number of packets delivered which have CE marks in
> > the rate sample, using similar approach of delivery accounting.
>
> Is this expected to be used from BPF CC? I don't see a user..
Great question. Yes the commit message could be more clear that this
intends for both ebpf-CC or other third party module that use ECN. For
example bbr2 uses it heavily (bbr2 upstream WIP). This feature is
useful for congestion control research which many use ECN as core
signals now.