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

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Message-ID: <20210924164242.1095c674@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Fri, 24 Sep 2021 16:42:42 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yuchung Cheng <ycheng@...gle.com>
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, 24 Sep 2021 16:30:28 -0700 Yuchung Cheng wrote:
> 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.

Interesting, thanks for explaining! :)