netdev - Re: [PATCH net-next 0/2] tcp: add sysctl_tcp_rto_min

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Message-ID: <CANn89iKb4nWKvByBwGFveLb5KL_F_Eh_7gPpJ-3fPkfQF7Zf0g@mail.gmail.com>
Date: Wed, 29 May 2024 09:39:02 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: Kevin Yang <yyd@...gle.com>, Paolo Abeni <pabeni@...hat.com>, 
	David Miller <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 0/2] tcp: add sysctl_tcp_rto_min_us

On Wed, May 29, 2024 at 9:00 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Wed, May 29, 2024 at 2:43 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > Hello Kevin,
> >
> > On Wed, May 29, 2024 at 1:13 AM Kevin Yang <yyd@...gle.com> wrote:
> > >
> > > Adding a sysctl knob to allow user to specify a default
> > > rto_min at socket init time.
> >
> > I wonder what the advantage of this new sysctl knob is since we have
> > had BPF or something like that to tweak the rto min already?
> >
> > There are so many places/parameters of the TCP stack that can be
> > exposed to the user side and adjusted by new sysctls...
> >
> > Thanks,
> > Jason
> >
> > >
> > > After this patch series, the rto_min will has multiple sources:
> > > route option has the highest precedence, followed by the
> > > TCP_BPF_RTO_MIN socket option, followed by this new
> > > tcp_rto_min_us sysctl.
> > >
> > > Kevin Yang (2):
> > >   tcp: derive delack_max with tcp_rto_min helper
> > >   tcp: add sysctl_tcp_rto_min_us
> > >
> > >  Documentation/networking/ip-sysctl.rst | 13 +++++++++++++
> > >  include/net/netns/ipv4.h               |  1 +
> > >  net/ipv4/sysctl_net_ipv4.c             |  8 ++++++++
> > >  net/ipv4/tcp.c                         |  3 ++-
> > >  net/ipv4/tcp_ipv4.c                    |  1 +
> > >  net/ipv4/tcp_output.c                  | 11 ++---------
> > >  6 files changed, 27 insertions(+), 10 deletions(-)
> > >
> > > --
> > > 2.45.1.288.g0e0cd299f1-goog
> > >
> > >
>
> Oh, I think you should have added Paolo as well.
>
> +Paolo Abeni

Many cloud customers do not have any BPF expertise.
If they use existing BPF programs (added by a product), they might not
have the ability to change it.

We tried advising them to use route attributes, after
commit bbf80d713fe75cfbecda26e7c03a9a8d22af2f4f ("tcp: derive
delack_max from rto_min")

Alas, dhcpd was adding its own routes, without the "rto_min 5"
attribute, then systemd came...
Lots of frustration, lots of wasted time, for something that has been
used for more than a decade
in Google DC.

With a sysctl, we could have saved months of SWE, and helped our
customers sooner.

Reviewed-by: Eric Dumazet <edumazet@...gle.com>