netdev - Re: [PATCH bpf-next 0/7] bpf: Propagate cn to TCP

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Message-ID: <20190323154124.gorqpaqex7ihfs6d@ast-mbp>
Date:   Sat, 23 Mar 2019 08:41:26 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     brakmo <brakmo@...com>, netdev <netdev@...r.kernel.org>,
        Martin Lau <kafai@...com>, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 0/7] bpf: Propagate cn to TCP

On Sat, Mar 23, 2019 at 02:12:39AM -0700, Eric Dumazet wrote:
> 
> 
> On 03/23/2019 01:05 AM, brakmo wrote:
> > This patchset adds support for propagating congestion notifications (cn)
> > to TCP from cgroup inet skb egress BPF programs.
> > 
> > Current cgroup skb BPF programs cannot trigger TCP congestion window
> > reductions, even when they drop a packet. This patch-set adds support
> > for cgroup skb BPF programs to send congestion notifications in the
> > return value when the packets are TCP packets. Rather than the
> > current 1 for keeping the packet and 0 for dropping it, they can
> > now return:
> >     NET_XMIT_SUCCESS    (0)    - continue with packet output
> >     NET_XMIT_DROP       (1)    - drop packet and do cn
> >     NET_XMIT_CN         (2)    - continue with packet output and do cn
> >     -EPERM                     - drop packet
> >
> 
> I believe I already mentioned this model is broken, if you have any virtual
> device before the cgroup BPF program.
> 
> Please think about offloading the pacing/throttling in the NIC,
> there is no way we will report back to tcp stack instant notifications.

I don't think 'offload to google proprietary nic' is a suggestion
that folks can practically follow.
Very few NICs can offload pacing to hw and there are plenty of limitations.
This patch set represents a pure sw solution that works and scales to millions of flows.

> This patch series is going way too far for my taste.

I would really appreciate if you can do a technical review of the patches.
Our previous approach didn't quite work due to complexity around locked/non-locked socket.
This is a cleaner approach.
Either we go with this one or will add a bpf hook into __tcp_transmit_skb.
This approach is better since it works for other protocols and can be
used by qdiscs w/o any bpf.

> This idea is not new, you were at Google when it was experimented by Nandita and
> others, and we know it is not worth the pain.

google networking needs are different from the rest of the world.

Thank you.