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

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Date:   Sat, 23 Mar 2019 22:36:24 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        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 03/23/2019 08:41 AM, Alexei Starovoitov wrote:
> 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.
> 

This has nothing to do with Google against Facebook really, it is a bit sad you react like this Alexei.

We just upstreamed bpf_skb_ecn_set_ce(), so I doubt you already have numbers to show that this strategy
is not enough.

All recent discussions about ECN (TCP Prague and SCE) do not _require_ instant feedback to the sender.

Please show us experimental results before we have to carry these huge hacks.

Thank you.