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

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Date:   Sun, 24 Mar 2019 09:19:13 -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 10:36:24PM -0700, Eric Dumazet wrote:
> 
> 
> 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.

I have a feeling we're looking at different patches.
All of your comments seems to be talking about something else.
I have a hard time connecting them to this patch set.
Have you read the cover letter and patches of _this_ set?

The results are in the cover letter and there is no change in behavior in networking stack.
Patches 1, 2, 3 are simple refactoring of bpf-cgroup return codes.
Patch 4 is the only one that touches net/ipv4/ip_output.c only to pass
the return code to tcp/udp layer.
The concept works for both despite of what you're claiming being tcp only.

Cover letter also explains why bpf_skb_ecn_set_ce is not enough.
Please realize that existing qdiscs already doing this.
The patchset allows bpf-cgroup to do the same.

If you were arguing about sysctl knob in patch 5 that I would understand.
Instead of a knob we can hard code the value for now. But please explain
what issues you see with that knob.

Also to be fair I applied your
commit f11216b24219 ("bpf: add skb->tstamp r/w access from tc clsact and cg skb progs")
without demanding 'experimental results' because the feature makes sense.
Yet, you folks didn't produce a _single_ example or test result since then.
This is not cool.