netdev - Re: [PATCH v4 net-next 1/8] net: Add skb->mono_delivery_time to distinguish mono delivery

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Date:   Sat, 12 Feb 2022 11:13:53 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>, kernel-team@...com,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v4 net-next 1/8] net: Add skb->mono_delivery_time to
 distinguish mono delivery_time from (rcv) timestamp

On Thu, Feb 10, 2022 at 11:12:38PM -0800, Martin KaFai Lau wrote:
> skb->tstamp was first used as the (rcv) timestamp.
> The major usage is to report it to the user (e.g. SO_TIMESTAMP).
> 
> Later, skb->tstamp is also set as the (future) delivery_time (e.g. EDT in TCP)
> during egress and used by the qdisc (e.g. sch_fq) to make decision on when
> the skb can be passed to the dev.
> 
> Currently, there is no way to tell skb->tstamp having the (rcv) timestamp
> or the delivery_time, so it is always reset to 0 whenever forwarded
> between egress and ingress.
> 
> While it makes sense to always clear the (rcv) timestamp in skb->tstamp
> to avoid confusing sch_fq that expects the delivery_time, it is a
> performance issue [0] to clear the delivery_time if the skb finally
> egress to a fq@...-dev.  For example, when forwarding from egress to
> ingress and then finally back to egress:
> 
>             tcp-sender => veth@...ns => veth@...tns => fq@...0@...tns
>                                      ^              ^
>                                      reset          rest
> 
> This patch adds one bit skb->mono_delivery_time to flag the skb->tstamp
> is storing the mono delivery_time (EDT) instead of the (rcv) timestamp.
> 
> The current use case is to keep the TCP mono delivery_time (EDT) and
> to be used with sch_fq.  A later patch will also allow tc-bpf@...ress
> to read and change the mono delivery_time.

Can you be more specific? How is the fq in the hostns even visible to
container ns? More importantly, why the packets are always going out from
container to eth0?

>From the sender's point of view, it can't see the hostns and can't event
know whether the packets are routed to eth0 or other containers on the
same host. So I don't see how this makes sense.

Crossing netns is pretty much like delivering on wire, *generally speaking*
if the skb meta data is not preserved on wire, it probably should not for
crossing netns either.

Thanks.