netdev - Re: [PATCH v6 net-next 0/13] Preserve mono delivery time (EDT) in skb->tstamp

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Date:   Wed, 2 Mar 2022 16:36:28 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        kernel-team <kernel-team@...com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v6 net-next 0/13] Preserve mono delivery time (EDT) in skb->tstamp

On Wed, Mar 2, 2022 at 11:55 AM Martin KaFai Lau <kafai@...com> 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.
>
> This set is to keep the mono delivery time and make it available to
> the final egress interface.  Please see individual patch for
> the details.
>
> [0] (slide 22): https://linuxplumbersconf.org/event/11/contributions/953/attachments/867/1658/LPC_2021_BPF_Datapath_Extensions.pdf
>

For the whole series

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

Thanks !