Re: [RFC PATCH v3 net-next 0/4] Preserve mono delivery time (EDT) in skb->tstamp

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Fri, Jan 21, 2022 at 2:30 AM Martin KaFai Lau <kafai@fb.com> wrote:
>
> skb->tstamp was first used as the (rcv) timestamp in real time clock base.
> 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@phy-dev.
>
> v3:
> - Feedback from v2 is using shinfo(skb)->tx_flags could be racy.
> - Considered to reuse a few bits in skb->tstamp to represent
>   different semantics, other than more code churns, it will break
>   the bpf usecase which currently can write and then read back
>   the skb->tstamp.
> - Went back to v1 idea on adding a bit to skb and address the
>   feedbacks on v1:
> - Added one bit skb->mono_delivery_time to flag that
>   the skb->tstamp has the mono delivery_time (EDT), instead
>   of adding a bit to flag if the skb->tstamp has been forwarded or not.
> - Instead of resetting the delivery_time back to the (rcv) timestamp
>   during recvmsg syscall which may be too late and not useful,
>   the delivery_time reset in v3 happens earlier once the stack
>   knows that the skb will be delivered locally.
> - Handled the tapping@ingress case by af_packet
> - No need to change the (rcv) timestamp to mono clock base as in v1.
>   The added one bit to flag skb->mono_delivery_time is enough
>   to keep the EDT delivery_time during forward.
> - Added logic to the bpf side to make the existing bpf
>   running at ingress can still get the (rcv) timestamp
>   when reading the __sk_buff->tstamp.  New __sk_buff->mono_delivery_time
>   is also added.  Test is still needed to test this piece.
>
> Martin KaFai Lau (4):
>   net: Add skb->mono_delivery_time to distinguish mono delivery_time
>     from (rcv) timestamp
>   net: Add skb_clear_tstamp() to keep the mono delivery_time
>   net: Set skb->mono_delivery_time and clear it when delivering locally
>   bpf: Add __sk_buff->mono_delivery_time and handle __sk_buff->tstamp
>     based on tc_at_ingress
>
>  drivers/net/loopback.c                     |   2 +-
>  include/linux/filter.h                     |  31 ++++-
>  include/linux/skbuff.h                     |  64 ++++++++--
>  include/uapi/linux/bpf.h                   |   1 +
>  net/bridge/br_forward.c                    |   2 +-
>  net/bridge/netfilter/nf_conntrack_bridge.c |   5 +-
>  net/core/dev.c                             |   4 +-
>  net/core/filter.c                          | 140 +++++++++++++++++++--
>  net/core/skbuff.c                          |   8 +-
>  net/ipv4/ip_forward.c                      |   2 +-
>  net/ipv4/ip_input.c                        |   1 +
>  net/ipv4/ip_output.c                       |   5 +-
>  net/ipv4/tcp_output.c                      |  16 +--
>  net/ipv6/ip6_input.c                       |   1 +
>  net/ipv6/ip6_output.c                      |   7 +-
>  net/ipv6/netfilter.c                       |   5 +-
>  net/netfilter/ipvs/ip_vs_xmit.c            |   6 +-
>  net/netfilter/nf_dup_netdev.c              |   2 +-
>  net/netfilter/nf_flow_table_ip.c           |   4 +-
>  net/netfilter/nft_fwd_netdev.c             |   2 +-
>  net/openvswitch/vport.c                    |   2 +-
>  net/packet/af_packet.c                     |   4 +-
>  net/sched/act_bpf.c                        |   5 +-
>  net/sched/cls_bpf.c                        |   6 +-
>  net/xfrm/xfrm_interface.c                  |   2 +-
>  tools/include/uapi/linux/bpf.h             |   1 +
>  26 files changed, 265 insertions(+), 63 deletions(-)
>
> --

This overall looks great to me.

The only effect on timestamping is slightly delayed receive timestamp
for packets arriving over a virtual loop (loopback, veth, ..)
interface, as the timestamp is now captured at the network protocol
input.

Actually, while writing that: timestamping is a socket level option,
not specific to IP. Might this break receive timestamping over such
interfaces for other protocols?