[PATCH v4 net-next 0/8] Preserve mono delivery time (EDT) in skb->tstamp

[Martin KaFai Lau <kafai@fb.com>]

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@phy-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

v4:
netdev:
- Push the skb_clear_delivery_time() from
  ip_local_deliver() and ip6_input() to
  ip_local_deliver_finish() and ip6_input_finish()
  to accommodate the ipvs forward path.
  This is the notable change in v4 at the netdev side.

    - Patch 3/8 first does the skb_clear_delivery_time() after
      sch_handle_ingress() in dev.c and this will make the
      tc-bpf forward path work via the bpf_redirect_*() helper.

    - The next patch 4/8 (new in v4) will then postpone the
      skb_clear_delivery_time() from dev.c to
      the ip_local_deliver_finish() and ip6_input_finish() after
      taking care of the tstamp usage in the ip defrag case.
      This will make the kernel forward path also work, e.g.
      the ip[6]_forward().

- Fixed a case v3 which missed setting the skb->mono_delivery_time bit
  when sending TCP rst/ack in some cases (e.g. from a ctl_sk).
  That case happens at ip_send_unicast_reply() and
  tcp_v6_send_response().  It is fixed in patch 1/8 (and
  then patch 3/8) in v4.

bpf:
- Adding __sk_buff->delivery_time_type instead of adding
  __sk_buff->mono_delivery_time as in v3.  The tc-bpf can stay with
  one __sk_buff->tstamp instead of having two 'time' fields
  while one is 0 and another is not.
  tc-bpf can use the new __sk_buff->delivery_time_type to tell
  what is stored in __sk_buff->tstamp.
- bpf_skb_set_delivery_time() helper is added to set
  __sk_buff->tstamp from non mono delivery_time to
  mono delivery_time
- Most of the convert_ctx_access() bpf insn rewrite in v3
  is gone, so no new rewrite added for __sk_buff->tstamp.
  The only rewrite added is for reading the new
  __sk_buff->delivery_time_type.
- Added selftests, test_tc_dtime.c

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 (8):
  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 after
    sch_handle_ingress()
  net: Postpone skb_clear_delivery_time() until knowing the skb is
    delivered locally
  bpf: Keep the (rcv) timestamp behavior for the existing tc-bpf@ingress
  bpf: Clear skb->mono_delivery_time bit if needed after running
    tc-bpf@egress
  bpf: Add __sk_buff->delivery_time_type and bpf_skb_set_delivery_time()
  bpf: selftests: test skb->tstamp in redirect_neigh

 drivers/net/loopback.c                        |   2 +-
 include/linux/filter.h                        |  33 +-
 include/linux/skbuff.h                        |  64 ++-
 include/net/inet_frag.h                       |   1 +
 include/uapi/linux/bpf.h                      |  35 +-
 net/bridge/br_forward.c                       |   2 +-
 net/bridge/netfilter/nf_conntrack_bridge.c    |   5 +-
 net/core/dev.c                                |   4 +-
 net/core/filter.c                             |  85 +++-
 net/core/skbuff.c                             |   8 +-
 net/ipv4/inet_fragment.c                      |   1 +
 net/ipv4/ip_forward.c                         |   2 +-
 net/ipv4/ip_fragment.c                        |   1 +
 net/ipv4/ip_input.c                           |   1 +
 net/ipv4/ip_output.c                          |   6 +-
 net/ipv4/tcp_output.c                         |  16 +-
 net/ipv6/ip6_input.c                          |   1 +
 net/ipv6/ip6_output.c                         |   7 +-
 net/ipv6/netfilter.c                          |   5 +-
 net/ipv6/tcp_ipv6.c                           |   2 +-
 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                           |   7 +-
 net/sched/cls_bpf.c                           |   8 +-
 net/xfrm/xfrm_interface.c                     |   2 +-
 tools/include/uapi/linux/bpf.h                |  35 +-
 .../selftests/bpf/prog_tests/tc_redirect.c    | 434 ++++++++++++++++++
 .../selftests/bpf/progs/test_tc_dtime.c       | 348 ++++++++++++++
 32 files changed, 1078 insertions(+), 57 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_tc_dtime.c

-- 
2.30.2