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Martin KaFai Lau says: ==================== Preserve mono delivery time (EDT) in skb->tstamp 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 v6: - Add kdoc and use non-UAPI type in patch 6 (Jakub) v5: netdev: - Patch 3 in v4 is broken down into smaller patches 3, 4, and 5 in v5 - The mono_delivery_time bit clearing in __skb_tstamp_tx() is done in __net_timestamp() instead. This is patch 4 in v5. - Missed a skb_clear_delivery_time() for the 'skip_classify' case in dev.c in v4. That is fixed in patch 5 in v5 for correctness. The skb_clear_delivery_time() will be moved to a later stage in Patch 10, so it was an intermediate error in v4. - Added delivery time handling for nfnetlink_{log, queue}.c in patch 9 (Daniel) - Added delivery time handling in the IPv6 IOAM hop-by-hop option which has an experimental IANA assigned value 49 in patch 8 - Added delivery time handling in nf_conntrack for the ipv6 defrag case in patch 7 - Removed unlikely() from testing skb->mono_delivery_time (Daniel) bpf: - Remove the skb->tstamp dance in ingress. Depends on bpf insn rewrite to return 0 if skb->tstamp has delivery time in patch 11. It is to backward compatible with the existing tc-bpf@ingress in patch 11. - bpf_set_delivery_time() will also allow dtime == 0 and dtime_type == BPF_SKB_DELIVERY_TIME_NONE as argument in patch 12. 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. ==================== Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.