Subject: Re: [PATCH v2 net-next 00/14] Scheduled packet Transmission: ETF

From: Stephen Hemminger <[email-masked]>
Date: Fri, 6 Jul 2018 14:38:30 -0700

On Tue,  3 Jul 2018 15:42:46 -0700
Jesus Sanchez-Palencia <[email-masked]> wrote:

> Changes since v1:
>   - moved struct sock_txtime from socket.h to uapi net_tstamp.h;
>   - sk_clockid was changed from u16 to u8;
>   - sk_txtime_flags was changed from u16 to a u8 bit field in struct sock;
>   - the socket option flags are now validated in sock_setsockopt();
>   - added SO_EE_ORIGIN_TXTIME;
>   - sockc.transmit_time is now initialized from all IPv4 Tx paths;
>   - added support for the IPv6 Tx path;
> 
> 
> Overview
> ========
> 
> This work consists of a set of kernel interfaces that can be used by
> applications that require (time-based) Scheduled Tx of packets.
> It is comprised by 3 new components to the kernel:
> 
>   - SO_TXTIME: socket option + cmsg programming interfaces.
> 
>   - etf: the "earliest txtime first" qdisc, that provides per-queue
> 	 TxTime-based scheduling. This has been renamed from 'tbs' to
> 	 'etf' to better describe its functionality.
> 
>   - taprio: the "time-aware priority scheduler" qdisc, that provides
> 	    per-port Time-Aware scheduling;
> 
> This patchset is providing the first 2 components, which have been
> developed for longer. The taprio qdisc will be shared as an RFC separately
> (shortly).
> 
> Note that this series is a follow up of the "Time based packet
> transmission" RFCv3 [1].
> 
> 
> 
> etf (formerly known as 'tbs')
> =============================
> 
> For applications/systems that the concept of time slices isn't precise
> enough, the etf qdisc allows applications to control the instant when
> a packet should leave the network controller. When used in conjunction
> with taprio, it can also be used in case the application needs to
> control with greater guarantee the offset into each time slice a packet
> will be sent. Another use case of etf, is when only a small number of
> applications on a system are time sensitive, so it can then be used
> with a more traditional root qdisc (like mqprio).
> 
> The etf qdisc is designed so it buffers packets until a configurable
> time before their deadline (Tx time). The qdisc uses a rbtree internally
> so the buffered packets are always 'ordered' by their txtime (deadline)
> and will be dequeued following the earliest txtime first.
> 
> It relies on the SO_TXTIME API set for receiving the per-packet timestamp
> (txtime) as well as the config flags for each socket: the clockid to be
> used as a reference, if the expected mode of txtime for that socket is
> deadline or strict mode, and if packet drops should be reported on the
> socket's error queue or not.
> 
> The qdisc will drop any packets with a Tx time in the past, or if a
> packet expires while waiting for being dequeued. Drops can be reported
> as errors back to userspace through the socket's error queue.
> 
> Example configuration:
> 
> $ tc qdisc add dev enp2s0 parent 100:1 etf offload delta 200000 \
>             clockid CLOCK_TAI
> 
> Here, the Qdisc will use HW offload for the txtime control.
> Packets will be dequeued by the qdisc "delta" (200000) nanoseconds before
> their transmission time. Because this will be using HW offload and
> since dynamic clocks are not supported by hrtimers, the system clock
> and the PHC clock must be synchronized for this mode to behave as expected.
> 
> A more complete example can be found here, with instructions of how to
> test it:
> 
> https://gist.github.com/jeez/bd3afeff081ba64a695008dd8215866f [2]
> 
> 
> Note that we haven't modified the qdisc so it uses a timerqueue because
> the modification needed was increasing the number of cachelines of a sk_buff.
> 
> 
> 
> This series is also hosted on github and can be found at [3].
> The companion iproute2 patches can be found at [4].
> 
> 
> [1] https://patchwork.ozlabs.org/cover/882342/
> 
> [2] github doesn't make it clear, but the gist can be cloned like this:
> $ git clone https://gist.github.com/jeez/bd3afeff081ba64a695008dd8215866f scheduled-tx-tests
> 
> [3] https://github.com/jeez/linux/tree/etf-v2
> 
> [4] https://github.com/jeez/iproute2/tree/etf-v2
> 
> 
> 
> Jesus Sanchez-Palencia (10):
>   net: Clear skb->tstamp only on the forwarding path
>   net: ipv4: Hook into time based transmission
>   net: ipv6: Hook into time based transmission
>   net/sched: Add HW offloading capability to ETF
>   igb: Refactor igb_configure_cbs()
>   igb: Only change Tx arbitration when CBS is on
>   igb: Refactor igb_offload_cbs()
>   igb: Only call skb_tx_timestamp after descriptors are ready
>   igb: Add support for ETF offload
>   net/sched: Make etf report drops on error_queue
> 
> Richard Cochran (2):
>   net: Add a new socket option for a future transmit time.
>   net: packet: Hook into time based transmission.
> 
> Vinicius Costa Gomes (2):
>   net/sched: Allow creating a Qdisc watchdog with other clocks
>   net/sched: Introduce the ETF Qdisc
> 
>  arch/alpha/include/uapi/asm/socket.h          |   3 +
>  arch/ia64/include/uapi/asm/socket.h           |   3 +
>  arch/mips/include/uapi/asm/socket.h           |   3 +
>  arch/parisc/include/uapi/asm/socket.h         |   3 +
>  arch/s390/include/uapi/asm/socket.h           |   3 +
>  arch/sparc/include/uapi/asm/socket.h          |   3 +
>  arch/xtensa/include/uapi/asm/socket.h         |   3 +
>  .../net/ethernet/intel/igb/e1000_defines.h    |  16 +
>  drivers/net/ethernet/intel/igb/igb.h          |   1 +
>  drivers/net/ethernet/intel/igb/igb_main.c     | 256 ++++++---
>  include/linux/netdevice.h                     |   1 +
>  include/net/inet_sock.h                       |   1 +
>  include/net/pkt_sched.h                       |   7 +
>  include/net/sock.h                            |  11 +
>  include/uapi/asm-generic/socket.h             |   3 +
>  include/uapi/linux/errqueue.h                 |   4 +
>  include/uapi/linux/net_tstamp.h               |  18 +
>  include/uapi/linux/pkt_sched.h                |  18 +
>  net/core/skbuff.c                             |   2 +-
>  net/core/sock.c                               |  39 ++
>  net/ipv4/icmp.c                               |   2 +
>  net/ipv4/ip_output.c                          |   3 +
>  net/ipv4/ping.c                               |   1 +
>  net/ipv4/raw.c                                |   2 +
>  net/ipv4/udp.c                                |   1 +
>  net/ipv6/ip6_output.c                         |  11 +-
>  net/ipv6/raw.c                                |   7 +-
>  net/ipv6/udp.c                                |   1 +
>  net/packet/af_packet.c                        |   6 +
>  net/sched/Kconfig                             |  11 +
>  net/sched/Makefile                            |   1 +
>  net/sched/sch_api.c                           |  11 +-
>  net/sched/sch_etf.c                           | 484 ++++++++++++++++++
>  33 files changed, 864 insertions(+), 75 deletions(-)
>  create mode 100644 net/sched/sch_etf.c
> 

Why support different clockid's in the API? 
I think the clock used in API should be either nanoseconds or USER_HZ (ie 100)
and the kernel components should use ktime. If you need to translate that to some
other value in the hardware driver,  then let the device driver do it.

Exposing multiple choices in userspace API, leads to more error paths and does
not provide direct benefits.