[Intel-wired-lan] [PATCH net-next v1 0/7] net/sched: Add txtime assist support for taprio

[Murali Karicheri]

Hi Vedang,

On 05/28/2019 01:46 PM, Vedang Patel wrote:
> Currently, we are seeing packets being transmitted outside their timeslices. We
> can confirm that the packets are being dequeued at the right time. So, the
> delay is induced after the packet is dequeued, because taprio, without any
> offloading, has no control of when a packet is actually transmitted.
> 
> In order to solve this, we are making use of the txtime feature provided by ETF
> qdisc. Hardware offloading needs to be supported by the ETF qdisc in order to
> take advantage of this feature. The taprio qdisc will assign txtime (in
> skb->tstamp) for all the packets which do not have the txtime allocated via the
> SO_TXTIME socket option. For the packets which already have SO_TXTIME set,
> taprio will validate whether the packet will be transmitted in the correct
> interval.
> 
> In order to support this, the following parameters have been added:
> - offload (taprio): This is added in order to support different offloading
>    modes which will be added in the future.
> - txtime-delay (taprio): this is the time the packet takes to traverse through
>    the kernel to adapter card.

I am very new to this TC and QoS handling of Linux kernel and TSN. So 
sorry some of the  questions below are very basic in nature. I would 
soon would be working to add this support in our platform based on this.
So please answer.

Is txtime-delay from the instance qdisc de-queue the packet to the time
packets get onto the wire? I am wondering if this time is deterministic
or we have some way to ensure this can be tuned to meet a max value?
Also how would one calculate this value or have to measure it?

> - skip_sock_check (etf): etf qdisc currently drops any packet which does not
>    have the SO_TXTIME socket option set. This check can be skipped by specifying
>    this option.
> 
> Following is an example configuration:
> 
> $ tc qdisc replace dev $IFACE parent root handle 100 taprio \\
>        num_tc 3 \\
>        map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \\
>        queues 1 at 0 1 at 0 1 at 0 \\
>        base-time $BASE_TIME \\
>        sched-entry S 01 300000 \\
>        sched-entry S 02 300000 \\
>        sched-entry S 04 400000 \\
>        offload 2 \\
>        txtime-delay 40000 \\
>        clockid CLOCK_TAI
> 

Could you tell me what is CLOCK_TAI?? I see below in the source code for
the definition in include/uapi/linux/time.h

/*
  * The driver implementing this got removed. The clock ID is kept as a
  * place holder. Do not reuse!
  */
#define CLOCK_SGI_CYCLE			10
#define CLOCK_TAI			11

So why do I see such defines being used in the example that is being
removed?

AFAIK, From the usage point of view, TSN requires the network being
synchronized through linux PTP. For synchronization phc2sys is used to
synchronize system time to the PHC. So why don't one use system time for
this?

So application will use clock_gettime() to know current time and
schedule the packet for transmission as well as user would use scripts
or such to check current system time and issue tc command above. So the
command should use CLOCK_REALTIME in that case. So all h/w and software
are aligned to the same time source. Or Have I understood it wrong? I am
assuming that for the offloaded case, h/w schedule Gate open, send
packet etc are synchronized to the PHC or use a translated time w.r.t 
the common time (network time. Assuming PHC tracks this).

Thanks in advance for clarifying this.

> $ tc qdisc replace dev $IFACE parent 100:1 etf \\
>        offload delta 200000 clockid CLOCK_TAI skip_sock_check
> 
> Here, the "offload" parameter is indicating that the TXTIME_OFFLOAD mode is
> enabled. Also, that all the traffic classes have been assigned the same queue.
> This is to prevent the traffic classes in the lower priority queues from
> getting starved. Note that this configuration is specific to the i210 ethernet
> card. Other network cards where the hardware queues are given the same
> priority, might be able to utilize more than one queue.
> 
> Following are some of the other highlights of the series:
> - Fix a bug where hardware timestamping and SO_TXTIME options cannot be used
>    together. (Patch 1)
> - Introduce hardware offloading. This patch introduces offload parameter which
>    can be used to enable the txtime offload mode. It will also support enabling
>    full offload when the support is available in drivers. (Patch 3)
> - Make TxTime assist mode work with TCP packets. (Patch 6 & 7)
> 
> 
> The following changes are recommended to be done in order to get the best
> performance from taprio in this mode:
> # ip link set dev enp1s0 mtu 1514

May I know why MTU is changed to 1514 to include the Ethernet header?

> # ethtool -K eth0 gso off
> # ethtool -K eth0 tso off
> # ethtool --set-eee eth0 eee off

Could you please explain why these are needed?

Thanks

Murali
> 
> Thanks,
> Vedang Patel
> 
> Vedang Patel (6):
>    igb: clear out tstamp after sending the packet.
>    etf: Add skip_sock_check
>    taprio: calculate cycle_time when schedule is installed
>    taprio: Add support for txtime offload mode.
>    taprio: make clock reference conversions easier
>    taprio: Adjust timestamps for TCP packets.
> 
> Vinicius Costa Gomes (1):
>    taprio: Add the skeleton to enable hardware offloading
> 
>   drivers/net/ethernet/intel/igb/igb_main.c |   1 +
>   include/uapi/linux/pkt_sched.h            |   6 +
>   net/sched/sch_etf.c                       |  10 +
>   net/sched/sch_taprio.c                    | 548 ++++++++++++++++++++--
>   4 files changed, 532 insertions(+), 33 deletions(-)
>