[Intel-wired-lan] [PATCH net-next v1 0/7] net/sched: Add txtime assist support for taprio
Murali Karicheri
m-karicheri2 at ti.com
Mon Jun 3 13:54:56 UTC 2019
On 06/03/2019 09:50 AM, Murali Karicheri wrote:
> 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.
Actually the tc class is mapped to the same queue in the previous
command, not this one, right?
Murali
>> 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(-)
>>
>
>
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