[xdp-hints] Re: [PATCH bpf-next v2 0/3] xsk: TX metadata txtime support

["Song, Yoong Siang" <yoong.siang.song@intel.com>]

On Saturday, December 2, 2023 10:16 PM, Willem de Bruijn wrote:
>Jesper Dangaard Brouer wrote:
>>
>>
>> On 12/1/23 16:09, Willem de Bruijn wrote:
>> > Song, Yoong Siang wrote:
>> >> On Friday, December 1, 2023 6:46 PM, Jesper Dangaard Brouer
><hawk@kernel.org> wrote:
>> >>> On 12/1/23 07:24, Song Yoong Siang wrote:
>> >>>> This series expands XDP TX metadata framework to include ETF HW offload.
>> >>>>
>> >>>> Changes since v1:
>> >>>> - rename Time-Based Scheduling (TBS) to Earliest TxTime First (ETF)
>> >>>> - rename launch-time to txtime
>> >>>>
>> >>>
>> >>> I strongly disagree with this renaming (sorry to disagree with Willem).
>> >>>
>> >>> The i210 and i225 chips call this LaunchTime in their programmers
>> >>> datasheets, and even in the driver code[1].
>> >>>
>> >>> Using this "txtime" name in the code is also confusing, because how can
>> >>> people reading the code know the difference between:
>> >>>   - tmo_request_timestamp and tmo_request_txtime
>> >>>
>> >>
>> >> Hi Jesper and Willem,
>> >>
>> >> How about using "launch_time" for the flag/variable and
>> >> "Earliest TxTime First" for the description/comments?
>> >
>>
>> I don't follow why you are calling the feature:
>>   - "Earliest TxTime First" (ETF).
>>   - AFAIK this just reference an qdisc name (that most don't know exists)
>>
>>
>> > I don't particularly care which term we use, as long as we're
>> > consistent. Especially, don't keep introducing new synonyms.
>> >
>> > The fact that one happens to be one vendor's marketing term does not
>> > make it preferable, IMHO. On the contrary.
>> >
>>
>> These kind of hardware features are defined as part of Time Sensitive
>> Networking (TSN).
>> I believe these TSN features are defined as part of IEEE 802.1Qbv (2015)
>> and according to Wikipedia[2] incorporated into IEEE 802.1Q.
>>
>> [2] https://en.wikipedia.org/wiki/Time-Sensitive_Networking
>>
>>
>> > SO_TXTIME is in the ABI, and EDT has been used publicly in kernel
>> > patches and conference talks, e.g., Van Jacobson's Netdev 0x12
>> > keynote. Those are vendor agnostic commonly used terms.
>> >
>>
>> I agree that EDT (Earliest Departure Time) have become a thing and term
>> in our community.
>> We could associate this feature with this.
>> I do fear what hardware behavior will be it if I e.g. ask it to send a
>> packet 2 sec in the future on i225 which max support 1 sec.
>> Will hardware send it at 1 sec?
>> Because then I'm violating the *Earliest* Departure Time.
>
>That should definitely not happen. At least not on a device that
>implements EDT semantics.
>
>This relates to Jakub's question in the previous thread on whether
>this mechanism allows out-of-order transmission or maintains FIFO
>behavior. That really is device specific.
>
>Older devices only support this for low rate (PTP) and with a small
>fixed number of outstanding requests. For pacing offload, devices need
>to support up to linerate and out-of-order.
>
>I don't think we want to enforce either in software, as the hardware
>is already out there. But it would be good if drivers can somehow
>label these capabilities. Including programmable horizon.
>
>It is up to the qdisc to ensure that it does not pass packets to the
>device beyond its horizon.
>
>ETF and FQ already have a concept of horizon. And a way to queue
>errors for packets out of bound (SO_EE_CODE_TXTIME_..).
>
>>
>> > But as long as Launch Time is not an Intel only trademark, fine to
>> > select that.
>>
>> The IEEE 802.1Qbv is sometimes called Time-Aware Shaper (TAS), but I
>> don't like to for us to name this after this.  This features is simply
>> taking advantage of exposing one of the hardware building blocks
>> (controlling/setting packet "launch time") that can be used for
>> implementing a TAS.
>>
>> I like the name "launch time" because it doesn't get easily confused
>> with other timestamps, and intuitively describes packet will be send at
>> a specific time (likely in future).
>>
>> --Jesper
>
>Understood on your point that txtime and tx_timestamp are too similar.
>As said, I don't care strongly. Launch time sounds fine to me. Others
>can speak up if they disagree.
>
>I take launch time as a less strict than EDT: it is a request to send
>at a certain time, with no strict definition on uncertainty. While EDT
>more strictly ensures that a packet is not sent before the timestamp.

Thanks for the deep discussion and information. I agree with launch time too.
I will submit v3 with launch time so that others can review on the
new naming and provide their feedback.