Subject: Re: [Intel-wired-lan] [PATCH iwl-next v4 0/5] iavf: Add devlink and devlink rate support'

From: Jiri Pirko <[email-masked]>
Date: Fri, 15 Dec 2023 13:30:32 +0100

Fri, Dec 15, 2023 at 12:06:52PM CET, [email-masked] wrote:
>On Thu, 2023-12-14 at 17:46 -0800, Jakub Kicinski wrote:
>> On Thu, 14 Dec 2023 21:29:51 +0100 Paolo Abeni wrote:
>> > Together with Simon, I spent some time on the above. We think the
>> > ndo_setup_tc(TC_SETUP_QDISC_TBF) hook could be used as common basis for
>> > this offloads, with some small extensions (adding a 'max_rate' param,
>> > too).
>> 
>> uAPI aside, why would we use ndo_setup_tc(TC_SETUP_QDISC_TBF)
>> to implement common basis?
>> 
>> Is it not cleaner to have a separate driver API, with its ops
>> and capabilities?
>
>We understand one of the end goal is consolidating the existing rate-
>related in kernel interfaces.  Adding a new one does not feel a good
>starting to reach that goal, see [1] & [2] ;). ndo_setup_tc() feels
>like the natural choice for H/W offload and TBF is the existing
>interface IMHO nearest to the requirements here.
>
>The devlink rate API could be a possible alternative...

Again, devlink rate was introduced for the rate configuration of the
entity that is not present (by netdev for example) on a host.
If we have netdev, let's use it.

>
>> > The idea would be:
>> > - 'fixing' sch_btf so that the s/w path became a no-op when h/w offload
>> > is enabled
>> > - extend sch_btf to support max rate
>> > - do the relevant ice implementation
>> > - ndo_set_tx_maxrate could be replaced with the mentioned ndo call (the
>> > latter interface is a strict super-set of former)
>> > - ndo_set_vf_rate could also be replaced with the mentioned ndo call
>> > (with another small extension to the offload data)
>> > 
>> > I think mqprio deserves it's own separate offload interface, as it
>> > covers multiple tasks other than shaping (grouping queues and mapping
>> > priority to classes)
>> > 
>> > In the long run we could have a generic implementation of the
>> > ndo_setup_tc(TC_SETUP_QDISC_TBF) in term of devlink rate adding a
>> > generic way to fetch the devlink_port instance corresponding to the
>> > given netdev and mapping the TBF features to the devlink_rate API.
>> > 
>> > Not starting this due to what Jiri mentioned [1].
>> 
>> Jiri, AFAIU, is against using devlink rate *uAPI* to configure network
>> rate limiting. That's separate from the internal representation.
>
>... with a couples of caveats:
>
>1) AFAICS devlink (and/or devlink_port) does not have fine grained, per
>queue representation and intel want to be able to configure shaping on
>per queue basis. I think/hope we don't want to bring the discussion to
>extending the devlink interface with queue support, I fear that will
>block us for a long time. Perhaps I’m missing or misunderstanding
>something here. Otherwise in retrospect this looks like a reasonable
>point to completely avoid devlink here.
>
>2) My understanding of Jiri statement was more restrictive. @Jiri it
>would great if could share your genuine interpretation: are you ok with
>using the devlink_port rate API as a basis to replace
>ndo_set_tx_maxrate() (via dev->devlink_port->devlink->) and possibly

Does not make any sense to me.

>ndo_set_vf_rate(). Note the given the previous point, this option would

ndo_set_vf_rate() (and the rest of ndo_[gs]et_vf_*() ndo) is the
legacy way. Devlink rate replaced that when switchdev eswich mode is
configured by:
$ sudo devlink dev eswitch set pci/0000:08:00.1 mode switchdev

In drivers, ndo_set_vf_rate() and devlink rate are implemented in the
same way. See mlx5 for example:
mlx5_esw_qos_set_vport_rate()
mlx5_esw_devlink_rate_leaf_tx_share_set()

>still feel problematic.
>
>Cheers,
>
>Paolo
>
>[1] https://xkcd.com/927/
>[2] https://www.youtube.com/watch?v=f8kO_L-pDwo
>