Re: [dpdk-dev] [PATCH v2 3/3] ethdev: improve flow mark Rx offload deprecation notice

[Jerin Jacob <jerinjacobk@gmail.com>]

On Mon, Dec 9, 2019 at 2:47 PM Andrew Rybchenko
<arybchenko@solarflare.com> wrote:
>
> On 12/5/19 11:12 AM, Jerin Jacob wrote:
> > On Mon, Dec 2, 2019 at 5:27 PM Andrew Rybchenko
> > <arybchenko@solarflare.com> wrote:
> >>


>> >>>
> >>> Ack.
> >>
> >> Yes, I agree as well, but in general we already have an
> >> exception MBUF_FAST_FREE which is just a nice wrap for
> >> enabled by default support for mbufs from different
> >> mempools and support for mbuf reference counters.
> >> I don't suggest to change it. Just want to highlight
> >> that we already have exceptions (nicely wrapped).
> >> That's why I would not touch packet type parsing
> >> "offload". Packet type parsing is not just on/off and
> >> adding on/off in addition to existing API looks overkill.
> >> Yes, it is one more exception, but nicely wrapped as well.
> >
> > I am all for making offloads disabled by default.
> >
> >>
> >>>>> And what would be DEFAULT behavior for the mbuf MARK updation feature?
> >>>>> (That's where this thread started).
> >>>>
> >>>> As all other features, mark is disabled by default.
> >>>> Using a rte_flow rule, it can be enabled.
> >>>> No need to pre-enable it.
> >>>
> >>> Ok.
> >>
> >> But it returns us to the point where we started [1]:
> >>
> >> The problem:
> >> ~~~~~~~~~~~~
> >> PMD wants to know before port start if application wants to
> >> to use flow MARK/FLAG in the future. It is required because:
> >>
> >> 1. HW may be configured in a different way to reserve resources
> >>    for MARK/FLAG delivery.
> >>
> >> 2. Datapath implementation choice may depend on it (e.g. vPMD
> >>    is faster, but does not support MARK).
> >>
> >> opt-in/opt-out solution has drawbacks mentioned above.
> >> Also I'm not sure if opt-in/opt-out is per-queue or per-port.
> >> (Offloads may be naturally per-queue and it is a big advantage).
> >>
> >> IMHO feature which should be opt-out is almost equivalent to
> >> offload enabled by default. It has the same negative properties
> >> as enabled by default offloads.
> >>
> >> Am I missing something again?
> >>
> >> From my point of view I see no problem in necessity to say
> >> in advance (before device start) that application would like
> >> to use some features at run time.
> >
> > I agree with your problem definition and solution as offload.
> >
> > I think, our constraint is, we can not change functional ABI behavior
> > for the next year. i.e The existing application should work for the
> > next year without
> > changing the code.
> >
> > I think, it all boiling down to adhere to that constraint or not for
> > this specific case.
>
> May be the escape is to avoid consistency checks in generic
> code (not sure that such checks are doable/required in this
> case, but anyway) and make the behaviour change vendor/driver-
> specific. I understand that it is far from ideal solution.
>
> May be offload should be combined with opt-out as a way to
> disable. I.e. offload is positive (not negative), but enabled
> by default (i.e. automatically added to offloads as we do
> for RSS_HASH) with an experimental opt-out to disable it.
>
> As the result:
> 1. There is no changes in behaviour from application point of
>    view.
> 2. Application which care about performance and ready to use
>    experimental opt-out to optimize performance can do it.
>    (i.e. use opt-out to avoid the offload enabled by default).
> 3. Later when window to normalize behaviour opens, opt-out
>    becomes NOP (i.e. it still could be preserved for some
>    time to simplify transition).
> 4. The offload is enabled by default during transition
>    period only since it represents a feature which had
>    no offload flag before and was always enabled before.
> 5. As an offload the feature may be controlled per-device
>    and per-queue natively.

Looks good to me.
It makes sense to have a generic opt API to have for year ABI,
which works on

- per queue/per port
- Enable by default to keep backward compatible.
- Have a generic signature to allow probe() all the enabled opt-in features
and then disable if required by the application.
- I think, rte_eth_dev_set_ptypes()  needs to change to generic API as
it comes under opt-in/out scheme.


>
> It still does not sort out "necessity to enable twice"
> concern which for specified above "the problem", IMO,
> contradicts to "disabled by default offloads" (I read
> it as "the best performance" by default).
>
> > Once that is decided, we can wrap it in offload flags vs opt scheme
> > (by default enabled scheme).
>
> Yes. May be I don't understand all the details of the opt
> scheme right now, but I don't like what I can imagine as
> described above.
>
> >>
> >> Yes, all features which may be controlled at run-time are
> >> headache for optimizations (VLAN offloads).
> >>
> >> [1]
> >> http://inbox.dpdk.org/dev/f170105b-9c60-1b04-cb18-52e0951ddcdb@solarflare.com/
>