Re: [dpdk-dev] [PATCH 1/2] ethdev: add flow action type update as an offload

[Thomas Monjalon <thomas@monjalon.net>]

19/11/2019 10:24, Andrew Rybchenko:
> On 11/8/19 4:30 PM, Thomas Monjalon wrote:
> > 08/11/2019 14:27, Andrew Rybchenko:
> >> On 11/8/19 4:17 PM, Thomas Monjalon wrote:
> >>> 08/11/2019 13:00, Andrew Rybchenko:
> >>>> On 11/8/19 2:03 PM, Thomas Monjalon wrote:
> >>>>> 08/11/2019 11:42, Andrew Rybchenko:
> >>>>>> On 11/8/19 1:28 PM, Thomas Monjalon wrote:
> >>>>>>> 08/11/2019 09:35, Andrew Rybchenko:
> >>>>>>>> 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)
> >>>>>>> Thank you for the clear problem statement.
> >>>>>>> I agree with it. This is a real design issue.
> >>>>>>>
> >>>>>>>
> >>>>>>>> Discussed solutions:
> >>>>>>>> ~~~~~~~~~~~~~~~~~~~~
> >>>>>> May be it is not 100% clear since below are alternatives.
> >>>>>>
> >>>>>>>> A. Explicit Rx offload suggested by the patch.
> >>>>>>>>
> >>>>>>>> B. Implicit by validation of a flow rule with MARK/FLAG actions used.
> >>>>>>>>
> >>>>>>>> C. Use dynamic field/flag (i.e. application registers dynamic field
> >>>>>>>>    and/or flag and PMD uses lookup to solve the problem) plus part
> >>>>>>>>    of (B) to discover if a feature is supported.
> >>>>>>> The dynamic field should be registered via a new API function
> >>>>>>> named '<feature>_init'.
> >>>>>>> It means the application must explicit request the feature.
> >>>>>>> I agree this is the way to go.
> >>>>>> If I understand your statement correctly, but (C) is not ideal since it
> >>>>>> looks global. If registered dynamic field of mbuf and is flag that
> >>>>>> the feature should be enabled, it is a flag to all ports/devices.
> >>>>>>
> >>>>>>>> All solutions require changes in applications which use these
> >>>>>>>> features. There is a deprecation notice in place which advertises
> >>>>>>>> DEV_RX_OFFLOAD_FLOW_MARK addition, but may be it is OK to substitute
> >>>>>>>> it with solution (B) or (C). Solution (C) requires changes since
> >>>>>>>> it should be combined with (B) in order to understand if
> >>>>>>>> the feature is supported.
> >>>>>>> I don't understand.
> >>>>>>> Application request and PMD support are two different things.
> >>>>>>> PMD support must be via rte_flow validation on a case by case anyway.
> >>>>>> I mean that application wants to understand if the feature is
> >>>>>> supported. Then, it wants to enable it. In the case of (B),
> >>>>>> if I understand the solution correctly, there is no explicit
> >>>>>> way to enable, PMD just detects it because of discovery is done
> >>>>>> (that's what I mean by "implicit" and it is a drawback from my
> >>>>>> point of view, but still could be considered). (C) solves the
> >>>>>> problem of (B).
> >>>>>>
> >>>>>>>> Advantages and drawbacks of solutions:
> >>>>>>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >>>>>>>> 1. The main drawback of (A) is a "duplication" since we already
> >>>>>>>>    have a way to request flow MARK using rte_flow API.
> >>>>>>>>    I don't fully agree that it is a duplication, but I agree
> >>>>>>>>    that it sounds like duplication and complicates a bit flow
> >>>>>>>>    MARK usage by applications. (B) complicates it as well.
> >>>>>>>>
> >>>>>>>> 2. One more drawback of the solution (A) is the necessity of
> >>>>>>>>    similar solution for META and it eats one more offload bit.
> >>>>>>>>    Yes, that's true and I think it is not a problem.
> >>>>>>>>    It would make it easier for applications to find out if
> >>>>>>>>    either MARK or META is supported.
> >>>>>>>>
> >>>>>>>> 3. The main advantage of the solution (A) is simplicity.
> >>>>>>>>    It is simple for application to understand if it supported.
> >>>>>>>>    It is simple in PMD to understand that it is required.
> >>>>>>>>    It is simple to disable it - just reconfigure.
> >>>>>>>>    Also it is easier to document it - just mention that
> >>>>>>>>    the offload should be supported and enabled.
> >>>>>>>>
> >>>>>>>> 4. The main advantage of the solution (B) is no "duplication".
> >>>>>>>>    I agree that it is valid argument. Solving the problem
> >>>>>>>>    without extra entities is always nice, but unfortunately
> >>>>>>>>    it is too complex in this case.
> >>>>>>>>
> >>>>>>>> 5. The main drawback of the solution (B) is the complexity.
> >>>>>>>>    It is necessary to choose a flow rule which should be used
> >>>>>>>>    as a criteria. It could be hardware dependent.
> >>>>>>>>    Complex logic is require in PMD if it wants to address the
> >>>>>>>>    problem and control MARK delivery based on validated flow
> >>>>>>>>    rules. It adds dependency between start/stop processing and
> >>>>>>>>    flow rules validation code.
> >>>>>>>>    It is pretty complicated to document it.
> >>>>>>>>
> >>>>>>>> 6. Useless enabling of the offload in the case of solution (A)
> >>>>>>>>    if really used flow rules do not support MARK looks like
> >>>>>>>>    drawback as well, but easily mitigated by a combination
> >>>>>>>>    with solution (B) and only required if the application wants
> >>>>>>>>    to dive in the level of optimization and complexity and
> >>>>>>>>    makes sense if application knows required flow rules in
> >>>>>>>>    advance. So, it is not a problem in this case.
> >>>>>>>>
> >>>>>>>> 7. Solution (C) has drawbacks of the solution (B) for
> >>>>>>>>    applications to understand if these features are supported,
> >>>>>>>>    but no drawbacks in PMD, since explicit criteria is used to
> >>>>>>>>    enable/disable (dynamic field/flag lookup).
> >>>>>>>>
> >>>>>>>> 8. Solution (C) is nice since it avoids "duplication".
> >>>>>>>>
> >>>>>>>> 9. The main drawback of the solution (C) is asymmetry.
> >>>>>>>>    As it was discussed in the case of RX_TIMESTAMP
> >>>>>>>>    (if I remember it correctly):
> >>>>>>>>     - PMD advertises RX_TIMESTAMP offload capability
> >>>>>>>>     - application enables the offload
> >>>>>>>>     - PMD registers dynamic field for timestamp
> >>>>>>>>    Solution (C):
> >>>>>>>>      - PMD advertises nothing
> >>>>>>>>      - application uses solution (B) to understand if
> >>>>>>>>        these features are supported
> >>>>>>>>      - application registers dynamic field/flag
> >>>>>>>>      - PMD does lookup and solve the problem
> >>>>>>>>    The asymmetry could be partially mitigated if RX_TIMESTAMP
> >>>>>>>>    solution is changed to require an application to register
> >>>>>>>>    dynamic fields and PMD to do lookup if the offload is
> >>>>>>>>    enabled. So, the only difference will be in no offload
> >>>>>>>>    in the case of flow MARK/FLAG and usage of complex logic
> >>>>>>>>    to understand if it is supported or no.
> >>>>>>>>    May be it would be really good since it will allow to
> >>>>>>>>    have dynamic fields registered before mempool population.
> >>>>>>>>
> >>>>>>>> 10. Common drawback of solutions (B) and (C) is no granularity.
> >>>>>>>>     Solution (A) may be per queue while (B) and (C) cannot be
> >>>>>>>>     per queue. Moreover (C) looks global - for all devices.
> >>>>>>>>     It could be really painful.
> >>>>>>>>
> >>>>>>>> (C) is nice, but I still vote for simplicity and
> >>>>>>>> granularity of (A).
> >>>>>>> I vote for clear separation of application needs and PMD support,
> >>>>>>> by using the method C (dynamic fields).
> >>>>>>> I agree timestamp must use the same path.
> >>>>>>> I agree it's complicate because we don't know in advance whether
> >>>>>>> a flow rule will be accepted, but that's the reality, config is complex.
> >>>>>> Do you think that global nature of the (C) is acceptable?
> >>>>> That's a good question.
> >>>>> Maybe the feature request should be per port.
> >>>>> In this case, we are back to solution A with a flag per port?
> >>>> Offloads are natively per-queue as well, so (A) keeps the choice
> >>>> between per-port vs per-queue to PMDs as usual.
> >>>>
> >>>>> Note that A and C will not guarantee that the offload will be possible.
> >>>> Yes, definitely.
> >>>>
> >>>>> We need B (flow rule validation) anyway.
> >>>> Strictly speaking (B) (checking flow rules before device
> >>>> startup) is required if an application can predict flow
> >>>> rules and wants to ensure that MARK offload will be usable.
> >>>> Otherwise, it may be skipped.
> >>> No no, I mean flow rule validation MUST be used anyway
> >>> during the runtime before applying a rule.
> >>> I agree it is hard to predict. I speak only about real rules.
> >>
> >> OK, I see. Of course, flow rule validation is required at runtime.
> >> I was rather concentrated on the stated problem solutions.
> >>
> >>>>> It seems A, B, C are not alternatives but all required
> >>>>> as pieces of a puzzle...
> >>>> Unfortunately true in the most complex case.
> >>>> Right now it will be A with B if required as explained above.
> >>>> C will come a bit later when the field migrates to dynamic.
> >>>>
> >>>> May be it is even better if application registers dynamic
> >>>> fields before an attempt to enable offload to be sure that
> >>>> it will not fail because of impossibility to register
> >>>> dynamic field (lack of space). I'm not sure, but it is not
> >>>> not that important.
> >>> Yes of course, lack of mbuf space is another reason for
> >>> disabling the feature.
> >>>
> >>>> If we finally go way A, should we add offloads for META back?
> >>>> I guess separate Rx and Tx are required.
> >>> I would prefer to add it as dynamic flags.
> >>> Why rushing on a very temporary solution while it is not a new issue?
> >>
> >> Basically it means that we go just (B)+(C) in the case of META.
> >> I have no strong opinion but thought that it could be better to
> >> align the solution. Of course, we can wait with it. As I understand
> >> META is an experimental feature.
> > 
> > Yes it is experimental and I think it is too late to align now.
> > 
> > Anyway, we will probably to discuss again these offloads TAG/MARK/META,
> > as requested by several people.
> > 
> 
> The series implements (A) to help to solve the problem described above.
> What is the fate of the series in v19.11 in accordance with the
> discussion?

I am against adding anything related to a feature union'ed in mbuf.
The feature must move to dynamic field first.

In addition, such capability is very weak.
I am not sure it is a good idea to have some weak capabilities,
meaning a feature could be available but not in all cases.
I think we should discuss more generally how we want to handle
the rte_flow capabilities conveniently and reliably.

So regarding 19.11, as this feature is not new, it can wait 20.02.