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

[Thomas Monjalon <thomas@monjalon.net>]

19/11/2019 11:59, Andrew Rybchenko:
> On 11/19/19 12:50 PM, Thomas Monjalon wrote:
> > 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.
> 
> It is really unexpected outcome from the above discussion.

I'm sorry, I thought I was clear in my request to switch to dynamic first.


> It is just possibility to deliver and handle marks on datapath and
> request to have it. It says almost nothing about rte_flow rules
> supported etc. I'll be happy to take part in the discussion.
> 
> > So regarding 19.11, as this feature is not new, it can wait 20.02.
> 
> OK, it is not critical for me, so I don't mind, however, I've seen
> patches which try to use it [1] except net/octeontx2 in the second
> patch of the series.
> 
> [1] https://patches.dpdk.org/patch/62415/