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

[Thomas Monjalon <thomas@monjalon.net>]

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.

> > 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?