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

[Andrew Rybchenko <arybchenko@solarflare.com>]

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?