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

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 11/19/2019 11:09 AM, Thomas Monjalon wrote:
> 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/
> 

Sorry, I have to resurrect this old (long) discussion because the patches are
still active in the patchwork [1] and the deprecation notice is still there [2].

Andrew has a good summary in the thread [3], after a year nothing seems changed.


Pavan, Thomas, Andrew, Ori,

What is our plan with this series, lets try to have a conclusion.




[1]
https://patches.dpdk.org/user/todo/dpdk/?series=7076

[2]
http://lxr.dpdk.org/dpdk/v20.05/source/doc/guides/rel_notes/deprecation.rst#L88

[3]
http://inbox.dpdk.org/dev/f170105b-9c60-1b04-cb18-52e0951ddcdb@solarflare.com/