Re: [EXT] Re: [PATCH 1/2] doc: add enqueue depth for new event type

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

On 2022-07-13 12:40, Pavan Nikhilesh Bhagavatula wrote:
> 
> 
>> -----Original Message-----
>> From: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>> Sent: Wednesday, July 13, 2022 2:39 PM
>> To: Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>; Thomas
>> Monjalon <thomas@monjalon.net>
>> Cc: Jerin Jacob Kollanukkaran <jerinj@marvell.com>; Ray Kinsella
>> <mdr@ashroe.eu>; dev@dpdk.org; timothy.mcdaniel@intel.com; Hemant
>> Agrawal <hemant.agrawal@nxp.com>; sachin.saxena@oss.nxp.com;
>> liangma@liangbit.com; peter.mccarthy@intel.com;
>> harry.van.haaren@intel.com; erik.g.carrillo@intel.com;
>> abhinandan.gujjar@intel.com; jay.jayatheerthan@intel.com;
>> anatoly.burakov@intel.com
>> Subject: Re: [EXT] Re: [PATCH 1/2] doc: add enqueue depth for new event
>> type
>>
>> On 2022-07-12 20:11, Pavan Nikhilesh Bhagavatula wrote:
>>> +Cc
>>> timothy.mcdaniel@intel.com;
>>> hemant.agrawal@nxp.com;
>>> sachin.saxena@oss.nxp.com;
>>> mattias.ronnblom@ericsson.com;
>>> jerinj@marvell.com;
>>> liangma@liangbit.com;
>>> peter.mccarthy@intel.com;
>>> harry.van.haaren@intel.com;
>>> erik.g.carrillo@intel.com;
>>> abhinandan.gujjar@intel.com;
>>> jay.jayatheerthan@intel.com;
>>> mdr@ashroe.eu;
>>> anatoly.burakov@intel.com;
>>>
>>>
>>>> -----Original Message-----
>>>> From: Thomas Monjalon <thomas@monjalon.net>
>>>> Sent: Tuesday, July 12, 2022 8:31 PM
>>>> To: Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>
>>>> Cc: Jerin Jacob Kollanukkaran <jerinj@marvell.com>; Ray Kinsella
>>>> <mdr@ashroe.eu>; dev@dpdk.org; Pavan Nikhilesh Bhagavatula
>>>> <pbhagavatula@marvell.com>
>>>> Subject: [EXT] Re: [PATCH 1/2] doc: add enqueue depth for new event
>> type
>>>>
>>>> External Email
>>>>
>>>> ----------------------------------------------------------------------
>>>> I'm not your teacher, but please consider Cc'ing people outside of your
>>>> company.
>>>>
>>>
>>> I generally add --to-cmd=./devtools/get-maintainer.sh but looks like it's
>> useless for
>>> sending deprecation notices.
>>>
>>> Maybe we should update it to include lib/driver maintainers when diff sees
>> deprecation.rst
>>>
>>>>
>>>> 27/06/2022 11:57, pbhagavatula@marvell.com:
>>>>> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>>>>
>>>>> A new field ``max_event_port_enqueue_new_burst`` will be added to
>> the
>>>>> structure ``rte_event_dev_info``. The field defines the max enqueue
>>>>> burst size of new events (OP_NEW) supported by the underlying event
>>>>> device.
>>>>>
>>>>> Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>>>> ---
>>>>>    doc/guides/rel_notes/deprecation.rst | 5 +++++
>>>>>    1 file changed, 5 insertions(+)
>>>>>
>>>>> diff --git a/doc/guides/rel_notes/deprecation.rst
>>>> b/doc/guides/rel_notes/deprecation.rst
>>>>> index 4e5b23c53d..071317e8e3 100644
>>>>> --- a/doc/guides/rel_notes/deprecation.rst
>>>>> +++ b/doc/guides/rel_notes/deprecation.rst
>>>>> @@ -125,3 +125,8 @@ Deprecation Notices
>>>>>      applications should be updated to use the ``dmadev`` library instead,
>>>>>      with the underlying HW-functionality being provided by the ``ioat`` or
>>>>>      ``idxd`` dma drivers
>>>>> +
>>>>> +* eventdev: The structure ``rte_event_dev_info`` will be extended to
>>>> include the
>>>>> +  max enqueue burst size of new events supported by the underlying
>>>> event device.
>>>>> +  A new field ``max_event_port_enqueue_new_burst`` will be added
>> to
>>>> the structure
>>>>> +  ``rte_event_dev_info`` in DPDK 22.11.
>>>>>
>>>>
>>>>
>>>>
>>>>
>>
>> Why is this needed, or useful? Why isn't called something with
>> "enqueue_depth" in it, like the already-present field?
>>
> 
> This is for a case where enqueues with OP_FORWARD/OP_RELEASE only supports
> enqueue depth of 1.

I assume it's for other cases as well? Any case when the event device 
has a max forward enqueue depth != max new enqueue depth?

I don't see why an event device would have such low max limit on the 
number events enqueued. If the underlying hardware has some limitations, 
why not let the driver loop until back pressure occurs? Then you can 
amortize the function call overhead and potentially other software 
operations (credit system management etc) over multiple events. Plus, 
the application doesn't need a special case for new events versus 
forward type events, or this-versus-that event device.

> Where as OP_NEW supports higher burst size.
> 
> This is already tied into capability description :
>   
> #define RTE_EVENT_DEV_CAP_BURST_MODE          (1ULL << 4)
> /**< Event device is capable of operating in burst mode for enqueue(forward,
>   * release) and dequeue operation. If this capability is not set, application
>   * still uses the rte_event_dequeue_burst() and rte_event_enqueue_burst() but
>   * PMD accepts only one event at a time.
>   *
>   * @see rte_event_dequeue_burst() rte_event_enqueue_burst()
>   */
> 
>> I think I would rather remove all fields related to the max
>> enqueue/dequeue burst sizes. They serve no purpose, as far as I see. If
>> you have some HW limit on the maximum number of new events it can
>> accept, have the driver retry until backpressure occurs.
>