Re: [dpdk-dev] [RFC 1/3] eventdev: allow for event devices requiring maintenance

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

On 2020-04-10 15:00, Jerin Jacob wrote:
> On Thu, Apr 9, 2020 at 7:32 PM Mattias Rönnblom
> <mattias.ronnblom@ericsson.com> wrote:
>> On 2020-04-09 15:32, Jerin Jacob wrote:
>>> On Thu, Apr 9, 2020 at 5:51 PM Mattias Rönnblom
>>> <mattias.ronnblom@ericsson.com> wrote:
>>>> On 2020-04-08 21:36, Jerin Jacob wrote:
>>>>> On Wed, Apr 8, 2020 at 11:27 PM Mattias Rönnblom
>>>>> <mattias.ronnblom@ericsson.com> wrote:
>>>>>> Extend Eventdev API to allow for event devices which require various
>>>>>> forms of internal processing to happen, even when events are not
>>>>>> enqueued to or dequeued from a port.
>>>>>>
>>>>>> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>>>>>> ---
>>>>>>     lib/librte_eventdev/rte_eventdev.h     | 65 ++++++++++++++++++++++++++
>>>>>>     lib/librte_eventdev/rte_eventdev_pmd.h | 14 ++++++
>>>>>>     2 files changed, 79 insertions(+)
>>>>>>
>>>>>> diff --git a/lib/librte_eventdev/rte_eventdev.h b/lib/librte_eventdev/rte_eventdev.h
>>>>>> index 226f352ad..d69150792 100644
>>>>>> --- a/lib/librte_eventdev/rte_eventdev.h
>>>>>> +++ b/lib/librte_eventdev/rte_eventdev.h
>>>>>> @@ -289,6 +289,15 @@ struct rte_event;
>>>>>>      * single queue to each port or map a single queue to many port.
>>>>>>      */
>>>>>>
>>>>>> +#define RTE_EVENT_DEV_CAP_REQUIRES_MAINT (1ULL << 9)
>>>>>> +/**< Event device requires calls to rte_event_maintain() during
>>>>> This scheme would call for DSW specific API handling in fastpath.
>>>> Initially this would be so, but buffering events might yield performance
>>>> benefits for more event devices than DSW.
>>>>
>>>>
>>>> In an application, it's often convenient, but sub-optimal from a
>>>> performance point of view, to do single-event enqueue operations. The
>>>> alternative is to use an application-level buffer, and the flush this
>>>> buffer with rte_event_enqueue_burst(). If you allow the event device to
>>>> buffer, you get the simplicity of single-event enqueue operations, but
>>>> without taking any noticeable performance hit.
>>> IMO, It is better to aggregate the burst by the application,  as sending
>>> event by event to the driver to aggregate has performance due to cost
>>> function pointer overhead.
>>
>> That's a very slight overhead - but for optimal performance, sure. It'll
>> come at a cost in terms of code complexity. Just look at the adapters.
>> They do this already. I think some applications are ready to take the
>> extra 5-10 clock cycles or so it'll cost them to do the function call
>> (provided the event device had buffering support).
> So Is there any advantage of moving aggregation logic to PMD? it is costly.


What do you mean by aggregation logic?


>
>>
>>> Another concern is the frequency of calling rte_event_maintain() function by
>>> the application, as the timing requirements will vary differently by
>>> the driver to driver and application to application.
>>> IMO, It is not portable and I believe the application should not be
>>> aware of those details. If the driver needs specific maintenance
>>> function for any other reason then better to use DPDK SERVICE core infra.
>>
>> The only thing the application needs to be aware of, is that it needs to
>> call rte_event_maintain() as often as it would have called dequeue() in
>> your "typical worker" example. To make sure this call is cheap-enough is
>> up to the driver, and this needs to hold true for all event devices that
>> needs maintenance.
> Why not rte_event_maintain() can't do either in dequeue() or enqueue()
> in the driver context? Either one of them has to be called
> periodically by application
> in any case?


No, producer-only ports can go idle for long times. For applications 
that don't "go idle" need not worry about the maintain function.


>
>>
>> If you plan to use a non-buffering hardware device driver or a soft,
>> centralized scheduler that doesn't need this, it will also not set the
>> flag, and thus the application needs not care about the
>> rte_event_maintain() function. DPDK code such as the eventdev adapters
>> do need to care, but the increase in complexity is slight, and the cost
>> of calling rte_maintain_event() on a maintenance-free devices is very
>> low (since the then-NULL function pointer is in the eventdev struct,
>> likely on a cache-line already dragged in).
>>
>>
>> Unfortunately, DPDK doesn't have a per-core delayed-work mechanism.
>> Flushing event buffers (and other DSW "background work") can't be done
>> on a service core, since they would work on non-MT-safe data structures
>> on the worker thread's event ports.
> Yes. Otherwise, DSW needs to update to support MT safe.


I haven't been looking at this in detail, but I suspect it will be both 
complex and not very performant. One of problems that need to be solved 
in such a solution, is the "pausing" of flows during migration. All 
participating lcores needs to ACK that a flow is paused.


>
>>
>>>>>> + * periods when neither rte_event_dequeue_burst() nor
>>>>> The typical worker thread will be
>>>>> while (1) {
>>>>>                    rte_event_dequeue_burst();
>>>>>                     ..proess..
>>>>>                    rte_event_enqueue_burst();
>>>>> }
>>>>> If so, Why DSW driver can't do the maintenance in driver context in
>>>>> dequeue() call.
>>>>>
>>>> DSW already does maintenance on dequeue, and works well in the above
>>>> scenario. The typical worker does not need to care about the
>>>> rte_event_maintain() functions, since it dequeues events on a regular basis.
>>>>
>>>>
>>>> What this RFC addresses is the more atypical (but still fairly common)
>>>> case of a port being neither dequeued to or enqueued from on a regular
>>>> basis. The timer and ethernet rx adapters are examples of such.
>>> If it is an Adapter specific use case problem then maybe, we have
>>> an option to fix the problem in adapter specific API usage or in that area.
>>>
>> It's not adapter specific, I think. There might be producer-only ports,
>> for example, which doesn't provide a constant stream of events, but
>> rather intermittent bursts. A traffic generator is one example of such
>> an application, and there might be other, less synthetic ones as well.
> In that case, the application knows the purpose of the eventdev port.
> Is changing eventdev spec to configure "port" or "queue" for that use
> case help? Meaning, DSW or
> Any driver can get the hint and change the function pointers
> accordingly for fastpath.
> For instance, do maintenance on enqueue() for such ports or so.


This is what DSW does already today. A dequeue() call with a zero-length 
event array serves the purpose of rte_event_maintain(). It's a bit of a 
hack, in my opinion.


>
>>
>>>>>> + * rte_event_enqueue_burst() are called on a port. This will allow the
>>>>>> + * event device to perform internal processing, such as flushing
>>>>>> + * buffered events, return credits to a global pool, or process
>>>>>> + * signaling related to load balancing.
>>>>>> + */