Re: [dpdk-dev] [RFC v2 1/3] eventdev: allow for event devices requiring maintenance

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

On 2021-08-03 06:39, Jerin Jacob wrote:
> On Mon, Aug 2, 2021 at 9:45 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.
>>
>> RFC v2:
>>    - Change rte_event_maintain() return type to be consistent
>>      with the documentation.
>>    - Remove unused typedef from eventdev_pmd.h.
>>
>> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>> Tested-by: Richard Eklycke <richard.eklycke@ericsson.com>
>> Tested-by: Liron Himi <lironh@marvell.com>
>> ---
>>   lib/eventdev/rte_eventdev.h | 62 +++++++++++++++++++++++++++++++++++++
>>   1 file changed, 62 insertions(+)
>>
>> +/**
>> + * Maintain an event device.
>> + *
>> + * This function is only relevant for event devices which has the
>> + * RTE_EVENT_DEV_CAP_REQUIRES_MAINT flag set. Such devices requires
>> + * the application to call rte_event_maintain() on a port during periods
>> + * which it is neither enqueuing nor dequeuing events from this
>> + * port. No port may be left unattended.
>> + *
>> + * An event device's rte_event_maintain() is a low overhead function. In
>> + * situations when rte_event_maintain() must be called, the application
>> + * should do so often.
> 
> See rte_service_component_register() scheme, If a driver needs additional house
> keeping it can use DPDK's service core scheme to abstract different driver
> requirements.We may not need any public API for this.
> 

What DSW requires, and indeed any event device that does software-level 
event buffering, is a way schedule the execution of some function to 
some time later, on the lcore that currently "owns" that port.

Put differently; it's not that the driver "needs some cycles at time T", 
but "it needs some cycles at time T on the lcore thread that currently 
is the user of eventdev port X".

The DSW output buffers and other per-port data structures aren't, for 
simplicity and performance, MT safe. That's one of the reasons the 
processing can't be done by a random service lcore.

Pushing output buffering into the application (or whatever is accessing 
the event device) is not a solution to the DSW<->adapter integration 
issue, since DSW also requires per-port deferred work for the flow 
migration machinery. In addition, if you have a look at the RX adapter, 
for example, you'll see that the buffering logic adds complexity to the 
"application".

The services cores are a rather course-grained deferred work construct. 
A more elaborate one might well have been the basis of a better solution 
than the proposed rte_event_maintain(), user-driven API.

rte_event_maintain() is a crude way to make the Ethernet/Crypto/Timer 
adapters work with DSW. I would argue it still puts us in a better 
position than we are today, where the DSW+adapter combo doesn't work at all.

If/when a more fancy DPDK deferred work framework comes along, 
rte_event_maintain() may be deprecated. Something like work queues in 
Linux could work, run as a DPDK service. In such a case, you might also 
need to require a service-cores-only deployment, and thus disallow the 
use of user-launched lcore threads.

That, however, is not a couple of tiny patches.