Re: [dpdk-dev] eventdev: method for finding out unlink status

["Van Haaren, Harry" <harry.van.haaren@intel.com>]

> From: Elo, Matias (Nokia - FI/Espoo) [mailto:matias.elo@nokia.com]
> Sent: Wednesday, September 5, 2018 8:49 AM
> To: Van Haaren, Harry <harry.van.haaren@intel.com>
> Cc: Jerin Jacob <jerin.jacob@caviumnetworks.com>; dev@dpdk.org
> Subject: Re: [dpdk-dev] eventdev: method for finding out unlink status
> 
> 
> >>
> >> I'm not sure I understand the issue here.
> >> Is anybody suggesting to make unlink() blocking?
> >>
> >> For certain PMDs, perhaps it must be a synchronous handled unlink().
> >> For other PMDs (eg event/sw) there are multiple threads involved,
> >> so it must be async. Hence, APIs should be async to avoid blocking the
> caller.
> >>
> >> With an async API, if you don't want the async behaviuor, it is
> >> easy to build the sync version: call it in a loop, optionally with a
> delay().
> >
> > Correct. My point was, rte_event_port_unlink() can be blocking as it
> > is a slow path API(does not really matter how long it waits).
> > If you think, it can be called in fastpath and/or application can
> > leverage some cpu cycles on completing the async call then you can add
> > at the cost of new API unlinks_in_progress() and make sure to update the
> documentation
> > about unlink() that it can be async call(currently it is documented as a
> sync
> > call).
> 
> Did you come to an agreement how to solve this issue? Any solution (e.g.
> make
> rte_event_port_unlink() blocking with SW eventdev) would be welcomed since
> this
> issue is currently blocking my work with eventdev.


I'll work on a patch and document async behavior as Jerin suggested.

I think we may still need to drain the port after unlinks_in_progress() returns
zero, as there may still be events in the port buffers until all unlinks are done.

That would make the final application code-path as follows:


/* control path core */
unlink();
while(unlinks_in_progress() > 0) {
    delay();
}

/* we know there are no more events being scheduled to the port.
   Could flag to data-path core here that unlinks are acked */

while(dequeue_burst(events, ... ) > 0) {
    free_events();
}

/* now we know there are no events in the port itself either.
   Application can perform whatever it wants with the data-path core */

The final dequeueing could also be handled in the data-path core,
and only break out of its loop when the unlink-ack and zero packets returned from dequeue.


I'll send a v1 patchset for your reviews - I think that might make things clearer.

Thanks, -Harry