> -----Original Message-----
> From: Van Haaren, Harry
> Sent: Friday, March 23, 2018 11:57 AM
> To: Eads, Gage <[email protected]>; [email protected]
> Cc: [email protected]; [email protected]; Richardson,
> Bruce <[email protected]>; [email protected];
> [email protected]
> Subject: RE: [PATCH v4 1/2] eventdev: add device stop flush callback
>
> > From: Eads, Gage
> > Sent: Tuesday, March 20, 2018 2:13 PM
> > To: [email protected]
> > Cc: [email protected]; Van Haaren, Harry
> > <[email protected]>; [email protected]; Richardson,
> > Bruce <[email protected]>; [email protected];
> > [email protected]
> > Subject: [PATCH v4 1/2] eventdev: add device stop flush callback
> >
> > When an event device is stopped, it drains all event queues. These
> > events may contain pointers, so to prevent memory leaks eventdev now
> > supports a user-provided flush callback that is called during the queue
> > drain
> process.
> > This callback is stored in process memory, so the callback must be
> > registered by any process that may call rte_event_dev_stop().
> >
> > This commit also clarifies the behavior of rte_event_dev_stop().
> >
> > This follows this mailing list discussion:
> > http://dpdk.org/ml/archives/dev/2018-January/087484.html
> >
> > Signed-off-by: Gage Eads <[email protected]>
>
> <snip most of the code - looks good!>
>
> > /**
> > - * Stop an event device. The device can be restarted with a call to
> > - * rte_event_dev_start()
> > + * Stop an event device.
> > + *
> > + * This function causes all queued events to be drained. While
> > + draining
> > events
> > + * out of the device, this function calls the user-provided flush
> > + callback
> > + * (if one was registered) once per event.
> > + *
> > + * This function does not drain events from event ports; the
> > + application is
> > + * responsible for flushing events from all ports before stopping the
> > device.
>
>
> Question about how an application is expected to correctly cleanup all the
> events here. Note in particular the last part: "application is responsible for
> flushing events from all ports **BEFORE** stopping the device".
>
> Given the event device is still running, how can the application be sure it
> has
> flushed all the events (from the dequeue side in particular)?
>
Appreciate the feedback -- good points all around.
I was expecting that the application would unlink queues from the ports, and
then dequeue until each port has no events. However, there are PMDs for which
runtime port link/unlink is not supported, so I see that this is not a viable
approach. Plus, this adds the application burden that you describe below.
>
> In order to drain all events from the ports, I was expecting the following:
>
> // stop scheduling new events to worker cores
> rte_event_dev_stop()
> ---> callback gets called for each event
>
> // to dequeue events from each port, and app cleans them up?
> FOR_EACH_PORT( rte_event_dev_dequeue(..., port_id, ...) )
>
>
> I'd like to avoid the dequeue-each-port() approach in application, as it adds
> extra
> burden to clean up correctly...
Agreed, but for a different reason: that approach means we'd have to change the
documented eventdev behavior. rte_eventdev.h states that the "schedule, enqueue
and dequeue functions should not be invoked when the device is stopped," and
this patch reiterates that in the rte_event_dev_stop() documentation ("Threads
that continue to enqueue/dequeue while the device is stopped, or being stopped,
will result in undefined behavior"). Since a PMD's stop cleanup code could just
be repeated calls to a PMD's dequeue code, allowing applications to dequeue
simultaneously could be troublesome.
>
> What if we say that dequeue() returns zero after stop() (leaving events
> possibly
> in the port-dequeue side SW buffers), and these events which were about to be
> dequeued by the worker core are also passed to the dev_stop_flush callback?
I'd prefer to have dequeue-while-stopped be unsupported, so we don't need an
additional check or synchronization in the datapath, but passing the events in
a port to the callback should work (for the sw PMD, at least). How does that
sound?
Thanks,
Gage
