Re: [dpdk-dev] [PATCH v4 1/2] eventdev: add device stop flush callback

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

> From: Eads, Gage
> Sent: Monday, March 26, 2018 10:59 PM
> To: Van Haaren, Harry <harry.van.haaren@intel.com>; dev@dpdk.org
> Cc: jerin.jacob@caviumnetworks.com; hemant.agrawal@nxp.com; Richardson, Bruce
> <bruce.richardson@intel.com>; santosh.shukla@caviumnetworks.com;
> nipun.gupta@nxp.com
> Subject: RE: [PATCH v4 1/2] eventdev: add device stop flush callback
> 
> 
> 
> > -----Original Message-----
> > From: Van Haaren, Harry
> > Sent: Friday, March 23, 2018 11:57 AM
> > To: Eads, Gage <gage.eads@intel.com>; dev@dpdk.org
> > Cc: jerin.jacob@caviumnetworks.com; hemant.agrawal@nxp.com; Richardson,
> > Bruce <bruce.richardson@intel.com>; santosh.shukla@caviumnetworks.com;
> > nipun.gupta@nxp.com
> > Subject: RE: [PATCH v4 1/2] eventdev: add device stop flush callback
> >
> > > From: Eads, Gage
> > > Sent: Tuesday, March 20, 2018 2:13 PM
> > > To: dev@dpdk.org
> > > Cc: jerin.jacob@caviumnetworks.com; Van Haaren, Harry
> > > <harry.van.haaren@intel.com>; hemant.agrawal@nxp.com; Richardson,
> > > Bruce <bruce.richardson@intel.com>; santosh.shukla@caviumnetworks.com;
> > > nipun.gupta@nxp.com
> > > 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 <gage.eads@intel.com>
> >
> > <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.

+1.

> >
> > 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.

All +1 too, good point about the header stating it is undefined behavior.


> > 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?


That's fine with me, both from design point of view, and SW PMD.

@HW PMD maintainers, would the above approach work for you?