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

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

(+Liang Ma for OPDL maintainer)

Ping to maintainers, is the below suggestion acceptable for your PMDs?

Summary of suggestion:
- After event_dev_stop() dequeue() is no longer allowed on any thread
- All events in the system (queues, ports, intermediary buffers) will be passed to a user-supplied callback for cleaning up events.



> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Van Haaren, Harry
> Sent: Tuesday, March 27, 2018 9:20 AM
> To: Eads, Gage <gage.eads@intel.com>; jerin.jacob@caviumnetworks.com;
> hemant.agrawal@nxp.com
> Cc: Richardson, Bruce <bruce.richardson@intel.com>;
> santosh.shukla@caviumnetworks.com; nipun.gupta@nxp.com; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v4 1/2] eventdev: add device stop flush
> callback
> 
> > 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?
> 
> 
>