[dpdk-dev] [PATCH 2/4] eventdev: implement the northbound APIs
Eads, Gage
gage.eads at intel.com
Tue Nov 29 06:46:08 CET 2016
> -----Original Message-----
> From: Jerin Jacob [mailto:jerin.jacob at caviumnetworks.com]
> Sent: Monday, November 28, 2016 9:43 PM
> To: Eads, Gage <gage.eads at intel.com>
> Cc: dev at dpdk.org; Richardson, Bruce <bruce.richardson at intel.com>; Van
> Haaren, Harry <harry.van.haaren at intel.com>; hemant.agrawal at nxp.com
> Subject: Re: [dpdk-dev] [PATCH 2/4] eventdev: implement the northbound APIs
>
> On Mon, Nov 28, 2016 at 03:53:08PM +0000, Eads, Gage wrote:
> > (Bruce's adviced heeded :))
> >
> > > -----Original Message-----
> > > From: Jerin Jacob [mailto:jerin.jacob at caviumnetworks.com]
> > > Sent: Tuesday, November 22, 2016 5:44 PM
> > > To: Eads, Gage <gage.eads at intel.com>
> > > Cc: dev at dpdk.org; Richardson, Bruce <bruce.richardson at intel.com>;
> > > Van Haaren, Harry <harry.van.haaren at intel.com>;
> > > hemant.agrawal at nxp.com
> > > Subject: Re: [dpdk-dev] [PATCH 2/4] eventdev: implement the
> > > northbound APIs
> > >
> > > On Tue, Nov 22, 2016 at 10:48:32PM +0000, Eads, Gage wrote:
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Jerin Jacob [mailto:jerin.jacob at caviumnetworks.com]
> > > > > Sent: Tuesday, November 22, 2016 2:00 PM > > To: Eads, Gage
> > > <gage.eads at intel.com> > > Cc: dev at dpdk.org; Richardson, Bruce
> > > <bruce.richardson at intel.com>; > > Van Haaren, Harry
> > > <harry.van.haaren at intel.com>; > > hemant.agrawal at nxp.com > >
> > > Subject: Re: [dpdk-dev] [PATCH 2/4] eventdev: implement the > >
> > > northbound APIs > > > > On Tue, Nov 22, 2016 at 07:43:03PM +0000,
> > > Eads, Gage wrote:
> > > > > > > > > > > One open issue I noticed is the "typical workflow"
> > > > > > > description starting in > > rte_eventdev.h:204 conflicts
> > > with > > the > > centralized software PMD that Harry > > posted last
> week.
> > > > > > > Specifically, that PMD expects a single core to call the
> > > > > > > > > schedule function. We could extend the documentation to
> > > account > > for > > this > > alternative style of scheduler
> > > invocation, or > > discuss > > ways to make the software > >
> > > PMD work with the > > documented > > workflow. I prefer the
> > > former, but either way I > > > > think we > > ought to expose
> > > the scheduler's expected usage to > > the user -- > > perhaps > > through
> an RTE_EVENT_DEV_CAP flag?
> > > > > > > > > >
> > > > > > > > > > I prefer former too, you can propose the
> > > documentation > > > > change required for > > software PMD.
> > > > > > > >
> > > > > > > > Sure, proposal follows. The "typical workflow" isn't
> > > the > > most > > optimal by having a conditional in the
> > > fast-path, of > > course, but it > > demonstrates the idea simply.
> > > > > > > >
> > > > > > > > (line 204)
> > > > > > > > * An event driven based application has following
> > > typical > > > > workflow on > > fastpath:
> > > > > > > > * \code{.c}
> > > > > > > > * while (1) {
> > > > > > > > *
> > > > > > > > * if (dev_info.event_dev_cap &
> > > > > > > > * RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED)
> > > > > > > > * rte_event_schedule(dev_id);
> > > > > > >
> > > > > > > Yes, I like the idea of RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED.
> > > > > > > It can be input to application/subsystem to launch
> > > separate > > > > core(s) for schedule functions.
> > > > > > > But, I think, the "dev_info.event_dev_cap & > > > >
> > > RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED"
> > > > > > > check can be moved inside the implementation(to make the
> > > > > better > > decisions and avoiding consuming cycles on HW
> > > based schedulers.
> > > > > >
> > > > > > How would this check work? Wouldn't it prevent any core from
> > > > > running the software scheduler in the centralized case?
> > > > >
> > > > > I guess you may not need RTE_EVENT_DEV_CAP here, instead need
> > > flag > > for device configure here > > > > #define
> > > RTE_EVENT_DEV_CFG_DISTRIBUTED_SCHED (1ULL << 1) > > > > struct
> > > rte_event_dev_config config; config.event_dev_cfg = > >
> > > RTE_EVENT_DEV_CFG_DISTRIBUTED_SCHED;
> > > > > rte_event_dev_configure(.., &config); > > > > on the driver
> > > side on configure, > > if (config.event_dev_cfg &
> > > RTE_EVENT_DEV_CFG_DISTRIBUTED_SCHED)
> > > > > eventdev->schedule = NULL;
> > > > > else // centralized case
> > > > > eventdev->schedule = your_centrized_schedule_function;
> > > > >
> > > > > Does that work?
> > > >
> > > > Hm, I fear the API would give users the impression that they can
> > > select the scheduling behavior of a given eventdev, when a software
> > > scheduler is more likely to be either distributed or centralized -- not both.
> > >
> > > Even if it is capability flag then also it is per "device". Right ?
> > > capability flag is more of read only too. Am i missing something here?
> > >
> >
> > Correct, the capability flag I'm envisioning is per-device and read-only.
> >
> > > >
> > > > What if we use the capability flag, and define
> > > rte_event_schedule() as the scheduling function for centralized
> > > schedulers and rte_event_dequeue() as the scheduling function for
> > > distributed schedulers? That way, the datapath could be the simple
> > > dequeue -> process -> enqueue. Applications would check the
> > > capability flag at configuration time to decide whether or not to launch an
> lcore that calls rte_event_schedule().
> > >
> > > I am all for simple "dequeue -> process -> enqueue".
> > > rte_event_schedule() added for SW scheduler only, now it may not
> > > make sense to add one more check on top of "rte_event_schedule()"
> > > to see it is really need or not in fastpath?
> > >
> >
> > Yes, the additional check shouldn't be needed. In terms of the 'typical
> workflow' description, this is what I have in mind:
> >
> > *
> > * An event driven based application has following typical workflow on
> fastpath:
> > * \code{.c}
> > * while (1) {
> > *
> > * rte_event_dequeue(...);
> > *
> > * (event processing)
> > *
> > * rte_event_enqueue(...);
> > * }
> > * \endcode
> > *
> > * The events are injected to event device through the *enqueue*
> > operation by
> > * event producers in the system. The typical event producers are
> > ethdev
> > * subsystem for generating packet events, core(SW) for generating
> > events based
> > * on different stages of application processing, cryptodev for
> > generating
> > * crypto work completion notification etc
> > *
> > * The *dequeue* operation gets one or more events from the event ports.
> > * The application process the events and send to downstream event
> > queue through
> > * rte_event_enqueue() if it is an intermediate stage of event
> > processing, on
> > * the final stage, the application may send to different subsystem
> > like ethdev
> > * to send the packet/event on the wire using ethdev rte_eth_tx_burst() API.
> > *
> > * The point at which events are scheduled to ports depends on the
> > device. For
> > * hardware devices, scheduling occurs asynchronously. Software
> > schedulers can
> > * either be distributed (each worker thread schedules events to its
> > own port)
> > * or centralized (a dedicated thread schedules to all ports).
> > Distributed
> > * software schedulers perform the scheduling in rte_event_dequeue(),
> > whereas
> > * centralized scheduler logic is located in rte_event_schedule(). The
> > * RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED capability flag indicates
> > whether a
> > * device is centralized and thus needs a dedicated scheduling thread
> > that
>
> Since we are starting a dedicated thread in centralized case, How about name
> the flag as RTE_EVENT_DEV_CAP_CENTRALIZED_SCHED?
> instead of RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED.
> No strong opinion here. Just a thought.
>
Fine with me.
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