Re: [PATCH 2/4] eventdev: implement the northbound APIs

["Eads, Gage" <gage.eads@intel.com>]

(Bruce's adviced heeded :))

>  -----Original Message-----
>  From: Jerin Jacob [mailto:jerin.jacob@caviumnetworks.com]
>  Sent: Tuesday, November 22, 2016 5:44 PM
>  To: Eads, Gage <gage.eads@intel.com>
>  Cc: dev@dpdk.org; Richardson, Bruce <bruce.richardson@intel.com>; Van
>  Haaren, Harry <harry.van.haaren@intel.com>; hemant.agrawal@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@caviumnetworks.com]
>  > >  Sent: Tuesday, November 22, 2016 2:00 PM
>  > >  To: Eads, Gage <gage.eads@intel.com>
>  > >  Cc: dev@dpdk.org; Richardson, Bruce <bruce.richardson@intel.com>;
>  > > Van  Haaren, Harry <harry.van.haaren@intel.com>;
>  > > hemant.agrawal@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
 * repeatedly calls rte_event_schedule().
 *
 */