[dpdk-dev] [PATCH 2/4] eventdev: implement the northbound APIs
Jerin Jacob
jerin.jacob at caviumnetworks.com
Wed Nov 23 00:43:32 CET 2016
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?
>
> 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?
>
> >
> > >
> > > >
> > > > > *
> > > > > * rte_event_dequeue(...);
> > > > > *
> > > > > * (event processing)
> > > > > *
> > > > > * rte_event_enqueue(...);
> > > > > * }
> > > > > * \endcode
> > > > > *
> > > > > * The *schedule* operation is intended to do event scheduling,
> > > > and the > * *dequeue* operation returns the scheduled events. An
> > > > implementation > * is free to define the semantics between
> > > > *schedule* and *dequeue*. For > * example, a system based on a
> > > > hardware scheduler can define its > * rte_event_schedule() to be
> > > > an NOOP, whereas a software scheduler can use > * the *schedule*
> > > > operation to schedule events. The > *
> > > > RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED capability flag indicates
> > > > whether > * rte_event_schedule() should be called by all cores or
> > > > by a single (typically > * dedicated) core.
> > > > >
> > > > > (line 308)
> > > > > #define RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED (1ULL < 2) > /**<
> > > > Event scheduling implementation is distributed and all cores must
> > > > execute > * rte_event_schedule(). If unset, the implementation is
> > > > centralized and > * a single core must execute the schedule
> > > > operation.
> > > > > *
> > > > > * \see rte_event_schedule()
> > > > > */
> > > > >
> > > > > > >
> > > > > > > On same note, If software PMD based workflow need a
> > > > separate core(s) for > > > schedule function then, Can we hide
> > > > that from API specification and pass an > > > argument to SW pmd
> > > > to define the scheduling core(s)?
> > > > > > >
> > > > > > > Something like --vdev=eventsw0,schedule_cmask=0x2
> > > > >
> > > > > An API for controlling the scheduler coremask instead of (or
> > > > perhaps in addition to) the vdev argument would be good, to allow
> > > > runtime control. I can imagine apps that scale the number of cores
> > > > based on load, and in doing so may want to migrate the scheduler to a
> > different core.
> > > >
> > > > Yes, an API for number of scheduler core looks OK. But if we are
> > > > going to have service core approach then we just need to specify at
> > > > one place as application will not creating the service functions.
> > > >
> > > > >
> > > > > >
> > > > > > Just a thought,
> > > > > >
> > > > > > Perhaps, We could introduce generic "service" cores concept to
> > > > DPDK to hide > > the > > requirement where the implementation
> > > > needs dedicated core to do certain > > work. I guess it would
> > > > useful for other NPU integration in DPDK.
> > > > > >
> > > > >
> > > > > That's an interesting idea. As you suggested in the other thread,
> > > > this concept could be extended to the "producer" code in the
> > > > example for configurations where the NIC requires software to feed
> > > > into the eventdev. And to the other subsystems mentioned in your original
> > PDF, crypto and timer.
> > > >
> > > > Yes. Producers should come in service core category. I think, that
> > > > enables us to have better NPU integration.(same application code for
> > > > NPU vs non NPU)
> > > >
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