Re: Service lcores and Application lcores

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Jun 29, 2017 at 03:36:04PM +0100, Van Haaren, Harry wrote:
> Hi All,
> 
> 
> The recently posted service cores patchset[1], introduces service lcores to run services for DPDK applications. Services are just an ordinary function for eg: eventdev scheduling, NIC RX, statistics and monitoring, etc. A service is just a callback function, which a core invokes. An atomic ensures that services that are
> non-multi-thread-safe are never concurrently invoked.
> 
> The topic of discussion in this thread is how we can ensure that application lcores do not interfere with service cores. I have a solution described below, opinions welcome.
> 
> 
> Regards, -Harry
> 
> 
> PS: This discussion extends that in the ML thread here[2], participants of that thread added to CC.
> 
> [1] Service Cores v2 patchset http://dpdk.org/dev/patchwork/bundle/hvanhaar/service_cores_v2/
> [2] http://dpdk.org/ml/archives/dev/2017-June/069290.html
> 
> 
> ________________________
> 
> 
> 
> A proposal for Eventdev, to ensure Service lcores and Application lcores play nice;
> 
> 1) Application lcores must not directly call rte_eventdev_schedule()
> 2A) Service cores are the proper method to run services
> 2B) If an application insists on running a service "manually" on an app lcore, we provide a function for that:
>      rte_service_run_from_app_lcore(struct service *srv);
> 
> The above function would allow a pesky app to run services on its own (non-service core) lcores, but
> does so through the service-core framework, allowing the service-library atomic to keep access serialized as required for non-multi-thread-safe services.
> 
> The above solution maintains the option of running the eventdev PMD as now (single-core dedicated to a single service), while providing correct serialization by using the rte_service_run_from_app_lcore() function. Given the atomic is only used when required (multiple cores mapped to the service) there should be no performance delta.
> 
> Given that the application should not invoke rte_eventdev_schedule(), we could even consider removing it from the Eventdev API. A PMD that requires cycles registers a service, and an application can use a service core or the run_from_app_lcore() function if it wishes to invoke that service on an application owned lcore.
> 
> 
> Opinions?

I would be in favour of this proposal, except for the proposed name for
the new function. It would be useful for an app to be able to "adopt" a
service into it's main loop if so desired. If we do this, I think I'd
also support the removal of a dedicated schedule call from the eventdev
API, or alternatively, if it is needed by other PMDs, leave it as a
no-op in the sw PMD in favour of the service-cores managed function.

/Bruce