Service lcores and Application lcores

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

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?