27. Service Cores Sample Application

The service cores sample application demonstrates the service cores capabilities of DPDK. The service cores infrastructure is part of the DPDK EAL, and allows any DPDK component to register a service. A service is a work item or task, that requires CPU time to perform its duty.

This sample application registers 5 dummy services. These 5 services are used to show how the service_cores API can be used to orchestrate these services to run on different service lcores. This orchestration is done by calling the service cores APIs, however the sample application introduces a “profile” concept to contain the service mapping details. Note that the profile concept is application specific, and not a part of the service cores API.

27.1. Compiling the Application

Go to the example directory:

export RTE_SDK=/path/to/rte_sdk
cd ${RTE_SDK}/examples/service_cores

Set the target (a default target is used if not specified). For example:
```
export RTE_TARGET=x86_64-native-linuxapp-gcc
```
See the DPDK Getting Started Guide for possible RTE_TARGET values.
Build the application:
```
make
```

27.2. Running the Application

To run the example, just execute the binary. Since the application dynamically adds service cores in the application code itself, there is no requirement to pass a service core-mask as an EAL argument at startup time.

$ ./build/service_cores

27.3. Explanation

The following sections provide some explanation of code focusing on registering applications from an applications point of view, and modifying the service core counts and mappings at runtime.

27.3.1. Registering a Service

The following code section shows how to register a service as an application. Note that the service component header must be included by the application in order to register services: rte_service_component.h, in addition to the ordinary service cores header rte_service.h which provides the runtime functions to add, remove and remap service cores.

struct rte_service_spec service = {
        .name = "service_name",
};
int ret = rte_service_component_register(services, &id);
if (ret)
        return -1;

/* set the service itself to be ready to run. In the case of
* ethdev, eventdev etc PMDs, this will be set when the
* appropriate configure or setup function is called.
*/
rte_service_component_runstate_set(id, 1);

/* Collect statistics for the service */
rte_service_set_stats_enable(id, 1);

/* The application sets the service to running state. Note that this
 * function enables the service to run - while the 'component' version
 * of this function (as above) marks the service itself as ready */
ret = rte_service_runstate_set(id, 1);

27.3.2. Controlling A Service Core

This section demonstrates how to add a service core. The rte_service.h header file provides the functions for dynamically adding and removing cores. The APIs to add and remove cores use lcore IDs similar to existing DPDK functions.

These are the functions to start a service core, and have it run a service:

/* the lcore ID to use as a service core */
uint32_t service_core_id = 7;
ret = rte_service_lcore_add(service_core_id);
if(ret)
        return -1;

/* service cores are in "stopped" state when added, so start it */
ret = rte_service_lcore_start(service_core_id);
if(ret)
        return -1;

/* map a service to the service core, causing it to run the service */
uint32_t service_id; /* ID of a registered service */
uint32_t enable = 1; /* 1 maps the service, 0 unmaps */
ret = rte_service_map_lcore_set(service_id, service_core_id, enable);
if(ret)
        return -1;

27.3.3. Removing A Service Core

To remove a service core, the steps are similar to adding but in reverse order. Note that it is not allowed to remove a service core if the service is running, and the service-core is the only core running that service (see documentation for rte_service_lcore_stop function for details).

27.3.4. Conclusion

The service cores infrastructure provides DPDK with two main features. The first is to abstract away hardware differences: the service core can CPU cycles to a software fallback implementation, allowing the application to be abstracted from the difference in HW / SW availability. The second feature is a flexible method of registering functions to be run, allowing the running of the functions to be scaled across multiple CPUs.