Re: Service lcores and Application lcores

-----Original Message-----

Date: Thu, 29 Jun 2017 16:57:08 +0100
From: Bruce Richardson <redacted>
To: "Van Haaren, Harry" <redacted>
CC: "dev@dpdk.org" <redacted>, 'Jerin Jacob'
 [off-list ref], "thomas@monjalon.net"
 [off-list ref], "Wiles, Keith" [off-list ref]
Subject: Re: Service lcores and Application lcores
User-Agent: Mutt/1.8.1 (2017-04-11)

On Thu, Jun 29, 2017 at 03:36:04PM +0100, Van Haaren, Harry wrote:

quoted

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

+1

Agree with Harry and Bruce here.

I think, The adapter function should take "struct service *" and return
lcore_function_t so that it can run using exiting rte_eal_remote_launch()

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.

I would be in favor of removing eventdev schedule and
RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED capability so that it is completely
transparent to application whether scheduler runs on HW or SW or "combination
of both"

/Bruce