Re: [dpdk-dev] [PATCH v2 1/2] service: add API to retrieve service core active

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

+ Gage (as I referring to his commit below)

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> 
> > > > > +/**
> > > > > + * Reports if a service lcore is currently running.
> > > > > + * @retval 0 Service thread is not active, and has been returned to
> EAL.
> > > > > + * @retval 1 Service thread is in the service core polling loop.
> > > > > + * @retval -EINVAL Invalid *lcore_id* provided.
> > > > > + */
> > > > > +__rte_experimental
> > > > > +int32_t rte_service_lcore_active(uint32_t lcore_id);
> > > > Would 'rte_service_lcore_may_be_active' better? It would be inline
> > > > with
> > > 'rte_service_may_be_active'?
> 
> I think the implementation behind the API is different, so I think _may_be_ is
> not appropriate for service_lcore_active, keeping same function name for v3.
> 
> rte_service_lcore_active() checks at a particular point in the calling thread if
> another thread is active *at that time*. It is either active or not. This is
> defined, it is deterministic in that the result is either yes or no, and there is
> no ambiguity at any given check. You're right the value can change *just* after
> the check - but at the time of the check the answer was deterministic.
> 
> rte_service_may_be_active() checks if a service *could* be run by a service
> core. It is not deterministic. A service lcore only sets a service as "active on
> lcore" (or not active) when it polls it - this opens a window of
> nondeterministic result. When a runstate is set to off, there is a window of
> "unknown" before we know certainly that the service is not run on a service
> core anymore. That is why I believe the _may_be_ is appropriate for this API,
> it shows this non determinism.
> 

I am looking at this from the application usage perspective (not the implementation). I am pointing to the similarity that exists with the new API. i.e. when 'rte_service_lcore_stop' is called, it is not known if the service lcore has stopped. If 'rte_service_lcore_active' returns 1, it indicates the lcore 'may be' active (the reasoning could be different in this case), it is not guaranteed that it is active by the time caller checks it. But when the API returns 0, it guarantees that the service lcore (assuming it was started and verified that it was started), has stopped.

Looking at the commit e30dd31847d212cd1b766612cbd980c7d8240baa that added the 'rte_service_may_be_active', the use case is a mechanism to identify a quiescent state after a service was stopped.
The use case for the new API is also the same. We want to identify a quiescent state after a service lcore is stopped.

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