Re: Service lcores and Application lcores

[Thomas Monjalon <thomas@monjalon.net>]

30/06/2017 10:52, Van Haaren, Harry:
> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > 29/06/2017 18:35, Van Haaren, Harry:
> > > 3) The problem;
> > >    If a service core runs the SW PMD schedule() function (option 2) *AND*
> > >    the application lcore runs schedule() func (option 1), the result is that
> > >    two threads are concurrently running a multi-thread unsafe function.
> > 
> > Which function is multi-thread unsafe?
> 
> With the current design, the service-callback does not have to be multi-thread safe.
> For example, the eventdev SW PMD is not multi-thread safe.
> 
> The service library handles serializing access to the service-callback if multiple cores
> are mapped to that service. This keeps the atomic complexity in one place, and keeps
> services as light-weight to implement as possible.
> 
> (We could consider forcing all service-callbacks to be multi-thread safe by using atomics,
> but we would not be able to optimize away the atomic cmpset if it is not required. This
> feels heavy handed, and would cause useless atomic ops to execute.)

OK thank you for the detailed explanation.

> > Why the same function would be run by the service and by the scheduler?
> 
> The same function can be run concurrently by the application, and a service core.
> The root cause that this could happen is that an application can *think* it is the
> only one running threads, but in reality one or more service-cores may be running
> in the background.
> 
> The service lcores and application lcores existence without knowledge of the others
> behavior is the cause of concurrent running of the multi-thread unsafe service function.

That's the part I still don't understand.
Why an application would run a function on its own core if it is already
run as a service? Can we just have a check that the service API exists
and that the service is running?