> From: Thomas Monjalon [mailto:tho...@monjalon.net] > Sent: Friday, June 30, 2017 10:29 AM > To: Van Haaren, Harry <harry.van.haa...@intel.com> > Cc: dev@dpdk.org; 'Jerin Jacob' <jerin.ja...@caviumnetworks.com>; Wiles, Keith > <keith.wi...@intel.com>; Richardson, Bruce <bruce.richard...@intel.com> > Subject: Re: Service lcores and Application lcores > > 30/06/2017 10:52, Van Haaren, Harry: > > From: Thomas Monjalon [mailto:tho...@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?
The point is that really it is an application / service core mis-match. The application should never run a PMD that it knows also has a service core running it. However, porting applications to the service-core API has an over-lap time where an application on 17.05 will be required to call eg: rte_eventdev_schedule() itself, and depending on startup EAL flags for service-cores, it may-or-may-not have to call schedule() manually. This is pretty error prone, and mis-configuration would cause A) deadlock due to no CPU cycles, B) segfault due to two cores. As per the other thread re service-lcores[1], removing rte_eventdev_schedule() from the API would force apps to use the service-core infrastructure for eventdev instead of the possibility of mis-match. [1] http://dpdk.org/ml/archives/dev/2017-June/069492.html
