30/06/2017 12:18, Van Haaren, Harry: > From: Thomas Monjalon [mailto:tho...@monjalon.net] > > 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.
Yes > 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. Yes service cores may be unavailable, depending of user configuration. That's why it must be possible to request the service core API to know whether a service is run or not. When porting an application to service core, you just have to run this check, which is known to be available for DPDK 17.08 (check rte_version.h). > This is pretty error prone, and mis-configuration would cause A) deadlock due > to no CPU cycles, B) segfault due to two cores.
