-----Original Message----- > Date: Fri, 30 Jun 2017 11:14:39 +0000 > From: "Van Haaren, Harry" <harry.van.haa...@intel.com> > To: Thomas Monjalon <tho...@monjalon.net> > CC: "dev@dpdk.org" <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 > > > From: Thomas Monjalon [mailto:tho...@monjalon.net] > > Sent: Friday, June 30, 2017 11:39 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 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. > > Yep - an application can check if a service is running by calling > rte_service_is_running(struct service_spec*); > It returns true if a service-core is running, mapped to the service, and the > service is start()-ed.
If I understand it correctly, driver should check the the _required_ service has been running or not ? Not the _application_. Right? > > > 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). > > Ok, so as part of porting to service-cores, applications are expected to > sanity check the services vs their own lcore config. > If there's no disagreement, I will add it to the releases notes of the V+1 > service-cores patchset. > > There is still a need for the rte_service_iterate() function as discussed in > the other branch of this thread. > I'll wait for consensus on that and post the next revision then. > > Thanks for the questions / input! > > > > > This is pretty error prone, and mis-configuration would cause A) deadlock > > > due to no CPU > > cycles, B) segfault due to two cores.
