From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Van Haaren, Harry" Subject: Re: Service lcores and Application lcores Date: Fri, 30 Jun 2017 11:14:39 +0000 Message-ID: References: <2363216.DczB0HHKeo@xps> <1614665.GlQH7FWj5q@xps> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Cc: "dev@dpdk.org" , 'Jerin Jacob' , "Wiles, Keith" , "Richardson, Bruce" To: Thomas Monjalon Return-path: Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by dpdk.org (Postfix) with ESMTP id CB4072C37 for ; Fri, 30 Jun 2017 13:14:42 +0200 (CEST) In-Reply-To: <1614665.GlQH7FWj5q@xps> Content-Language: en-US List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" > From: Thomas Monjalon [mailto:thomas@monjalon.net] > Sent: Friday, June 30, 2017 11:39 AM > To: Van Haaren, Harry > Cc: dev@dpdk.org; 'Jerin Jacob' ; Wiles, = Keith > ; Richardson, Bruce > Subject: Re: Service lcores and Application lcores >=20 > 30/06/2017 12:18, Van Haaren, Harry: > > From: Thomas Monjalon [mailto: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 (optio= n 2) *AND* > > > > > > the application lcore runs schedule() func (option 1), the r= esult is that > > > > > > two threads are concurrently running a multi-thread unsafe f= unction. > > > > > > > > > > Which function is multi-thread unsafe? > > > > > > > > With the current design, the service-callback does not have to be m= ulti-thread safe. > > > > For example, the eventdev SW PMD is not multi-thread safe. > > > > > > > > The service library handles serializing access to the service-callb= ack 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 i= s 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 sche= duler? > > > > > > > > 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 *t= hink* 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 knowled= ge of the others > > > > behavior is the cause of concurrent running of the multi-thread uns= afe 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 alre= ady > > > run as a service? Can we just have a check that the service API exist= s > > > 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. >=20 > Yes >=20 > > However, porting applications to the service-core API has an over-lap t= ime 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 hav= e to call > schedule() manually. >=20 > 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_servi= ce_is_running(struct service_spec*); It returns true if a service-core is running, mapped to the service, and th= e service is start()-ed. > 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 sa= nity 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 i= n the other branch of this thread. I'll wait for consensus on that and post the next revision then.=20 Thanks for the questions / input! > > This is pretty error prone, and mis-configuration would cause A) deadlo= ck due to no CPU > cycles, B) segfault due to two cores.