From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Monjalon Subject: Re: Service lcores and Application lcores Date: Fri, 30 Jun 2017 11:29:17 +0200 Message-ID: <2363216.DczB0HHKeo@xps> References: <25008072.aWAPEDBPgL@xps> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Cc: dev@dpdk.org, 'Jerin Jacob' , "Wiles, Keith" , "Richardson, Bruce" To: "Van Haaren, Harry" Return-path: Received: from out1-smtp.messagingengine.com (out1-smtp.messagingengine.com [66.111.4.25]) by dpdk.org (Postfix) with ESMTP id 9710B567E for ; Fri, 30 Jun 2017 11:29:18 +0200 (CEST) In-Reply-To: List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" 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?