From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Monjalon Subject: Re: [PATCH v2 2/6] ethdev: add port ownership Date: Fri, 19 Jan 2018 21:19:18 +0100 Message-ID: <2018230.dXJnJTqQbo@xps> References: <20180118131017.GA1622@hmswarspite.think-freely.org> <1959306.0b6nHJGtEC@xps> <20180119194739.GF9519@hmswarspite.think-freely.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Cc: dev@dpdk.org, Matan Azrad , Bruce Richardson , "Ananyev, Konstantin" , Gaetan Rivet , "Wu, Jingjing" To: Neil Horman Return-path: Received: from out1-smtp.messagingengine.com (out1-smtp.messagingengine.com [66.111.4.25]) by dpdk.org (Postfix) with ESMTP id A75851B31A for ; Fri, 19 Jan 2018 21:19:54 +0100 (CET) In-Reply-To: <20180119194739.GF9519@hmswarspite.think-freely.org> List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" 19/01/2018 20:47, Neil Horman: > On Fri, Jan 19, 2018 at 07:12:36PM +0100, Thomas Monjalon wrote: > > 19/01/2018 18:43, Neil Horman: > > > On Fri, Jan 19, 2018 at 06:17:51PM +0100, Thomas Monjalon wrote: > > > > 19/01/2018 16:27, Neil Horman: > > > > > On Fri, Jan 19, 2018 at 03:13:47PM +0100, Thomas Monjalon wrote: > > > > > > 19/01/2018 14:30, Neil Horman: > > > > > > > So it seems like the real point of contention that we need to settle here is, > > > > > > > what codifies an 'owner'. Must it be a specific execution context, or can we > > > > > > > define any arbitrary section of code as being an owner? I would agrue against > > > > > > > the latter. > > > > > > > > > > > > This is the first thing explained in the cover letter: > > > > > > "2. The port usage synchronization will be managed by the port owner." > > > > > > There is no intent to manage the threads synchronization for a given port. > > > > > > It is the responsibility of the owner (a code object) to configure its > > > > > > port via only one thread. > > > > > > It is consistent with not trying to manage threads synchronization > > > > > > for Rx/Tx on a given queue. > > > > > > > > > > > > > > > > > Yes, in his cover letter, and I contend that notion is an invalid design point. > > > > > By codifying an area of code as an 'owner', rather than an execution context, > > > > > you're defining the notion of heirarchy, not ownership. That is to say, > > > > > you want to codify the notion that there are top level ports that the > > > > > application might see, and some of those top level ports are parents to > > > > > subordinate ports, which only the parent port should access directly. If thats > > > > > all you want to encode, there are far easier ways to do it: > > > > > > > > > > struct rte_eth_shared_data { > > > > > < existing bits > > > > > > struct rte_eth_port_list { > > > > > struct rte_eth_port_list *children; > > > > > struct rte_eth_port_list *parent; > > > > > }; > > > > > }; > > > > > > > > > > > > > > > Build an api around a structure like that, so that the parent/child relationship > > > > > is globally clear, and this would be much easier, especially if you want to > > > > > continue asserting that the notion of synchronization/exclusion is an exercise > > > > > left to the application. > > > > > > > > Not only Neil. > > > > An owner can be something else than a port. > > > > An owner can be an app process (multi-processes). > > > > An owner can be a library. > > > > The intent is really to solve the generic problem of which code > > > > is managing a port. > > > > > > > I don't see how this precludes any part of what you just said. Define the > > > rte_eth_port_list externally to the shared_data struct and allow any object you > > > want to allocate it, then anything you want to control a heirarchy of ports can > > > do so without issue, and the structure is far more clear than an opaque id that > > > carries subtle semantic ordering with it. > > > > Sorry, I don't understand. Please could you rephrase? > > > > Sure, I'm saying the fact that you want an owner to be an object > (library/port/process) rather than strictly an execution context > (process/thread) doesn't preclude what I'm proposing above. You can create a > generic version of the strcture I propose above like so: > > struct rte_obj_heirarchy { > struct rte_obj_heirarchy *children; > struct rte_obj_heirarchy *parent; > void *owner_data; /* optional */ > }; > > And embed that structure in any object you would like to give a representative > heirarchy to, you then have a fairly simple api > > struct rte_obj_heirarchy *heirarchy_alloc(); > bool heirarchy_set(struct rte_obj_heirarchy *parent, struct rte_obj_heirarcy *child) > void heirarchy_release(struct rte_obj_heirarchy *obj) > > That gives you the privately held list relationship I think you are in part > looking for (i.e. the ability for a failsafe device to iterate over the ports it > is in control of), without the awkwardness of the ordinal priority that the > current implementation imposes. What is the awkward ordinal priority? I see you discuss it below. So let's discuss it below. > In summary, if what you want is ownership in the strictest sense of the word > (i.e. mutually exclusive access, which I think makes sense), then using a lock > and flag is really the simplest way to go. If instead what you want is a > heirarchical relationship where you can iterate over a limited set of objects > (the failsafe child port example), then the above is what you want. We want only ownership. That's why it's called ownership :) The hierarchical relationship is private to the owner. For instance, failsafe implements its own list of sub-devices. So we just need to expose that the ports are already owned. > The soution Matan is providing does some of each of these things, but comes with > very odd side effects > > It offers a level of mutual exclusion, in that only one > object can own another at a time, but does so in a way that introduces this very > atypical ordinality (once an ownership object is created with owner_new, any > previously created ownership object will be denied the ability to take ownership > of a port) You mean only the last owner id can take an ownership? If yes, it looks like a bug. Please could you show what is responsible of this effect in the patch? > It also offers a level of filtering (in that if you can set the ownership id of > a given set of object to the value X, you can then iterate over them by > iterating over all objects of that type, and filtering on their id), but it > offers no clear in-memory relationship between parent and children (i.e. if you > were to look at at an object in a debugger and see that it was owned by owner id > X, it would provide you with no indicator of what object held the allocated > ownership object assigned id X. I think it is wrong. There is an owner name for debug/printing purpose. > My proposal trades a few bytes of data in > exchage for a global clear, definitive heirarcy relationship. And if you add an > api call and a spinlock, you can easily graft on mutual exclusion here, by > blocking access to objects that arent the immediate parent of a given object. For the hierarchical relationship, I think it is over-engineered. For blocking access, it means you need a caller id parameter in every functions in order to identify if the caller is the owner. My summary: - you think there is a bug - needs to show - you think about relationship needs that I don't see - you think about access permission which would be a huge change