Hi Neil, > ----- Message----- > From: Neil Horman [mailto:nhor...@tuxdriver.com] > Sent: Friday, January 19, 2018 7:48 PM > To: Thomas Monjalon <tho...@monjalon.net> > Cc: dev@dpdk.org; Matan Azrad <ma...@mellanox.com>; Richardson, Bruce > <bruce.richard...@intel.com>; Ananyev, Konstantin > <konstantin.anan...@intel.com>; Gaetan Rivet <gaetan.ri...@6wind.com>; Wu, > Jingjing <jingjing...@intel.com> > Subject: Re: [dpdk-dev] [PATCH v2 2/6] ethdev: add port ownership > > 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. > > 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. > > > 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)
Why is that? As I understand current code: any owner id between 1 and next_owner_id is considered as valid. Konstantin > > 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. 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. > > Neil > > > > subsequently created object
