Re: [dpdk-dev] [PATCHv3] librte_acl make it build/work for 'default' target

["Richardson, Bruce" <bruce.richardson@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ananyev, Konstantin
> Sent: Wednesday, August 27, 2014 8:19 PM
> To: Neil Horman
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCHv3] librte_acl make it build/work for 'default'
> target
> 
> 
> 
> > -----Original Message-----
> > From: Neil Horman [mailto:nhorman@tuxdriver.com]
> > Sent: Wednesday, August 27, 2014 7:57 PM
> > To: Ananyev, Konstantin
> > Cc: dev@dpdk.org; thomas.monjalon@6wind.com
> > Subject: Re: [PATCHv3] librte_acl make it build/work for 'default' target
> >
> > On Wed, Aug 27, 2014 at 11:25:04AM +0000, Ananyev, Konstantin wrote:
> > > > From: Neil Horman [mailto:nhorman@tuxdriver.com]
> > > > Sent: Tuesday, August 26, 2014 6:45 PM
> > > > To: Ananyev, Konstantin
> > > > Cc: dev@dpdk.org; thomas.monjalon@6wind.com
> > > > Subject: Re: [PATCHv3] librte_acl make it build/work for 'default' target
> > > >
> > > > On Mon, Aug 25, 2014 at 04:30:05PM +0000, Ananyev, Konstantin wrote:
> > > > > Hi Neil,
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: Neil Horman [mailto:nhorman@tuxdriver.com]
> > > > > > Sent: Thursday, August 21, 2014 9:15 PM
> > > > > > To: dev@dpdk.org
> > > > > > Cc: Ananyev, Konstantin; thomas.monjalon@6wind.com; Neil Horman
> > > > > > Subject: [PATCHv3] librte_acl make it build/work for 'default' target
> > > > > >
> > > > > > Make ACL library to build/work on 'default' architecture:
> > > > > > - make rte_acl_classify_scalar really scalar
> > > > > >  (make sure it wouldn't use sse4 instrincts through resolve_priority()).
> > > > > > - Provide two versions of rte_acl_classify code path:
> > > > > >   rte_acl_classify_sse() - could be build and used only on systems with
> sse4.2
> > > > > >   and upper, return -ENOTSUP on lower arch.
> > > > > >   rte_acl_classify_scalar() - a slower version, but could be build and used
> > > > > >   on all systems.
> > > > > > - keep common code shared between these two codepaths.
> > > > > >
> > > > > > v2 chages:
> > > > > >  run-time selection of most appropriate code-path for given ISA.
> > > > > >  By default the highest supprted one is selected.
> > > > > >  User can still override that selection by manually assigning new value
> to
> > > > > >  the global function pointer rte_acl_default_classify.
> > > > > >  rte_acl_classify() becomes a macro calling whatever
> rte_acl_default_classify
> > > > > >  points to.
> > > > > >
> > > > >
> > > > > I see you decided not to wait for me and fix everything by yourself :)
> > > > >
> > > > Yeah, sorry, I'm getting pinged about enabling these features in Fedora,
> and it
> > > > had been about 2 weeks, so I figured I'd just take care of it.
> > >
> > > No worries. I admit that it was a long delay from my side.
> > >
> > > >
> > > > > > V3 Changes
> > > > > >  Updated classify pointer to be a function so as to better preserve ABI
> > > > >
> > > > > As I said in my previous mail it generates extra jump...
> > > > > Though from numbers I got the performance impact is negligible: < 1%.
> > > > > So I suppose, I don't have a good enough reason to object :)
> > > > >
> > > > Yeah, I just don't see a way around it.  I was hoping that the compiler
> would
> > > > have been smart enough to see that the rte_acl_classify function was small
> and
> > > > in-linable, but apparently it won't do that.  As you note however the
> > > > performance change is minor (I'm guessing within a standard deviation of
> your
> > > > results).
> > > >
> > > > > Though I still think we better keep  rte_acl_classify_scalar() publically
> available (same as we do for rte acl_classify_sse()):
> > > > > First of all keep  rte_acl_classify_scalar() is already part of our public API.
> > > > > Also, as I remember, one of the customers explicitly asked for scalar
> version and they planned to call it directly.
> > > > > Plus using rte_acl_select_classify() to always switch between
> implementations is not always handy:
> > > >
> > > > I'm not exactly opposed to this, though it seems odd to me that a user
> might
> > > > want to call a particular version of the classifier directly.  But I certainly
> > > > can't predict everything a consumer wants to do.  If we really need to keep
> it
> > > > public then, it begs the question, is providing a generic entry point even
> > > > worthwhile?  Is it just as easy to expose the scalar/sse and any future
> versions
> > > > directly so the application can just embody the intellegence to select the
> best
> > > > path?  That saves us having to maintain another API point.  I can go with
> > > > consensus on that.
> > > >
> > > > > -  it is global, which means that we can't simultaneously use
> classify_scalar() and classify_sse() for 2 different ACL contexts.
> > > > > - to properly support such switching we then will need to support
> something like (see app/test/test_acl.c below):
> > > > >   old_alg = rte_acl_get_classify();
> > > > >   rte_acl_select_classify(new_alg);
> > > > >   ...
> > > > >   rte_acl_select_classify(old_alg);
> > > > >
> > > > We could attach the classification method to the acl context, so each
> > > > rte_acl_ctx can point to whatever classifier funtion it wants to.  That would
> > > > remove the global issues you point out above.
> > >
> > > I thought about that approach too.
> > > But there is one implication with DPDK MP model:
> > > Same ACL context can be shared by different DPDK processes,
> > > while acl_classify() could be loaded to the different addresses.
> > > Of course we can overcome it by creating a global table of function pointers
> indexed by calssify_alg and
> > > store inside ACL ctx alg instead of actual function pointer.
> > > But that means extra overhead of at least two loads per classify() call.
> > >
> > Hmm, how is the context shared around between processes?  Is it just shared
> as a
> > common cow data page resulting from a fork?  If so, then we should be good
> > because the DSO text will be at the same address (i.e. the pointer will still be
> > valid).  If you do some sort of message passing, then, yes, thats a problem.
> >
> 
> No, it is not parent-child relationship.
> There could be a group of  independently spawned processes.
> One of them should be 'primary' (starts first), other 'secondary's'.
> All hugepage memory pages mapped by the primary process, supposed to be
> mapped to the same VAs by each secondary.
> So all stuff that is allocated from hugepage memory is shared between all
> processes in the group.
> More  detailed  description: http://dpdk.org/doc/intel/dpdk-prog-guide-
> 1.7.0.pdf, section 23.
> 

Function pointers just don't work easily with multiprocess.  Again some history, since today seems to be my Intel DPDK history day...
For the PMDs, originally we allowed NIC access only by the primary process, but later removed that limitation by having the secondary processes do a driver load and pci scan on startup, and having the ethdev structure split between the function pointer part which is not shared and configured independently in the secondary process as part of the pci scan, and the data part which is in hugepage memory and is shared across all processes. For the hash library, we needed a different approach and we looked at having tables of functions, but discarded the idea as largely unworkable when we took user-specified functions into account. What we ended up doing was provide separate api's to call the add/delete/lookup function with a pre-computed hash, so that multi-process apps could explicitly call the hash function without using a fn pointer and then pass in the computed value to the rest of the API calls.

Apologies for the digression from the immediate topic at hand, but I think it's something that is good to make people generally aware of when working with DPDK libs.

Regards,
/Bruce