>-----Original Message----- >From: Honnappa Nagarahalli [mailto:honnappa.nagaraha...@arm.com] >Sent: Wednesday, October 3, 2018 8:54 PM >To: Honnappa Nagarahalli <honnappa.nagaraha...@arm.com>; Wang, Yipeng1 ><yipeng1.w...@intel.com>; Van Haaren, Harry ><harry.van.haa...@intel.com>; Richardson, Bruce <bruce.richard...@intel.com> >Cc: De Lara Guarch, Pablo <pablo.de.lara.gua...@intel.com>; dev@dpdk.org; >Gavin Hu (Arm Technology China) ><gavin...@arm.com>; Steve Capper <steve.cap...@arm.com>; Ola Liljedahl ><ola.liljed...@arm.com>; nd <n...@arm.com>; Gobriel, >Sameh <sameh.gobr...@intel.com> >Subject: RE: [dpdk-dev] [PATCH 3/4] hash: fix rw concurrency while moving keys > >> >> > >-----Original Message----- >> > >From: Van Haaren, Harry >> > >> > > > > /** >> > >> > > > > * Add a key to an existing hash table. >> > >> > > > >@@ -222,7 +222,7 @@ rte_hash_add_key(const struct rte_hash >> > >> > > > >*h, const void >> > >> > > *key); >> > >> > > > > * array of user data. This value is unique for this key. >> > >> > > > > */ >> > >> > > > > int32_t >> > >> > > > >-rte_hash_add_key_with_hash(const struct rte_hash *h, const >> > >> > > > >void *key, >> > >> > > hash_sig_t sig); >> > >> > > > >+rte_hash_add_key_with_hash(struct rte_hash *h, const void >> > >> > > > >+*key, >> > >> > > hash_sig_t sig); >> > >> > > > > >> > >> > > > > / >> > >> > > > >> > >> > > > I think the above changes will break ABI by changing the >> > >> > > > parameter >> > >> type? >> > >> > > Other people may know better on this. >> > >> > > >> > >> > > Just removing a const should not change the ABI, I believe, >> > >> > > since the const is just advisory hint to the compiler. Actual >> > >> > > parameter size and count remains unchanged so I don't believe there >> is an issue. >> > >> > > [ABI experts, please correct me if I'm wrong on this] >> > >> > >> > >> > >> > >> > [Certainly no ABI expert, but...] >> > >> > >> > >> > I think this is an API break, not ABI break. >> > >> > >> > >> > Given application code as follows, it will fail to compile - even >> > >> > though >> > >> running >> > >> > the new code as a .so wouldn't cause any issues (AFAIK). >> > >> > >> > >> > void do_hash_stuff(const struct rte_hash *h, ...) { >> > >> > /* parameter passed in is const, but updated function >> > >> > prototype is >> > >> non- >> > >> > const */ >> > >> > rte_hash_add_key_with_hash(h, ...); } >> > >> > >> > >> > This means that we can't recompile apps against latest patch >> > >> > without application code changes, if the app was passing a const >> > >> > rte_hash struct >> > >> as >> > >> > the first parameter. >> > >> > >> > >> Agree. Do we need to do anything for this? >> > > >> > >I think we should try to avoid breaking API wherever possible. >> > >If we must, then I suppose we could follow the ABI process of a >> > >deprecation notice. >> > > >> > >From my reading of the versioning docs, it doesn't document this case: >> > >https://doc.dpdk.org/guides/contributing/versioning.html >> > > >> > >I don't recall a similar situation in DPDK previously - so I suggest >> > >you ask Tech board for input here. >> > > >> > >Hope that helps! -Harry >> > [Wang, Yipeng] >> > Honnappa, how about use a pointer to the counter in the rte_hash >> > struct instead of the counter? Will this avoid API change? >> I think it defeats the purpose of 'const' parameter to the API and provides >> incorrect information to the user. >Yipeng, I think I have misunderstood your comment. I believe you meant; we >could allocate memory to the counter and store the >pointer in the structure. Please correct me if I am wrong. >This could be a solution, though it will be another cache line access. It >might be ok given that it is a single cache line for entire hash >table. [Wang, Yipeng] Yeah that is what I meant. It is an additional memory access but probably it will be in local cache. Since time is tight, it could be a simple workaround for this version and in future you can extend this pointed counter to a counter array as Ola suggested and the Cuckooo switch paper did for scaling issue.
> >> IMO, DPDK should have guidelines on how to handle the API compatibility >> breaks. I will send an email to tech board on this. >> We can also solve this by having counters on the bucket. I was planning to do >> this little bit later. I will look at the effort involved and may be do it >> now.
