Re: [dpdk-dev] [PATCH v5 1/3] rcu: add RCU library supporting QSBR mechanism

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> > > > > > > > > > > >
> > > > > > > > > > > > After evaluating long term API/ABI issues, I think
> > > > > > > > > > > > you need to get rid of almost all use of inline and
> > > > > > > > > > > > visible structures. Yes it might be marginally
> > > > > > > > > > > > slower, but you thank me
> > > > > > the first time you have to fix something.
> > > > > > > > > > > >
> > > > > > > > > > > Agree, I was planning on another version to address
> > > > > > > > > > > this (I am yet
> > > > > > to take a look at your patch addressing the ABI).
> > > > > > > > > > > The structure visibility definitely needs to be addressed.
> > > > > > > > > > > For the inline functions, is the plan to convert all
> > > > > > > > > > > the inline functions in DPDK? If yes, I think we need
> > > > > > > > > > > to consider the performance
> > > > > > > > > > difference. May be consider L3-fwd application, change
> > > > > > > > > > all the
> > > > > > inline functions in its path and run a test?
> > > > > > > > > >
> > > > > > > > > > Every function that is not in the direct datapath should
> > > > > > > > > > not be
> > > > > > inline.
> > > > > > > > > > Exceptions or things like rx/tx burst, ring
> > > > > > > > > > enqueue/dequeue, and packet alloc/free
> > > > > I do not understand how DPDK can claim ABI compatibility if we
> > > > > have
> > > > inline functions (unless we freeze any development in these inline
> > > > functions forever).
> > > > >
> > > > > > > > >
> > > > > > > > > Plus synchronization routines: spin/rwlock/barrier, etc.
> > > > > > > > > I think rcu should be one of such exceptions - it is just
> > > > > > > > > another synchronization mechanism after all (just a bit
> > > > > > > > > more
> > > > sophisticated).
> > > > > > > > > Konstantin
> > > > > > > >
> > > > > > > > If you look at the other userspace RCU, you wil see that the
> > > > > > > > only inlines are the rcu_read_lock,rcu_read_unlock and
> > > > > > rcu_reference/rcu_assign_pointer.
> > > > > > > >
> > > > > > > > The synchronization logic is all real functions.
> > > > > > >
> > > > > > > In fact, I think urcu provides both flavors:
> > > > > > > https://github.com/urcu/userspace-
> > > > > > rcu/blob/master/include/urcu/static/
> > > > > > > urcu-qsbr.h I still don't understand why we have to treat it
> > > > > > > differently then let say spin-lock/ticket-lock or rwlock.
> > > > > > > If we gone all the way to create our own version of rcu, we
> > > > > > > probably want it to be as fast as possible (I know that main
> > > > > > > speedup should come from the fact that readers don't have to
> > > > > > > wait for writer to finish, but still...)
> > > > > > >
> > > > > > > Konstantin
> > > > > > >
> > > > > >
> > > > > > Having locking functions inline is already a problem in current
> > releases.
> > > > > > The implementation can not be improved without breaking ABI (or
> > > > > > doing special workarounds like lock v2)
> > > > > I think ABI and inline function discussion needs to be taken up in
> > > > > a
> > > > different thread.
> > > > >
> > > > > Currently, I am looking to hide the structure visibility. I looked
> > > > > at your
> > > > patch [1], it is a different case than what I have in this patch. It
> > > > is a pretty generic use case as well (similar situation exists in
> > > > other libraries). I think a generic solution should be agreed upon.
> > > > >
> > > > > If we have to hide the structure content, the handle to QS
> > > > > variable
> > > > returned to the application needs to be opaque. I suggest using 'void *'
> > > > behind which any structure can be used.
> > > > >
> > > > > typedef void * rte_rcu_qsbr_t;
> > > > > typedef void * rte_hash_t;
> > > > >
> > > > > But it requires typecasting.
> > > > >
> > > > > [1] http://patchwork.dpdk.org/cover/52609/
> > > >
> > > > C allows structure to be defined without knowing what is in it
> > therefore.
> > > >
> > > > typedef struct rte_rcu_qsbr rte_rcu_qsbr_t;
> > > >
> > > > is preferred (or do it without typedef)
> > > >
> > > > struct rte_rcu_qsbr;
> > >
> > > I see that rte_hash library uses the same approach (struct rte_hash in
> > rte_hash.h, though it is marking as internal). But the ABI Laboratory tool
> > [1] seems to be reporting incorrect numbers for this library even though
> > the internal structure is changed.
> > >
> > > [1]
> > > https://abi-
> > laboratory.pro/index.php?view=compat_report&l=dpdk&v1=19.0
> > > 2&v2=current&obj=66794&kind=abi
> >
> > The problem is rte_hash structure is exposed as part of ABI in
> > rte_cuckoo_hash.h This was a mistake.
> Do you mean, due to the use of structure with the same name? I am wondering if it is just a tools issue. The application is not supposed to
> include rte_cuckoo_hash.h.
> 
> For the RCU library, we either need to go all functions or leave it the way it is. I do not see a point in trying to hide the internal structure
> while having inline functions.
> 
> I converted the inline functions to function calls.
> 
> Testing on Arm platform (results *are* repeatable) shows very minimal drop (0.1% to 0.2%) in performance while using lock-free rte_hash
> data structure. But one of the test cases which is just spinning shows good amount of drop (41%).
> 
> Testing on x86 (Xeon Gold 6132 CPU @ 2.60GHz, results *are* pretty repeatable) shows performance improvements (7% to 8%) while using
> lock-free rte_hash data structure. The test cases which is just spinning show significant drop (14%, 155%, 231%).
> Konstantin, any thoughts on the results?

The fact that function show better result than inline (even for hash) is sort of surprise to me.
Don't have any good explanation off-hand, but the actual numbers for hash test are huge by itself...

In general, I still think that sync primitives better to stay inlined - there is no much point to create ones
and then figure out that no-one using them because they are too slow.
Though if there is no real perf difference between inlined and normal - no point to keep it inlined.
About RCU lib, my thought to have inlined version for 19.05 and do further perf testing with it
(as I remember there were suggestions about using it in l3fwd for guarding routing table or so).
If we'll find there is no real difference - move it to not-inlined version in 19.08.
It is experimental for now  - so could be changed without formal ABI breakage.

 Konstantin