[dpdk-dev] [PATCH 3/4] hash: fix rw concurrency while moving keys

Honnappa Nagarahalli Honnappa.Nagarahalli at arm.com
Thu Oct 4 05:32:08 CEST 2018


> >
> >> >-----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.
> >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.
> [Wang, Yipeng]
> I think with ABI/API change, you might need to announce it one release cycle
> ahead.
> 
> In the cuckoo switch paper: Scalable, High Performance Ethernet Forwarding
> with CUCKOOSWITCH it separates the version counter array and the hash
> table. You can strike a balance between granularity of the version counter and
> the cache/memory requirement.
> Is it a better way?
This will introduce another cache line access. It would be good to stay within the single cacheline.

> 
> Another consideration is current bucket is 64-byte exactly with the partial-
> key-hashing.
> To add another counter, we need to think about changing certain variables to
> still align cache line.
The 'flags' structure member is not being used. I plan to remove that. That will give us 8B, I will use 4B out of it for the counter.


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