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

Wang, Yipeng1 yipeng1.wang at intel.com
Thu Oct 4 21:16:17 CEST 2018


>-----Original Message-----
>From: Honnappa Nagarahalli [mailto:Honnappa.Nagarahalli at arm.com]
>Sent: Wednesday, October 3, 2018 8:54 PM
>To: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>; Wang, Yipeng1 <yipeng1.wang at intel.com>; Van Haaren, Harry
><harry.van.haaren at intel.com>; Richardson, Bruce <bruce.richardson at intel.com>
>Cc: De Lara Guarch, Pablo <pablo.de.lara.guarch at intel.com>; dev at dpdk.org; Gavin Hu (Arm Technology China)
><Gavin.Hu at arm.com>; Steve Capper <Steve.Capper at arm.com>; Ola Liljedahl <Ola.Liljedahl at arm.com>; nd <nd at arm.com>; Gobriel,
>Sameh <sameh.gobriel at 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.


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