Re: [PATCH] lib/hash,lib/rcu: feature hidden key count in hash

["Abdullah Ömer Yamaç" <aomeryamac@gmail.com>]

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I appreciate that you gave me suggestions and comments. I will make changes
according to all your recommendations, but before that, I want to make
everyone's minds clear. Then, I will apply modifications.

On Tue, Feb 20, 2024 at 2:35 AM Honnappa Nagarahalli <
Honnappa.Nagarahalli@arm.com> wrote:

>
>
> > On Feb 19, 2024, at 3:28 PM, Abdullah Ömer Yamaç <aomeryamac@gmail.com>
> wrote:
> >
> > Hello,
> >
> > Let me explain a use case;
> >
> > I have a hash table whose key value is IP addresses, and data (let's say
> the username of the IP) is related to the IP address. The key point is
> matching these data with flows. Flows are dynamic, and this hash table is
> dynamic, as well; both can change anytime. For example, when a flow starts,
> we look up the hash table with the corresponding IP and retrieve the
> username. We need to hold this username until the flow terminates, although
> we removed this IP key from the hash table (multithread). That's why we
> have RCU and defer queue is necessary for high performance. In my
> application, I need to know the number of IP-username entries. These
> numbers can be calculated by rte_hash_count - defer queue size.
> The entries in the defer queue are not reclaimed (there is a probability
> that all of them can be reclaimed) and hence they are not available for
> allocation. So, rte_hash_count - defer queue size might not give you the
> correct number you are expecting.
>
> Currently, there is no API in hash library that forces a reclaim. Does it
> makes sense to have an API that just does the reclaim (and returns the
> number of entries pending in the defer queue)? A call to rte_hash_count
> should provide the exact count you are looking for.

You are right; no API in the hash library forces a reclaim. In my
application, I periodically call rte_count to retrieve hash size, and this
data is shown in my GUI. So that means I need to call regularly reclaim. I
am trying to figure out which is better, calling reclaim or retrieving the
defer queue size. Any comment about this?

> >
> > I think if you need a non-blocking and multithreaded hash table, an
> RCU-enabled hash table is necessary. Also, this API is necessary if you
> need to get the actual matchable size.
> >
> >
> >
> >
> >
> > On Mon, Feb 19, 2024 at 8:36 PM Medvedkin, Vladimir <
> vladimir.medvedkin@intel.com> wrote:
> > Hi Abdullah,
> >
> > Could you please tell more about use cases where this API may be useful?
> >
> > >a new API to get the hidden key count in the hash table if the rcu qsbr
> is enabled
> >
> > Here in commit message and down below in doxygen comments, I think this
> > statement should be more specific because rcu can be created with
> > RTE_HASH_QSBR_MODE_SYNC mode i.e. without defer queue.
> >
> > Also, new API must be reflected in release notes
> >
> > On 07/02/2024 15:33, Abdullah Ömer Yamaç wrote:
> > > This patch introduce a new API to get the hidden key count in the hash
> > > table if the rcu qsbr is enabled. When using rte_hash_count with rcu
> > > qsbr enabled, it will return the number of elements that are not in the
> > > free queue. Unless rte_rcu_qsbr_dq_reclaim is called, the number of
> > > elements in the defer queue will not be counted and freed. Therefore I
> > > added a new API to get the number of hidden (defer queue) elements
> > > in the hash table. Then the user can calculate the total number of
> > > elements that are available in the hash table.
> > >
> > > Signed-off-by: Abdullah Ömer Yamaç <aomeryamac@gmail.com>
> > >
> > > ---
> > > Cc: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > > Cc: Yipeng Wang <yipeng1.wang@intel.com>
> > > Cc: Sameh Gobriel <sameh.gobriel@intel.com>
> > > Cc: Bruce Richardson <bruce.richardson@intel.com>
> > > Cc: Vladimir Medvedkin <vladimir.medvedkin@intel.com>
> > > ---
> > >   lib/hash/rte_cuckoo_hash.c |  9 +++++++++
> > >   lib/hash/rte_hash.h        | 13 +++++++++++++
> > >   lib/hash/version.map       |  1 +
> > >   lib/rcu/rte_rcu_qsbr.c     |  8 ++++++++
> > >   lib/rcu/rte_rcu_qsbr.h     | 11 +++++++++++
> > >   lib/rcu/version.map        |  1 +
> > >   6 files changed, 43 insertions(+)
> > >
> > > diff --git a/lib/hash/rte_cuckoo_hash.c b/lib/hash/rte_cuckoo_hash.c
> > > index 70456754c4..3553f3efc7 100644
> > > --- a/lib/hash/rte_cuckoo_hash.c
> > > +++ b/lib/hash/rte_cuckoo_hash.c
> > > @@ -555,6 +555,15 @@ rte_hash_max_key_id(const struct rte_hash *h)
> > >               return h->entries;
> > >   }
> > >
> > > +int32_t
> > > +rte_hash_dq_count(const struct rte_hash *h)
> > > +{
> > > +     if (h->dq == NULL)
> > input arguments must be checked since this is a public API, the same is
> > true for rte_rcu_qsbr_dq_count()
> > > +             return -EINVAL;
> > why not just return 0?
> > > +
> > > +     return rte_rcu_qsbr_dq_count(h->dq);
> > > +}
> > > +
> > >   int32_t
> > >   rte_hash_count(const struct rte_hash *h)
> > >   {
> > > diff --git a/lib/hash/rte_hash.h b/lib/hash/rte_hash.h
> > > index 7ecc021111..8ea97e297d 100644
> > > --- a/lib/hash/rte_hash.h
> > > +++ b/lib/hash/rte_hash.h
> > > @@ -193,6 +193,19 @@ rte_hash_free(struct rte_hash *h);
> > >   void
> > >   rte_hash_reset(struct rte_hash *h);
> > >
> > > +
> > > +/**
> > > + * Return the number of records in the defer queue of the hash table
> > > + * if RCU is enabled.
> > > + * @param h
> > > + *  Hash table to query from
> > > + * @return
> > > + *   - -EINVAL if parameters are invalid
> > > + *   - A value indicating how many records were inserted in the table.
> > did you mean how many records are kept in defer queue?
> > > + */
> > > +int32_t
> > > +rte_hash_dq_count(const struct rte_hash *h);
> > > +
> > >   /**
> > >    * Return the number of keys in the hash table
> > >    * @param h
> > > diff --git a/lib/hash/version.map b/lib/hash/version.map
> > > index 6b2afebf6b..7f7b158cf1 100644
> > > --- a/lib/hash/version.map
> > > +++ b/lib/hash/version.map
> > > @@ -9,6 +9,7 @@ DPDK_24 {
> > >       rte_hash_add_key_with_hash;
> > >       rte_hash_add_key_with_hash_data;
> > >       rte_hash_count;
> > > +     rte_hash_dq_count;
> > new API must introduced as an experimental API. The same is true for
> > rte_rcu_qsbr_dq_count()
> > >       rte_hash_crc32_alg;
> > >       rte_hash_crc_set_alg;
> > >       rte_hash_create;
> > > diff --git a/lib/rcu/rte_rcu_qsbr.c b/lib/rcu/rte_rcu_qsbr.c
> > > index bd0b83be0c..89f8da4c4c 100644
> > > --- a/lib/rcu/rte_rcu_qsbr.c
> > > +++ b/lib/rcu/rte_rcu_qsbr.c
> > > @@ -450,6 +450,14 @@ rte_rcu_qsbr_dq_reclaim(struct rte_rcu_qsbr_dq
> *dq, unsigned int n,
> > >       return 0;
> > >   }
> > >
> > > +/**
> > > + * Return the number of entries in a defer queue.
> > > + */
> > > +unsigned int rte_rcu_qsbr_dq_count(struct rte_rcu_qsbr_dq *dq)
> > > +{
> Please validate dq here.
>
> > > +     return rte_ring_count(dq->r);
> > > +}
> > > +
> > >   /* Delete a defer queue. */
> > >   int
> > >   rte_rcu_qsbr_dq_delete(struct rte_rcu_qsbr_dq *dq)
> > > diff --git a/lib/rcu/rte_rcu_qsbr.h b/lib/rcu/rte_rcu_qsbr.h
> > > index 23c9f89805..ed5a590edd 100644
> > > --- a/lib/rcu/rte_rcu_qsbr.h
> > > +++ b/lib/rcu/rte_rcu_qsbr.h
> > > @@ -794,6 +794,17 @@ int
> > >   rte_rcu_qsbr_dq_reclaim(struct rte_rcu_qsbr_dq *dq, unsigned int n,
> > >       unsigned int *freed, unsigned int *pending, unsigned int
> *available);
> > >
> > > +/**
> > > + * Return the number of entries in a defer queue.
> > > + *
> > > + * @param dq
> > > + *   Defer queue.
> > > + * @return
> > > + *   The number of entries in the defer queue.
> > > + */
> > > +unsigned int
> > > +rte_rcu_qsbr_dq_count(struct rte_rcu_qsbr_dq *dq);
> Agree on the need for this API in RCU
>
> > > +
> > >   /**
> > >    * Delete a defer queue.
> > >    *
> > > diff --git a/lib/rcu/version.map b/lib/rcu/version.map
> > > index 982ffd59d9..f410ab41e7 100644
> > > --- a/lib/rcu/version.map
> > > +++ b/lib/rcu/version.map
> > > @@ -5,6 +5,7 @@ DPDK_24 {
> > >       rte_rcu_qsbr_dq_create;
> > >       rte_rcu_qsbr_dq_delete;
> > >       rte_rcu_qsbr_dq_enqueue;
> > > +     rte_rcu_qsbr_dq_count;
> > >       rte_rcu_qsbr_dq_reclaim;
> > >       rte_rcu_qsbr_dump;
> > >       rte_rcu_qsbr_get_memsize;
> >
> > --
> > Regards,
> > Vladimir
> >
>
>

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