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From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
To: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
"stephen@networkplumber.org" <stephen@networkplumber.org>,
"paulmck@linux.ibm.com" <paulmck@linux.ibm.com>
CC: "Wang, Yipeng1" <yipeng1.wang@intel.com>, "Medvedkin, Vladimir"
<vladimir.medvedkin@intel.com>, "Ruifeng Wang (Arm Technology China)"
<Ruifeng.Wang@arm.com>, Dharmik Thakkar <Dharmik.Thakkar@arm.com>,
"dev@dpdk.org" <dev@dpdk.org>, nd <nd@arm.com>, nd <nd@arm.com>
Thread-Topic: [PATCH v3 1/3] lib/ring: add peek API
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Subject: Re: [dpdk-dev] [PATCH v3 1/3] lib/ring: add peek API
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> > Subject: [PATCH v3 1/3] lib/ring: add peek API
> >
> > From: Ruifeng Wang <ruifeng.wang@arm.com>
> >
> > The peek API allows fetching the next available object in the ring
> > without dequeuing it. This helps in scenarios where dequeuing of
> > objects depend on their value.
> >
> > Signed-off-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
> > Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> > ---
> > lib/librte_ring/rte_ring.h | 30 ++++++++++++++++++++++++++++++
> > 1 file changed, 30 insertions(+)
> >
> > diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
> > index 2a9f768a1..d3d0d5e18 100644
> > --- a/lib/librte_ring/rte_ring.h
> > +++ b/lib/librte_ring/rte_ring.h
> > @@ -953,6 +953,36 @@ rte_ring_dequeue_burst(struct rte_ring *r, void
> **obj_table,
> > r->cons.single, available);
> > }
> >
> > +/**
> > + * Peek one object from a ring.
> > + *
> > + * The peek API allows fetching the next available object in the ring
> > + * without dequeuing it. This API is not multi-thread safe with
> > +respect
> > + * to other consumer threads.
> > + *
> > + * @param r
> > + * A pointer to the ring structure.
> > + * @param obj_p
> > + * A pointer to a void * pointer (object) that will be filled.
> > + * @return
> > + * - 0: Success, object available
> > + * - -ENOENT: Not enough entries in the ring.
> > + */
> > +__rte_experimental
> > +static __rte_always_inline int
> > +rte_ring_peek(struct rte_ring *r, void **obj_p)
>=20
> As it is not MT safe, then I think we need _sc_ in the name, to follow ot=
her
> rte_ring functions naming conventions
> (rte_ring_sc_peek() or so).
Agree
>=20
> As a better alternative what do you think about introducing a serialized
> versions of DPDK rte_ring dequeue functions?
> Something like that:
>=20
> /* same as original ring dequeue, but:
> * 1) move cons.head only if cons.head =3D=3D const.tail
> * 2) don't update cons.tail
> */
> unsigned int
> rte_ring_serial_dequeue_bulk(struct rte_ring *r, void **obj_table, unsign=
ed
> int n,
> unsigned int *available);
>=20
> /* sets both cons.head and cons.tail to cons.head + num */ void
> rte_ring_serial_dequeue_finish(struct rte_ring *r, uint32_t num);
>=20
> /* resets cons.head to const.tail value */ void
> rte_ring_serial_dequeue_abort(struct rte_ring *r);
>=20
> Then your dq_reclaim cycle function will look like that:
>=20
> const uint32_t nb_elt =3D dq->elt_size/8 + 1; uint32_t avl, n; uintptr_t
> elt[nb_elt]; ...
>=20
> do {
>=20
> /* read next elem from the queue */
> n =3D rte_ring_serial_dequeue_bulk(dq->r, elt, nb_elt, &avl);
> if (n =3D=3D 0)
> break;
>=20
> /* wrong period, keep elem in the queue */ if (rte_rcu_qsbr_check(dr->v=
,
> elt[0]) !=3D 1) {
> rte_ring_serial_dequeue_abort(dq->r);
> break;
> }
>=20
> /* can reclaim, remove elem from the queue */
> rte_ring_serial_dequeue_finish(dr->q, nb_elt);
>=20
> /*call reclaim function */
> dr->f(dr->p, elt);
>=20
> } while (avl >=3D nb_elt);
>=20
> That way, I think even rte_rcu_qsbr_dq_reclaim() can be MT safe.
> As long as actual reclamation callback itself is MT safe of course.
I think it is a great idea. The other writers would still be polling for th=
e current writer to update the tail or update the head. This makes it a blo=
cking solution.
We can make the other threads not poll i.e. they will quit reclaiming if th=
ey see that other writers are dequeuing from the queue. The other way is to=
use per thread queues.
The other requirement I see is to support unbounded-size data structures wh=
ere in the data structures do not have a pre-determined number of entries. =
Also, currently the defer queue size is equal to the total number of entrie=
s in a given data structure. There are plans to support dynamically resizab=
le defer queue. This means, memory allocation which will affect the lock-fr=
ee-ness of the solution.
So, IMO:
1) The API should provide the capability to support different algorithms - =
may be through some flags?
2) The requirements for the ring are pretty unique to the problem we have h=
ere (for ex: move the cons-head only if cons-tail is also the same, skip po=
lling). So, we should probably implement a ring with-in the RCU library?
>From the timeline perspective, adding all these capabilities would be diffi=
cult to get done with in 19.11 timeline. What I have here satisfies my curr=
ent needs. I suggest that we make provisions in APIs now to support all the=
se features, but do the implementation in the coming releases. Does this so=
und ok for you?
>=20
> > +{
> > + uint32_t prod_tail =3D r->prod.tail;
> > + uint32_t cons_head =3D r->cons.head;
> > + uint32_t count =3D (prod_tail - cons_head) & r->mask;
> > + unsigned int n =3D 1;
> > + if (count) {
> > + DEQUEUE_PTRS(r, &r[1], cons_head, obj_p, n, void *);
> > + return 0;
> > + }
> > + return -ENOENT;
> > +}
> > +
> > #ifdef __cplusplus
> > }
> > #endif
> > --
> > 2.17.1