RE: [PATCH v2] mbuf: add fast free bulk function

["Morten Brørup" <mb@smartsharesystems.com>]

> From: huangdengdui [mailto:huangdengdui@huawei.com]
> Sent: Wednesday, 15 January 2025 07.52
> 
> On 2025/1/15 0:39, Morten Brørup wrote:
> > mbuf: add fast free bulk function
> >
> > When putting an mbuf back into its mempool, there are certain
> requirements
> > to the mbuf. Specifically, some of its fields must be initialized.
> >
> > These requirements are in fact invariants about free mbufs, held in
> > mempools, and thus also apply when allocating an mbuf from a mempool.
> > With this in mind, the additional assertions in rte_mbuf_raw_free()
> were
> > moved to __rte_mbuf_raw_sanity_check().
> > Furthermore, the assertion regarding pinned external buffer was
> enhanced;
> > it now also asserts that the referenced pinned external buffer has
> > refcnt == 1.
> >
> > The description of RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE was updated to
> > include the remaining requirements, which were missing here.
> >
> > And finally:
> > A new rte_mbuf_fast_free_bulk() inline function was added for the
> benefit
> > of ethdev drivers supporting fast release of mbufs.
> > It asserts these requirements and that the mbufs belong to the
> specified
> > mempool, and then calls rte_mempool_put_bulk().
> >
> > Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> > ---
> > v2:
> > * Fixed missing inline.
> > ---
> >  lib/ethdev/rte_ethdev.h |  6 ++++--
> >  lib/mbuf/rte_mbuf.h     | 39 +++++++++++++++++++++++++++++++++++++--
> >  2 files changed, 41 insertions(+), 4 deletions(-)
> >
> > diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
> > index 1f71cad244..e9267fca79 100644
> > --- a/lib/ethdev/rte_ethdev.h
> > +++ b/lib/ethdev/rte_ethdev.h
> > @@ -1612,8 +1612,10 @@ struct rte_eth_conf {
> >  #define RTE_ETH_TX_OFFLOAD_MULTI_SEGS       RTE_BIT64(15)
> >  /**
> >   * Device supports optimization for fast release of mbufs.
> > - * When set application must guarantee that per-queue all mbufs
> comes from
> > - * the same mempool and has refcnt = 1.
> > + * When set application must guarantee that per-queue all mbufs come
> from the same mempool,
> > + * are direct, have refcnt=1, next=NULL and nb_segs=1, as done by
> rte_pktmbuf_prefree_seg().
> > + *
> > + * @see rte_mbuf_fast_free_bulk()
> >   */
> >  #define RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE   RTE_BIT64(16)
> >  #define RTE_ETH_TX_OFFLOAD_SECURITY         RTE_BIT64(17)
> > diff --git a/lib/mbuf/rte_mbuf.h b/lib/mbuf/rte_mbuf.h
> > index 0d2e0e64b3..7590d82689 100644
> > --- a/lib/mbuf/rte_mbuf.h
> > +++ b/lib/mbuf/rte_mbuf.h
> > @@ -568,6 +568,10 @@ __rte_mbuf_raw_sanity_check(__rte_unused const
> struct rte_mbuf *m)
> >  	RTE_ASSERT(rte_mbuf_refcnt_read(m) == 1);
> >  	RTE_ASSERT(m->next == NULL);
> >  	RTE_ASSERT(m->nb_segs == 1);
> > +	RTE_ASSERT(!RTE_MBUF_CLONED(m));
> > +	RTE_ASSERT(!RTE_MBUF_HAS_EXTBUF(m) ||
> > +			(RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
> > +			rte_mbuf_ext_refcnt_read(m->shinfo) == 1));
> >  	__rte_mbuf_sanity_check(m, 0);
> >  }
> >
> > @@ -623,12 +627,43 @@ static inline struct rte_mbuf
> *rte_mbuf_raw_alloc(struct rte_mempool *mp)
> >  static __rte_always_inline void
> >  rte_mbuf_raw_free(struct rte_mbuf *m)
> >  {
> > -	RTE_ASSERT(!RTE_MBUF_CLONED(m) &&
> > -		  (!RTE_MBUF_HAS_EXTBUF(m) ||
> RTE_MBUF_HAS_PINNED_EXTBUF(m)));
> >  	__rte_mbuf_raw_sanity_check(m);
> >  	rte_mempool_put(m->pool, m);
> >  }
> >
> > +/**
> > + * Put a bulk of mbufs allocated from the same mempool back into the
> mempool.
> > + *
> > + * The caller must ensure that the mbufs come from the specified
> mempool,
> > + * are direct and properly reinitialized (refcnt=1, next=NULL,
> nb_segs=1), as done by
> > + * rte_pktmbuf_prefree_seg().
> > + *
> > + * This function should be used with care, when optimization is
> > + * required. For standard needs, prefer rte_pktmbuf_free_bulk().
> > + *
> > + * @see RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE
> > + *
> > + * @param mp
> > + *   The mempool to which the mbufs belong.
> > + * @param mbufs
> > + *   Array of pointers to packet mbufs.
> > + *   The array must not contain NULL pointers.
> > + * @param count
> > + *   Array size.
> > + */
> > +static __rte_always_inline void
> > +rte_mbuf_fast_free_bulk(struct rte_mempool *mp, struct rte_mbuf
> **mbufs, unsigned int count)
> > +{
> > +	for (unsigned int idx = 0; idx < count; idx++) {
> > +		const struct rte_mbuf *m = mbufs[idx];
> > +		RTE_ASSERT(m != NULL);
> > +		RTE_ASSERT(m->pool == mp);
> > +		__rte_mbuf_raw_sanity_check(m);
> > +	}
> 
> Is there some way to avoid executing a loop in non-debug mode? Like the
> following or other better way
> 
> #ifdef RTE_LIBRTE_MBUF_DEBUG
> 	{
> 		for (unsigned int idx = 0; idx < count; idx++) {
> 			const struct rte_mbuf *m = mbufs[idx];
> 			RTE_ASSERT(m != NULL);
> 			RTE_ASSERT(m->pool == mp);
> 			__rte_mbuf_raw_sanity_check(m);
> 		}
> 	}
> #endif

The loop is already omitted in non-debug mode:
RTE_ASSERT() [1] is omitted unless RTE_ENABLE_ASSERT is set.
__rte_mbuf_raw_sanity_check() [2] consist of some RTE_ASSERTs and __rte_mbuf_sanity_check().
__rte_mbuf_sanity_check() [3] is omitted unless RTE_LIBRTE_MBUF_DEBUG is set.

So the compiler will detect that the loop has no effect, and optimize away the loop.

[1]: https://elixir.bootlin.com/dpdk/v24.11.1/source/lib/eal/include/rte_debug.h#L46
[2]: https://elixir.bootlin.com/dpdk/v24.11.1/source/lib/mbuf/rte_mbuf.h#L566
[3]: https://elixir.bootlin.com/dpdk/v24.11.1/source/lib/mbuf/rte_mbuf.h#L348

> 
> > +
> > +	rte_mempool_put_bulk(mp, (void **)mbufs, count);
> 
> Can the mp be obtained from the mbuf?

Passing "mp" as a parameter avoids a potential CPU cache miss by not dereferencing the first mbuf, if the driver/application already has the mempool pointer readily available (and hot in the CPU cache) from somewhere else.
If the driver/or application doesn't have the mempool pointer readily available, it can obtain it from the mbuf when calling this function.

And as a bonus side effect, passing "mp" as a parameter allows calling the function with count=0 without special handling inside the function.

Obviously, if the driver/application gets "mp" from mbuf[0]->pool, it needs to first check that count>0; but that would be the situation for the driver/application whenever it accesses an mbuf array, regardless what it is doing with that array.