[PATCH v8 3/3] mbuf: optimize reset of reinitialized mbufs

[Morten Brørup]

> From: Rahul Bhansali [mailto:rbhansali at marvell.com]
> Sent: Friday, 6 March 2026 13.19
> 
> Please see inline.
> 
> > From: Bruce Richardson <bruce.richardson at intel.com>
> > Sent: Monday, October 20, 2025 2:17 PM
> >
> > On Sun, Oct 19, 2025 at 01: 45: 45PM -0700, Stephen Hemminger wrote:
> > On Thu, 9 Oct 2025 18: 15: 12 +0100 > Bruce Richardson
> > <bruce. richardson@ intel. com> wrote: > > > On Sat, Aug 23, 2025 at
> 06: 30: 02AM +0000, Morten Brørup
> > On Sun, Oct 19, 2025 at 01:45:45PM -0700, Stephen Hemminger wrote:
> > > On Thu, 9 Oct 2025 18:15:12 +0100
> > > Bruce Richardson <mailto:bruce.richardson at intel.com> wrote:
> > >
> > > > On Sat, Aug 23, 2025 at 06:30:02AM +0000, Morten Brørup wrote:
> > > > > An optimized function for resetting a bulk of newly allocated
> > > > > reinitialized mbufs (a.k.a. raw mbufs) was added.
> > > > >
> > > > > Compared to the normal packet mbuf reset function, it takes
> advantage of
> > > > > the following two details:
> > > > > 1. The 'next' and 'nb_segs' fields are already reset, so
> resetting them
> > > > > has been omitted.
> > > > > 2. When resetting the mbuf, the 'ol_flags' field must indicate
> whether the
> > > > > mbuf uses an external buffer, and the 'data_off' field must not
> exceed the
> > > > > data room size when resetting the data offset to include the
> default
> > > > > headroom.
> > > > > Unlike the normal packet mbuf reset function, which reads the
> mbuf itself
> > > > > to get the information required for resetting these two fields,
> this
> > > > > function gets the information from the mempool.
> > > > >
> > > > > This makes the function write-only of the mbuf, unlike the
> normal packet
> > > > > mbuf reset function, which is read-modify-write of the mbuf.
> > > > >
> > > > > Signed-off-by: Morten Brørup <mailto:mb at smartsharesystems.com>
> > > > > ---
> > > > >  lib/mbuf/rte_mbuf.h | 74 ++++++++++++++++++++++++++++---------
> --------
> > > > >  1 file changed, 46 insertions(+), 28 deletions(-)
> > > > >
> > > > > diff --git a/lib/mbuf/rte_mbuf.h b/lib/mbuf/rte_mbuf.h
> > > > > index 49c93ab356..6f37a2e91e 100644
> > > > > --- a/lib/mbuf/rte_mbuf.h
> > > > > +++ b/lib/mbuf/rte_mbuf.h
> > > > > @@ -954,6 +954,50 @@ static inline void
> rte_pktmbuf_reset_headroom(struct rte_mbuf *m)
> > > > >  					(uint16_t)m->buf_len);
> > > > >  }
> > > > >
> > > > > +/**
> > > > > + * Reset the fields of a bulk of packet mbufs to their default
> values.
> > > > > + *
> > > > > + * The caller must ensure that the mbufs come from the
> specified mempool,
> > > > > + * are direct and properly reinitialized (refcnt=1, next=NULL,
> nb_segs=1),
> 
> [Rahul] For Marvell's CNxx SoCs, mbuf pointers alloc and free are
> offloaded to HW for Rx/Tx, so these fields "next and nb_segs" will not
> be reset to default values by HW.
> When packets are coming from wire, we reset these fields in Rx
> fastpath, but in case of SW allocated mbuf, we cannot do it in
> Marvell's mempool driver as that is unaware of mbuf.

It has always been an invariant that mbufs stored in a mempool have their "next" and "nb_segs" fields reset.
This means that these fields must be reset before free.

In an ethdev driver's normal Tx path, the driver calls rte_pktmbuf_prefree_seg() before freeing an mbuf.
Does your ethdev driver not do that?

> Is it possible to reset these also in rte_mbuf_raw_reset_bulk() itself
> for mbuf alloc requests ?

Due to the invariant (about mbufs stored in a mempool having their "next" and "nb_segs" fields reset), resetting them again in rte_mbuf_raw_reset_bulk() after fetching the mbufs from the mempool (i.e. after calling rte_mempool_get_bulk()) is considered unnecessary.

PS:
I wish for a roadmap towards eliminating this invariant, and instead require the ethdev drivers to reset the "nb_segs" and "next" fields in the Rx fastpath instead - where the driver is initializing many other mbuf fields anyway, and the additional cost is near-zero.
One of the steps in such a roadmap could be to reset the "nb_segs" and "next" fields in the rte_mbuf_raw_reset_bulk() function, for ethdev drivers which hasn't implemented it yet.

> 
> > > > > + * as done by rte_pktmbuf_prefree_seg().
> > > > > + *
> > > > > + * This function should be used with care, when optimization
> is required.
> > > > > + * For standard needs, prefer rte_pktmbuf_reset().
> > > > > + *
> > > > > + * @param mp
> > > > > + *   The mempool to which the mbuf belongs.
> > > > > + * @param mbufs
> > > > > + *   Array of pointers to packet mbufs.
> > > > > + *   The array must not contain NULL pointers.
> > > > > + * @param count
> > > > > + *   Array size.
> > > > > + */
> > > > > +static inline void
> > > > > +rte_mbuf_raw_reset_bulk(struct rte_mempool *mp, struct
> rte_mbuf **mbufs, unsigned int count)
> > > > > +{
> > > > > +	uint64_t ol_flags = (rte_pktmbuf_priv_flags(mp) &
> RTE_PKTMBUF_POOL_F_PINNED_EXT_BUF) ?
> > > > > +			RTE_MBUF_F_EXTERNAL : 0;
> > > > > +	uint16_t data_off = RTE_MIN_T(RTE_PKTMBUF_HEADROOM,
> rte_pktmbuf_data_room_size(mp),
> > > > > +			uint16_t);
> > > > > +
> > > > > +	for (unsigned int idx = 0; idx < count; idx++) {
> > > > > +		struct rte_mbuf *m = mbufs[idx];
> > > > > +
> > > > > +		m->pkt_len = 0;
> > > > > +		m->tx_offload = 0;
> > > > > +		m->vlan_tci = 0;
> > > > > +		m->vlan_tci_outer = 0;
> > > > > +		m->port = RTE_MBUF_PORT_INVALID;
> > > >
> > > > Have you considered doing all initialization using 64-bit stores?
> It's
> > > > generally cheaper to do a single 64-bit store than e.g. set of
> 16-bit ones.
> > > > This also means that we could remove the restriction on having
> refcnt and
> > > > nb_segs already set. As in PMDs, a single store can init
> data_off, ref_cnt,
> > > > nb_segs and port.
> > > >
> > > > Similarly for packet_type and pkt_len, and data_len/vlan_tci and
> rss fields
> > > > etc. For max performance, the whole of the mbuf cleared here can
> be done in
> > > > 40 bytes, or 5 64-bit stores. If we do the stores in order,
> possibly the
> > > > compiler can even opportunistically coalesce more stores, so we
> could even
> > > > end up getting 128-bit or larger stores depending on the ISA
> compiled for.
> > > > [Maybe the compiler will do this even if they are not in order,
> but I'd
> > > > like to maximize my chances here! :-)]
> > > >
> > > > /Bruce
> > >
> > > Although it is possible to use less CPU instructions, the
> performance
> > > limiting factor is which fields are in cache.
> >
> > Yes, the cache presence of the target of the stores has a massive
> effect on
> > how well the code will perform. However, the number of stores can
> make a
> > difference too - especially if you are in store-heavy code. Consider
> the
> > number of store operations which would be generated by storing
> > field-by-field to a burst of 32 packets. With the previous work we
> have
> > done on our PMDs, and vectorizing them, we got a noticible benefit
> from
> > doing larger vector stores compared to smaller ones!
> >
> > /Bruce