From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
Subject: Re: [PATCH 3/9] mbuf: set mbuf fields while in pool
Date: Fri, 31 Mar 2017 11:51:13 +0000
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Cc: "dev@dpdk.org" <dev@dpdk.org>, "mb@smartsharesystems.com"
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To: "Richardson, Bruce" <bruce.richardson@intel.com>, Olivier Matz
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> -----Original Message-----
> From: Richardson, Bruce
> Sent: Friday, March 31, 2017 12:22 PM
> To: Olivier Matz <olivier.matz@6wind.com>
> Cc: dev@dpdk.org; Ananyev, Konstantin <konstantin.ananyev@intel.com>; mb@=
smartsharesystems.com; Chilikin, Andrey
> <andrey.chilikin@intel.com>; jblunck@infradead.org; nelio.laranjeiro@6win=
d.com; arybchenko@solarflare.com
> Subject: Re: [PATCH 3/9] mbuf: set mbuf fields while in pool
>=20
> On Wed, Mar 08, 2017 at 10:41:55AM +0100, Olivier Matz wrote:
> > Set the value of m->refcnt to 1, m->nb_segs to 1 and m->next
> > to NULL when the mbuf is stored inside the mempool (unused).
> > This is done in rte_pktmbuf_prefree_seg(), before freeing or
> > recycling a mbuf.
> >
> > Before this patch, the value of m->refcnt was expected to be 0
> > while in pool.
> >
> > The objectives are:
> >
> > - to avoid drivers to set m->next to NULL in the early Rx path, since
> >   this field is in the second 64B of the mbuf and its access could
> >   trigger a cache miss
> >
> > - rationalize the behavior of raw_alloc/raw_free: one is now the
> >   symmetric of the other, and refcnt is never changed in these function=
s.
> >
> > Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> > ---
> >  drivers/net/mlx5/mlx5_rxtx.c     |  5 ++---
> >  drivers/net/mpipe/mpipe_tilegx.c |  1 +
> >  lib/librte_mbuf/rte_mbuf.c       |  2 ++
> >  lib/librte_mbuf/rte_mbuf.h       | 42 +++++++++++++++++++++++++++++---=
--------
> >  4 files changed, 36 insertions(+), 14 deletions(-)
> >
> <snip>
> >  /**
> > @@ -1244,9 +1262,13 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> >  	__rte_mbuf_sanity_check(m, 0);
> >
> >  	if (likely(rte_mbuf_refcnt_update(m, -1) =3D=3D 0)) {
> > -		/* if this is an indirect mbuf, it is detached. */
> >  		if (RTE_MBUF_INDIRECT(m))
> >  			rte_pktmbuf_detach(m);
> > +
> > +		m->next =3D NULL;
> > +		m->nb_segs =3D 1;
> > +		rte_mbuf_refcnt_set(m, 1);
> > +
> >  		return m;
> >  	}
> >  	return NULL;
>=20
> Do we need to make this change to prefree_seg? If we update the detach
> function to set the next point to null on detaching a segment, and if we
> change the "free" function which frees a whole chain of mbufs, we should
> be covered, should we not? If we are freeing a standalone segment, that
> segment should already have it's nb_segs and next pointers correct.

detach() is invoked only for indirect mbufs.
We can have a chain of direct mbufs too.
About free() - most PMD use either rte_pktmbuf_free_seg()
or rte_pktmbuf_prefree_seg();rte_mempool_put_bulk(); directly.
Konstantin