Hi Bruce,
On Tue, 24 Jan 2017 15:50:49 +0000, Bruce Richardson
<bruce.richard...@intel.com> wrote:
> On Tue, Jan 24, 2017 at 04:19:28PM +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
> > functions.
> >
> > Signed-off-by: Olivier Matz <olivier.m...@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 | 45
> > +++++++++++++++++++++++++++++----------- 4 files changed, 38
> > insertions(+), 15 deletions(-)
> <snip>
> > /* compat with older versions */
> > __rte_deprecated
> > -static inline void __attribute__((always_inline))
> > +static inline void
> > __rte_mbuf_raw_free(struct rte_mbuf *m)
> > {
> > rte_mbuf_raw_free(m);
> > @@ -1218,8 +1232,12 @@ static inline void rte_pktmbuf_detach(struct
> > rte_mbuf *m) m->data_len = 0;
> > m->ol_flags = 0;
> >
> > - if (rte_mbuf_refcnt_update(md, -1) == 0)
> > + if (rte_mbuf_refcnt_update(md, -1) == 0) {
>
> Minor nit, but in the case that we only have a single reference to the
> mbufs, we are always setting that to zero just to re-increment it to 1
> again.
>
> > + md->next = NULL;
> > + md->nb_segs = 1;
> > + rte_mbuf_refcnt_set(md, 1);
> > rte_mbuf_raw_free(md);
> > + }
> > }
> >
> > /**
I'm trying to gather the comments that have been made on this patchset.
About this one, I think it would be more complex to change the code
to avoid to set the refcnt twice:
- we would need to duplicate code from rte_mbuf_refcnt_update(), which
I think is not a very good idea, due to the big comment
- it would make the detach code less readable
- it's even not sure that it will be more performant: since
rte_mbuf_refcnt_update() is inline, the compiler is probably able to
do the simplification by itself.
Olivier
