-----Original Message-----
From: Richardson, Bruce
Sent: Friday, March 31, 2017 12:22 PM
To: Olivier Matz <redacted>
Cc: dev@dpdk.org; Ananyev, Konstantin <redacted>; mb@smartsharesystems.com; Chilikin, Andrey
[off-list ref]; jblunck@infradead.org; nelio.laranjeiro@6wind.com; arybchenko@solarflare.com
Subject: Re: [PATCH 3/9] mbuf: set mbuf fields while in pool

On Wed, Mar 08, 2017 at 10:41:55AM +0100, Olivier Matz wrote:

quoted

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 <redacted>
---
 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>

quoted

 /**

@@ -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) == 0)) {
-		/* if this is an indirect mbuf, it is detached. */
 		if (RTE_MBUF_INDIRECT(m))
 			rte_pktmbuf_detach(m);
+
+		m->next = NULL;
+		m->nb_segs = 1;
+		rte_mbuf_refcnt_set(m, 1);
+
 		return m;
 	}
 	return NULL;

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