From mboxrd@z Thu Jan  1 00:00:00 1970
From: Bruce Richardson <bruce.richardson@intel.com>
Subject: Re: [RFC 3/8] mbuf: set mbuf fields while in pool
Date: Tue, 24 Jan 2017 15:50:49 +0000
Message-ID: <20170124155049.GA172024@bricha3-MOBL3.ger.corp.intel.com>
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To: Olivier Matz <olivier.matz@6wind.com>
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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.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       | 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);
> +	}
>  }
>  
>  /**