Re: [dpdk-dev] [PATCH] mbuf: cleanup rte_pktmbuf_lastseg(), fix atomic usage

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Hanon,

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Hanoh Haim
> Sent: Wednesday, November 15, 2017 8:36 AM
> To: dev@dpdk.org
> Cc: Hanoh Haim <hhaim@cisco.com>
> Subject: [dpdk-dev] [PATCH] mbuf: cleanup rte_pktmbuf_lastseg(), fix atomic usage
> 
> Signed-off-by: Hanoh Haim <hhaim@cisco.com>
> ---
>  lib/librte_mbuf/rte_mbuf.h | 27 +++++++++++++--------------
>  1 file changed, 13 insertions(+), 14 deletions(-)
> 
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index 7e326bb..ab110f8 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -1159,6 +1159,15 @@ static inline void rte_pktmbuf_reset(struct rte_mbuf *m)
> 	__rte_mbuf_sanity_check(m, 1);
>  }
> 
> +
> +static __rte_always_inline void rte_pktmbuf_reset_before_free(struct rte_mbuf *m)
> +{
> +	if (m->next != NULL) {
> +		m->next = NULL;
> +		m->nb_segs = 1;
> +	}
> +}
> +
>  /**
>   * Allocate a new mbuf from a mempool.
>   *
> @@ -1323,8 +1332,7 @@ static inline void rte_pktmbuf_detach(struct rte_mbuf *m)
> 	m->ol_flags = 0;
> 
> 	if (rte_mbuf_refcnt_update(md, -1) == 0) {
> -		md->next = NULL;
> -		md->nb_segs = 1;
> +		rte_pktmbuf_reset_before_free(md);
> 		rte_mbuf_refcnt_set(md, 1);
> 		rte_mbuf_raw_free(md);
> 	}
> @@ -1354,25 +1362,16 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> 		if (RTE_MBUF_INDIRECT(m))
> 			rte_pktmbuf_detach(m);
> 
> -		if (m->next != NULL) {
> -			m->next = NULL;
> -			m->nb_segs = 1;
> -		}
> -
> +		rte_pktmbuf_reset_before_free(m);
> 		return m;
> 
> -       } else if (rte_atomic16_add_return(&m->refcnt_atomic, -1) == 0) {
> -
> +	} else if (rte_mbuf_refcnt_update(m, -1) == 0) {

That would cause extra read; cmp (and possible slowdown) for atomic refcnt.
If that really need to be fixed - probably we need to introduce a new function
that would do update without trying to read refctn first - rte_mbuf_refcnt_update_blind() or so.
Konstantin


> 
> 		if (RTE_MBUF_INDIRECT(m))
> 			rte_pktmbuf_detach(m);
> 
> -		if (m->next != NULL) {
> -			m->next = NULL;
> -			m->nb_segs = 1;
> -		}
> +		rte_pktmbuf_reset_before_free(m);
> 		rte_mbuf_refcnt_set(m, 1);
> -
> 		return m;
> 	}
> 	return NULL;
> --
> 2.9.3