Re: [PATCH] net/null: Add fast mbuf release TX offload

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 24 Jun 2025 18:14:16 +0000
Morten Brørup <mb@smartsharesystems.com> wrote:

> Added fast mbuf release, re-using the existing mbuf pool pointer
> in the queue structure.
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>

Makes sense.

> ---
>  drivers/net/null/rte_eth_null.c | 30 +++++++++++++++++++++++++++---
>  1 file changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/null/rte_eth_null.c b/drivers/net/null/rte_eth_null.c
> index 8a9b74a03b..12c0d8d1ff 100644
> --- a/drivers/net/null/rte_eth_null.c
> +++ b/drivers/net/null/rte_eth_null.c
> @@ -34,6 +34,17 @@ struct pmd_internals;
>  struct null_queue {
>  	struct pmd_internals *internals;
>  
> +	/**
> +	 * For RX queue:
> +	 *  Mempool to allocate mbufs from.
> +	 *
> +	 * For TX queue:
> +	 *  Mempool to free mbufs to, if fast release of mbufs is enabled.
> +	 *  UINTPTR_MAX if the mempool for fast release of mbufs has not yet been detected.
> +	 *  NULL if fast release of mbufs is not enabled.
> +	 *
> +	 *  @see RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE
> +	 */
>  	struct rte_mempool *mb_pool;

Do all drivers to it this way?
Is it documented in ethdev?

>  	void *dummy_packet;
>  
> @@ -151,7 +162,16 @@ eth_null_tx(void *q, struct rte_mbuf **bufs, uint16_t nb_bufs)
>  	for (i = 0; i < nb_bufs; i++)
>  		bytes += rte_pktmbuf_pkt_len(bufs[i]);
>  
> -	rte_pktmbuf_free_bulk(bufs, nb_bufs);
> +	if (h->mb_pool != NULL) { /* RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE */
> +		if (unlikely(h->mb_pool == (void *)UINTPTR_MAX)) {
> +			if (unlikely(nb_bufs == 0))
> +				return 0; /* Do not dereference uninitialized bufs[0]. */
> +			h->mb_pool = bufs[0]->pool;
> +		}
> +		rte_mbuf_raw_free_bulk(h->mb_pool, bufs, nb_bufs);
> +	} else {
> +		rte_pktmbuf_free_bulk(bufs, nb_bufs);
> +	}
>  	rte_atomic_fetch_add_explicit(&h->tx_pkts, nb_bufs, rte_memory_order_relaxed);
>  	rte_atomic_fetch_add_explicit(&h->tx_bytes, bytes, rte_memory_order_relaxed);
>  
> @@ -259,7 +279,7 @@ static int
>  eth_tx_queue_setup(struct rte_eth_dev *dev, uint16_t tx_queue_id,
>  		uint16_t nb_tx_desc __rte_unused,
>  		unsigned int socket_id __rte_unused,
> -		const struct rte_eth_txconf *tx_conf __rte_unused)
> +		const struct rte_eth_txconf *tx_conf)
>  {
>  	struct rte_mbuf *dummy_packet;
>  	struct pmd_internals *internals;
> @@ -284,6 +304,9 @@ eth_tx_queue_setup(struct rte_eth_dev *dev, uint16_t tx_queue_id,
>  
>  	internals->tx_null_queues[tx_queue_id].internals = internals;
>  	internals->tx_null_queues[tx_queue_id].dummy_packet = dummy_packet;
> +	internals->tx_null_queues[tx_queue_id].mb_pool =
> +			tx_conf->offloads & RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE ?
> +			(void *)UINTPTR_MAX : NULL;
>  
>  	return 0;
>  }
> @@ -309,7 +332,8 @@ eth_dev_info(struct rte_eth_dev *dev,
>  	dev_info->max_rx_queues = RTE_DIM(internals->rx_null_queues);
>  	dev_info->max_tx_queues = RTE_DIM(internals->tx_null_queues);
>  	dev_info->min_rx_bufsize = 0;
> -	dev_info->tx_offload_capa = RTE_ETH_TX_OFFLOAD_MULTI_SEGS | RTE_ETH_TX_OFFLOAD_MT_LOCKFREE;
> +	dev_info->tx_offload_capa = RTE_ETH_TX_OFFLOAD_MULTI_SEGS |
> +			RTE_ETH_TX_OFFLOAD_MT_LOCKFREE | RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE;
>  
>  	dev_info->reta_size = internals->reta_size;
>  	dev_info->flow_type_rss_offloads = internals->flow_type_rss_offloads;