[PATCH v2] net/null: Add fast mbuf release TX offload

[Ivan Malov]

Hi Morten,

Good patch. Please see below.

On Sat, 26 Jul 2025, Morten Brørup wrote:

> Added fast mbuf release, re-using the existing mbuf pool pointer
> in the queue structure.
>
> Signed-off-by: Morten Brørup <mb at smartsharesystems.com>
> ---
> v2:
> * Also announce the offload as a per-queue capability.
> * Added missing test of per-device offload configuration when configuring
>  the queue.
> ---
> drivers/net/null/rte_eth_null.c | 33 ++++++++++++++++++++++++++++++---
> 1 file changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/null/rte_eth_null.c b/drivers/net/null/rte_eth_null.c
> index 8a9b74a03b..09cfc74494 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:

Perhaps spell it 'Rx', 'Tx', but this is up to you.

> +	 *  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
> +	 */

May be it would be better to have a separate 'tx_pkt_burst' callback, to avoid
conditional checks below. Though, I understand this will downgrade the per-queue
capability to the per-port only, so feel free to disregard this point.

> 	struct rte_mempool *mb_pool;
> 	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 +304,10 @@ 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 =
> +			(dev->data->dev_conf.txmode.offloads | tx_conf->offloads) &
> +			RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE ?
> +			(void *)UINTPTR_MAX : NULL;

Given the fact that FAST_FREE and MULTI_SEGS are effectively conflicting,
wouldn't it be better to have a check for the presence of both flags here? I'm
not sure whether this is already checked at the generic layer above the PMD.

Thank you.

>
> 	return 0;
> }
> @@ -309,7 +333,10 @@ 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_queue_offload_capa = RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE;
> +	dev_info->tx_offload_capa = RTE_ETH_TX_OFFLOAD_MULTI_SEGS |
> +			RTE_ETH_TX_OFFLOAD_MT_LOCKFREE |
> +			dev_info->tx_queue_offload_capa;
>
> 	dev_info->reta_size = internals->reta_size;
> 	dev_info->flow_type_rss_offloads = internals->flow_type_rss_offloads;
> -- 
> 2.43.0
>
>