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

[Morten Brørup]

> From: Ivan Malov [mailto:ivan.malov at arknetworks.am]
> Sent: Saturday, 26 July 2025 08.15
> 
> 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.

I just checked, and it seems all three spellings "rx", "Rx" and "RX" are being used in DPDK.
I personally prefer RX, so I'll keep that.

> 
> > +	 *  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.

I considered this, and I can imagine an application using FAST_FREE for its primary queues, and normal free for some secondary queues.
Looking at other drivers - which have implemented a runtime check, not separate callbacks for FAST_FREE - I guess they came to the same conclusion.

> 
> > 	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.

Interesting thought - got me looking deeper into this.

It seems MULTI_SEGS is primarily a capability flag.
The description of the MULTI_SEGS flag [1] could be interpreted that way too:
/** Device supports multi segment send. */

[1]: https://elixir.bootlin.com/dpdk/v25.07/source/lib/ethdev/rte_ethdev.h#L1614

E.g. the i40e driver offers MULTI_SEGS capability per-device, but not per-queue. And it doesn't use the MULTI_SEGS flag for any purpose (beyond capability reporting).

Furthermore, enabling MULTI_SEGS on TX (per device or per queue) wouldn't mean that all transmitted packets are segmented; it only means that the driver must be able to handle segmented packets.
I.e. MULTI_SEGS could be enabled on a device, and yet it would be acceptable to enable FAST_FREE on a queue on that device.

> 
> Thank you.

Thank you for reviewing.

> 
> >
> > 	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
> >
> >