[RFC] ethdev: TX mbuf fast release optimization

[Morten Brørup]

> From: Bruce Richardson [mailto:bruce.richardson at intel.com]
> Sent: Thursday, 3 July 2025 16.14
> 
> On Thu, Jul 03, 2025 at 03:59:14PM +0200, Morten Brørup wrote:
> > For TX mbuf fast release offload, I propose to add the mbuf mempool
> > pointer to the ethdev tx queue configuration structure,
> > so the ethdev TX burst operation doesn't need to fetch it from the
> > first mbuf of each burst being fast free'd to the mempool.
> >
> > This modification of the struct rte_eth_txconf, and the requirement
> > to set the mempool pointer if the RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE
> > flag is set, will be an API+ABI change in 25.11.
> > Should it be announced in the 25.07 release notes?
> >
> > Note: We could phase it in softly by letting the ethdev drivers
> > check if the pointer has been set, and fall back to fetching it
> > from mbuf[0] if not.
> >
> > /**
> >  * A structure used to configure a Tx ring of an Ethernet port.
> >  */
> > struct rte_eth_txconf {
> > 	struct rte_eth_thresh tx_thresh; /**< Tx ring threshold registers.
> */
> > 	uint16_t tx_rs_thresh; /**< Drives the setting of RS bit on TXDs.
> */
> > 	uint16_t tx_free_thresh; /**< Start freeing Tx buffers if there
> are
> > 				      less free descriptors than this value. */
> >
> > 	uint8_t tx_deferred_start; /**< Do not start queue with
> rte_eth_dev_start(). */
> > 	/**
> > 	 * Per-queue Tx offloads to be set  using RTE_ETH_TX_OFFLOAD_*
> flags.
> > 	 * Only offloads set on tx_queue_offload_capa or tx_offload_capa
> > 	 * fields on rte_eth_dev_info structure are allowed to be set.
> > 	 */
> > 	uint64_t offloads;
> >
> > +	/**
> > +	 * Per-queue mempool to release the mbufs to; required for
> > +	 * RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE offload.
> > +	 */
> > +	struct rte_mempool *mp;
> > +
> 
> Is this really best put in the txconf struct? Would it not be better
> just
> stored in the queue structure on free or Tx of the first packet? That
> would
> make it not change the ABI.

It seems natural to me to specify the mempool at configuration time,
because it is a configuration parameter.
The txconf struct seemed like the best place to put it.

Fetching it from the first mbuf of each burst at runtime, and possibly caching it, seems like a workaround for the missing configuration parameter.

If backwards ABI compatibility is important, we can modify the struct as follows:

struct rte_eth_txconf {
	struct rte_eth_thresh tx_thresh; /**< Tx ring threshold registers. */
	uint16_t tx_rs_thresh; /**< Drives the setting of RS bit on TXDs. */
	uint16_t tx_free_thresh; /**< Start freeing Tx buffers if there are
				      less free descriptors than this value. */

	uint8_t tx_deferred_start; /**< Do not start queue with rte_eth_dev_start(). */
	/**
	 * Per-queue Tx offloads to be set  using RTE_ETH_TX_OFFLOAD_* flags.
	 * Only offloads set on tx_queue_offload_capa or tx_offload_capa
	 * fields on rte_eth_dev_info structure are allowed to be set.
	 */
	uint64_t offloads;

+	/**
+	 * Per-queue mempool to release the mbufs to; required for
+	 * RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE offload.
+	 */
+	struct rte_mempool *mp;
+ #ifdef RTE_ARCH_32
+	uint32_t reserved_32s[1]; /**< Reserved for future fields */
+ #endif
-	uint64_t reserved_64s[2]; /**< Reserved for future fields */
+	uint64_t reserved_64s[1]; /**< Reserved for future fields */
	void *reserved_ptrs[2];   /**< Reserved for future fields */
};