[RFC] ethdev: TX mbuf fast release optimization

[Bruce Richardson]

On Thu, Jul 03, 2025 at 05:12:46PM +0200, Morten Brørup wrote:
> > 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.
> 
Not suggesting from each burst, just the first mbuf seen. One time only.

/Bruce