RE: [RFC] ethdev: TX mbuf fast release optimization

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Thursday, 3 July 2025 17.21
> 
> On Thu, Jul 03, 2025 at 05:12:46PM +0200, Morten Brørup wrote:
> > > From: Bruce Richardson [mailto:bruce.richardson@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.

That's what I meant by "possibly caching it". :-)

The check for the one-time cache update still takes up code space in the per-burst fast path. (Although very little.)

But I still prefer providing configuration information at configuration time, rather than snooping it at runtime.

> 
> /Bruce