> From: Konstantin Ananyev [mailto:konstantin.anan...@huawei.com]
> Sent: Monday, 30 June 2025 13.41
>
> > When fast releasing mbufs, the mbufs are not accessed, so do not prefetch
> > them.
> > This saves a mbuf load operation for each fast released TX mbuf.
> >
> > When fast release of mbufs is enabled for a TX queue, cache the mbuf
> > mempool pointer in the TX queue structure.
> > This saves one mbuf load operation for each burst of fast released TX
> > mbufs.
> >
> > The txep->mbuf pointer is not used after the mbuf has been freed, so do
> > not reset the pointer.
> > This saves a txep store operation for each TX mbuf freed.
> >
> > Signed-off-by: Morten Brørup <m...@smartsharesystems.com>
> > ---
> > drivers/net/intel/common/tx.h | 5 +++
> > .../i40e/i40e_recycle_mbufs_vec_common.c | 4 +-
> > drivers/net/intel/i40e/i40e_rxtx.c | 39 ++++++++++---------
> > 3 files changed, 28 insertions(+), 20 deletions(-)
> >
> > diff --git a/drivers/net/intel/common/tx.h b/drivers/net/intel/common/tx.h
> > index b0a68bae44..54c9b845f7 100644
> > --- a/drivers/net/intel/common/tx.h
> > +++ b/drivers/net/intel/common/tx.h
> > @@ -62,6 +62,11 @@ struct ci_tx_queue {
> > uint16_t tx_next_dd;
> > uint16_t tx_next_rs;
> > uint64_t offloads;
> > + /* Mempool pointer for fast release of mbufs.
> > + * NULL if disabled, UINTPTR_MAX if enabled and not yet known.
> > + * Initialized at first use.
> > + */
> > + struct rte_mempool *fast_free_mp;
> > uint64_t mbuf_errors;
> > rte_iova_t tx_ring_dma; /* TX ring DMA address */
> > bool tx_deferred_start; /* don't start this queue in dev start */
> > diff --git a/drivers/net/intel/i40e/i40e_recycle_mbufs_vec_common.c
> b/drivers/net/intel/i40e/i40e_recycle_mbufs_vec_common.c
> > index 2875c578af..a46605cee9 100644
> > --- a/drivers/net/intel/i40e/i40e_recycle_mbufs_vec_common.c
> > +++ b/drivers/net/intel/i40e/i40e_recycle_mbufs_vec_common.c
> > @@ -106,7 +106,9 @@ i40e_recycle_tx_mbufs_reuse_vec(void *tx_queue,
> > if (txq->offloads & RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE) {
> > /* Avoid txq contains buffers from unexpected mempool. */
> > if (unlikely(recycle_rxq_info->mp
> > - != txep[0].mbuf->pool))
> > + != (likely(txq->fast_free_mp != (void
> > *)UINTPTR_MAX)
> ?
> > + txq->fast_free_mp :
> > + (txq->fast_free_mp = txep[0].mbuf->pool))))
> > return 0;
> >
> > /* Directly put mbufs from Tx to Rx. */
> > diff --git a/drivers/net/intel/i40e/i40e_rxtx.c
> b/drivers/net/intel/i40e/i40e_rxtx.c
> > index c3ff2e05c3..679c1340b8 100644
> > --- a/drivers/net/intel/i40e/i40e_rxtx.c
> > +++ b/drivers/net/intel/i40e/i40e_rxtx.c
> > @@ -1332,7 +1332,7 @@ static __rte_always_inline int
> > i40e_tx_free_bufs(struct ci_tx_queue *txq)
> > {
> > struct ci_tx_entry *txep;
> > - uint16_t tx_rs_thresh = txq->tx_rs_thresh;
> > + const uint16_t tx_rs_thresh = txq->tx_rs_thresh;
> > uint16_t i = 0, j = 0;
> > struct rte_mbuf *free[RTE_I40E_TX_MAX_FREE_BUF_SZ];
> > const uint16_t k = RTE_ALIGN_FLOOR(tx_rs_thresh,
> RTE_I40E_TX_MAX_FREE_BUF_SZ);
> > @@ -1345,41 +1345,40 @@ i40e_tx_free_bufs(struct ci_tx_queue *txq)
> >
> > txep = &txq->sw_ring[txq->tx_next_dd - (tx_rs_thresh - 1)];
> >
> > - for (i = 0; i < tx_rs_thresh; i++)
> > - rte_prefetch0((txep + i)->mbuf);
> > -
> > if (txq->offloads & RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE) {
> > + struct rte_mempool * const fast_free_mp =
> > + likely(txq->fast_free_mp != (void
> > *)UINTPTR_MAX) ?
> > + txq->fast_free_mp :
> > + (txq->fast_free_mp = txep[0].mbuf->pool);
> > +
>
> Nit idea.
> Acked-by: Konstantin Ananyev <konstantin.anan...@huawei.com>
>
> Just as a suggestion for further improvement:
> can we update (& check) txq->fast_free_mp not at tx_free_bufs() time,
> but when we fill txep[] and filling txd[] based on mbuf values?
> In theory it should allow to remove the check above.
> Also, again in theory, it opens opportunity (with some extra effort) to use
> similar optimization rte_mempool_put_bulk)
> even for cases when RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE is not set.
Checking the TX mbufs to determine if rte_mempool_put_bulk() can be used even
when the RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE offload flag is not set...
That would require that the tx_burst() function checks all the TX mbufs for the
RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE requirements to set the txq->fast_free_mp
pointer.
And if the requirements are not met, it must never set the txq->fast_free_mp
pointer again. Otherwise, some previously transmitted mbufs, waiting to be
freed, might not meet the RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE requirements.
Relying on the explicit RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE offload flag (here or
elsewhere) only requires one check (of the offload flag and/or the mempool
pointer (whichever is hotter in cache)) per burst of packets.
PS: The drivers really should be using the new rte_mbuf_raw_free_bulk() instead
of rte_mempool_put_bulk(), so the freed mbufs are sanity checked in
RTE_LIBRTE_MBUF_DEBUG mode. But such changes belong in another patch series.
>
>
> > if (k) {
> > for (j = 0; j != k; j += RTE_I40E_TX_MAX_FREE_BUF_SZ) {
> > - for (i = 0; i < RTE_I40E_TX_MAX_FREE_BUF_SZ;
> > ++i,
> ++txep) {
> > + for (i = 0; i < RTE_I40E_TX_MAX_FREE_BUF_SZ;
> > ++i,
> ++txep)
> > free[i] = txep->mbuf;
> > - txep->mbuf = NULL;
> > - }
> > - rte_mempool_put_bulk(free[0]->pool, (void
> > **)free,
> > + rte_mempool_put_bulk(fast_free_mp, (void
> > **)free,
> > RTE_I40E_TX_MAX_FREE_BUF_SZ);
> > }
> > }
> >
> > if (m) {
> > - for (i = 0; i < m; ++i, ++txep) {
> > + for (i = 0; i < m; ++i, ++txep)
> > free[i] = txep->mbuf;
> > - txep->mbuf = NULL;
> > - }
> > - rte_mempool_put_bulk(free[0]->pool, (void **)free, m);
> > + rte_mempool_put_bulk(fast_free_mp, (void **)free, m);
> > }
> > } else {
> > - for (i = 0; i < txq->tx_rs_thresh; ++i, ++txep) {
> > + for (i = 0; i < tx_rs_thresh; i++)
> > + rte_prefetch0((txep + i)->mbuf);
> > +
> > + for (i = 0; i < tx_rs_thresh; ++i, ++txep)
> > rte_pktmbuf_free_seg(txep->mbuf);
> > - txep->mbuf = NULL;
> > - }
> > }
> >
> > - txq->nb_tx_free = (uint16_t)(txq->nb_tx_free + txq->tx_rs_thresh);
> > - txq->tx_next_dd = (uint16_t)(txq->tx_next_dd + txq->tx_rs_thresh);
> > + txq->nb_tx_free = (uint16_t)(txq->nb_tx_free + tx_rs_thresh);
> > + txq->tx_next_dd = (uint16_t)(txq->tx_next_dd + tx_rs_thresh);
> > if (txq->tx_next_dd >= txq->nb_tx_desc)
> > - txq->tx_next_dd = (uint16_t)(txq->tx_rs_thresh - 1);
> > + txq->tx_next_dd = (uint16_t)(tx_rs_thresh - 1);
> >
> > - return txq->tx_rs_thresh;
> > + return tx_rs_thresh;
> > }
> >
> > /* Populate 4 descriptors with data from 4 mbufs */
> > @@ -2546,6 +2545,8 @@ i40e_dev_tx_queue_setup(struct rte_eth_dev *dev,
> > txq->reg_idx = reg_idx;
> > txq->port_id = dev->data->port_id;
> > txq->offloads = offloads;
> > + txq->fast_free_mp = offloads & RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE ?
> > + (void *)UINTPTR_MAX : NULL;
> > txq->i40e_vsi = vsi;
> > txq->tx_deferred_start = tx_conf->tx_deferred_start;
> >
> > --
> > 2.43.0
