RE: [PATCH] net/i40e: Fast release optimizations

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> 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 <mb@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.ananyev@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. 
 

>  		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