RE: [PATCH v11 2/2] net/i40e: replace put function

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

> From: Kamalakshitha Aligeri [mailto:kamalakshitha.aligeri@arm.com]
> Sent: Wednesday, 5 July 2023 20.03
> 
> Integrated zero-copy put API in mempool cache in i40e PMD.
> On Ampere Altra server, l3fwd single core's performance improves by 5%
> with the new API
> 
> Signed-off-by: Kamalakshitha Aligeri <kamalakshitha.aligeri@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> Reviewed-by: Feifei Wang <feifei.wang2@arm.com>
> ---
>  drivers/net/i40e/i40e_rxtx_vec_common.h | 27 ++++++++++++++++++++-----
>  1 file changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/i40e/i40e_rxtx_vec_common.h
> b/drivers/net/i40e/i40e_rxtx_vec_common.h
> index fe1a6ec75e..35cdb31b2e 100644
> --- a/drivers/net/i40e/i40e_rxtx_vec_common.h
> +++ b/drivers/net/i40e/i40e_rxtx_vec_common.h
> @@ -95,18 +95,35 @@ i40e_tx_free_bufs(struct i40e_tx_queue *txq)
> 
>  	n = txq->tx_rs_thresh;
> 
> -	 /* first buffer to free from S/W ring is at index
> -	  * tx_next_dd - (tx_rs_thresh-1)
> -	  */
> +	/* first buffer to free from S/W ring is at index
> +	 * tx_next_dd - (tx_rs_thresh-1)
> +	 */
>  	txep = &txq->sw_ring[txq->tx_next_dd - (n - 1)];
> 
>  	if (txq->offloads & RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE) {
> +		struct rte_mempool *mp = txep[0].mbuf->pool;
> +		struct rte_mempool_cache *cache =
> rte_mempool_default_cache(mp, rte_lcore_id());
> +		void **cache_objs;
> +
> +		if (unlikely(!cache))
> +			goto fallback;
> +
> +		cache_objs = rte_mempool_cache_zc_put_bulk(cache, mp, n);
> +		if (unlikely(!cache_objs))
> +			goto fallback;
> +
>  		for (i = 0; i < n; i++) {
> -			free[i] = txep[i].mbuf;
> +			cache_objs[i] = txep[i].mbuf;
>  			/* no need to reset txep[i].mbuf in vector path */
>  		}
> -		rte_mempool_put_bulk(free[0]->pool, (void **)free, n);
>  		goto done;
> +
> +fallback:
> +		for (i = 0; i < n; i++)
> +			free[i] = txep[i].mbuf;
> +		rte_mempool_generic_put(mp, (void **)free, n, cache);

Patchwork, when building with gcc-debug for FreeBSD13-64 and RHEL92-64, complains that "free" may be used uninitialized in rte_mempool_check_cookies() called via RTE_MEMPOOL_CHECK_COOKIES() from rte_mempool_generic_put() here. But "free" is initialized by the preceding loop, so I don't understand why GCC thinks it may be uninitialized.

Does adding curly braces to the loop help GCC understand that the free array is initialized, or what is the proper workaround?

> +		goto done;
> +
>  	}
> 
>  	m = rte_pktmbuf_prefree_seg(txep[0].mbuf);
> --
> 2.25.1

Acked-by: Morten Brørup <mb@smartsharesystems.com>