Re: [PATCH] net/intel: do not bypass mbuf lib for mbuf fast-free

[Bruce Richardson <bruce.richardson@intel.com>]

On Sat, Apr 18, 2026 at 09:56:38AM +0000, Morten Brørup wrote:
> Freeing mbufs directly into the mempool meant that mbuf instrumentation,
> including mbuf history marking, was omitted.
> The mbufs are now freed via the rte_mbuf_raw_free_bulk() function instead.
> 
> Added a static_assert to ensure that type casting the array of struct
> ci_tx_entry_vec to an array of rte_mbuf pointers remains sound.
> 
> Performance note:
> The (n & 31) condition was not removed.
> For the default tx_rs_thresh value (32), the condition will be true.
> And due to inlining, the rte_mbuf_raw_free_bulk() ends up in an
> rte_memcpy(), where the optimizer takes advantage of knowing that the
> lower bits are not set.
> This should compensate somewhat for removing the handcoded optimization of
> copying in chunks of 32 mbufs.
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> ---

Ran a very quick perf test using a couple of 100G ports, no regression
seen with this patch, maybe even a slight perf bump. Therefore:

Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Bruce Richardson <bruce.richardson@intel.com>

One comment inline below:

>  doc/guides/rel_notes/release_26_07.rst |  4 +++
>  drivers/net/intel/common/tx.h          | 36 +++-----------------------
>  2 files changed, 7 insertions(+), 33 deletions(-)
> 
> diff --git a/doc/guides/rel_notes/release_26_07.rst b/doc/guides/rel_notes/release_26_07.rst
> index 060b26ff61..9367d38b13 100644
> --- a/doc/guides/rel_notes/release_26_07.rst
> +++ b/doc/guides/rel_notes/release_26_07.rst
> @@ -24,6 +24,10 @@ DPDK Release 26.07
>  New Features
>  ------------
>  
> +* **Updated Intel common driver.**
> +
> +  * Added missing mbuf history marking to vectorized Tx path for MBUF_FAST_FREE.
> +

I don't think this is a big enough change to require a release note update.
It's really more of a bug fix. If you are ok with it, I'd like to drop this
RN entry on apply of the patch?

>  .. This section should contain new features added in this release.
>     Sample format:
>  
> diff --git a/drivers/net/intel/common/tx.h b/drivers/net/intel/common/tx.h
> index 283bd58d5d..4a201da83c 100644
> --- a/drivers/net/intel/common/tx.h
> +++ b/drivers/net/intel/common/tx.h
> @@ -285,42 +285,12 @@ ci_tx_free_bufs_vec(struct ci_tx_queue *txq, ci_desc_done_fn desc_done, bool ctx
>  			(txq->fast_free_mp = txep[0].mbuf->pool);
>  
>  	if (mp != NULL && (n & 31) == 0) {
> -		void **cache_objs;
> -		struct rte_mempool_cache *cache = rte_mempool_default_cache(mp, rte_lcore_id());
> -
> -		if (cache == NULL)
> -			goto normal;
> -
> -		cache_objs = &cache->objs[cache->len];
> -
> -		if (n > RTE_MEMPOOL_CACHE_MAX_SIZE) {
> -			rte_mempool_ops_enqueue_bulk(mp, (void *)txep, n);
> -			goto done;
> -		}
> -
> -		/* The cache follows the following algorithm
> -		 *   1. Add the objects to the cache
> -		 *   2. Anything greater than the cache min value (if it
> -		 *   crosses the cache flush threshold) is flushed to the ring.
> -		 */
> -		/* Add elements back into the cache */
> -		uint32_t copied = 0;
> -		/* n is multiple of 32 */
> -		while (copied < n) {
> -			memcpy(&cache_objs[copied], &txep[copied], 32 * sizeof(void *));
> -			copied += 32;
> -		}
> -		cache->len += n;
> -
> -		if (cache->len >= cache->flushthresh) {
> -			rte_mempool_ops_enqueue_bulk(mp, &cache->objs[cache->size],
> -					cache->len - cache->size);
> -			cache->len = cache->size;
> -		}
> +		static_assert(sizeof(*txep) == sizeof(struct rte_mbuf *),
> +				"txep array is not similar to an array of rte_mbuf pointers");
> +		rte_mbuf_raw_free_bulk(mp, (void *)txep, n);
>  		goto done;
>  	}
>  
> -normal:
>  	m = rte_pktmbuf_prefree_seg(txep[0].mbuf);
>  	if (likely(m)) {
>  		free[0] = m;
> -- 
> 2.43.0
>