Re: [PATCH v4 6/6] net/ring: replace rte atomics with GCC builtin atomics

[Olivier Matz <olivier.matz@6wind.com>]

Hi Tyler,

Few comments below.

On Fri, Jun 02, 2023 at 12:45:07PM -0700, Tyler Retzlaff wrote:
> Replace the use of rte_atomic.h types and functions, instead use GCC
> supplied C++11 memory model builtins.
> 
> Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
> Acked-by: Morten Brørup <mb@smartsharesystems.com>
> Acked-by: Bruce Richardson <bruce.richardson@intel.com>
> ---
>  drivers/net/ring/rte_eth_ring.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/ring/rte_eth_ring.c b/drivers/net/ring/rte_eth_ring.c
> index e8bc9b6..43eb627 100644
> --- a/drivers/net/ring/rte_eth_ring.c
> +++ b/drivers/net/ring/rte_eth_ring.c
> @@ -44,8 +44,8 @@ enum dev_action {
>  
>  struct ring_queue {
>  	struct rte_ring *rng;
> -	rte_atomic64_t rx_pkts;
> -	rte_atomic64_t tx_pkts;
> +	uint64_t rx_pkts;
> +	uint64_t tx_pkts;
>  };
>  
>  struct pmd_internals {
> @@ -80,9 +80,10 @@ struct pmd_internals {
>  	const uint16_t nb_rx = (uint16_t)rte_ring_dequeue_burst(r->rng,
>  			ptrs, nb_bufs, NULL);
>  	if (r->rng->flags & RING_F_SC_DEQ)
> -		r->rx_pkts.cnt += nb_rx;
> +		r->rx_pkts += nb_rx;
>  	else
> -		rte_atomic64_add(&(r->rx_pkts), nb_rx);
> +		/* NOTE: review for potential ordering optimization */
> +		__atomic_fetch_add(&r->rx_pkts, nb_rx, __ATOMIC_SEQ_CST);

We can use __ATOMIC_RELAXED here (and below too), since there is no ordering
constraint. We only want statistics to be correct.

You can remove the other NOTEs from the patch.


>  	return nb_rx;
>  }
>  
> @@ -94,9 +95,10 @@ struct pmd_internals {
>  	const uint16_t nb_tx = (uint16_t)rte_ring_enqueue_burst(r->rng,
>  			ptrs, nb_bufs, NULL);
>  	if (r->rng->flags & RING_F_SP_ENQ)
> -		r->tx_pkts.cnt += nb_tx;
> +		r->tx_pkts += nb_tx;
>  	else
> -		rte_atomic64_add(&(r->tx_pkts), nb_tx);
> +		/* NOTE: review for potential ordering optimization */
> +		__atomic_fetch_add(&r->tx_pkts, nb_tx, __ATOMIC_SEQ_CST);
>  	return nb_tx;
>  }
>  
> @@ -184,13 +186,15 @@ struct pmd_internals {
>  
>  	for (i = 0; i < RTE_ETHDEV_QUEUE_STAT_CNTRS &&
>  			i < dev->data->nb_rx_queues; i++) {
> -		stats->q_ipackets[i] = internal->rx_ring_queues[i].rx_pkts.cnt;
> +		/* NOTE: review for atomic access */
> +		stats->q_ipackets[i] = internal->rx_ring_queues[i].rx_pkts;
>  		rx_total += stats->q_ipackets[i];
>  	}
>  
>  	for (i = 0; i < RTE_ETHDEV_QUEUE_STAT_CNTRS &&
>  			i < dev->data->nb_tx_queues; i++) {
> -		stats->q_opackets[i] = internal->tx_ring_queues[i].tx_pkts.cnt;
> +		/* NOTE: review for atomic access */
> +		stats->q_opackets[i] = internal->tx_ring_queues[i].tx_pkts;
>  		tx_total += stats->q_opackets[i];
>  	}
>  
> @@ -207,9 +211,11 @@ struct pmd_internals {
>  	struct pmd_internals *internal = dev->data->dev_private;
>  
>  	for (i = 0; i < dev->data->nb_rx_queues; i++)
> -		internal->rx_ring_queues[i].rx_pkts.cnt = 0;
> +		/* NOTE: review for atomic access */
> +		internal->rx_ring_queues[i].rx_pkts = 0;
>  	for (i = 0; i < dev->data->nb_tx_queues; i++)
> -		internal->tx_ring_queues[i].tx_pkts.cnt = 0;
> +		/* NOTE: review for atomic access */
> +		internal->tx_ring_queues[i].tx_pkts = 0;
>  
>  	return 0;
>  }
> -- 
> 1.8.3.1
>