Re: [dpdk-dev] [PATCH v1] net/memif: optimized with one-way barrier

["Phil Yang (Arm Technology China)" <Phil.Yang@arm.com>]

+ Gavin

> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Phil Yang
> Sent: Monday, August 26, 2019 7:00 PM
> To: jgrajcia@cisco.com; dev@dpdk.org
> Cc: thomas@monjalon.net; jerinj@marvell.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; damarion@cisco.com; nd
> <nd@arm.com>
> Subject: [dpdk-dev] [PATCH v1] net/memif: optimized with one-way barrier
> 
> Using 'rte_mb' to synchronize the shared ring head/tail between producer
> and consumer will stall the pipeline and damage performance on the weak
> memory model platforms, such like aarch64. Meanwhile update the shared
> ring head and tail are observable and ordered between CPUs on IA.
> 
> Optimized this full barrier with the one-way barrier can improve the
> throughput. On aarch64 n1sdp server this patch make testpmd throughput
> boost 2.1%. On Intel E5-2640, testpmd got 3.98% performance gain.
> 
> Signed-off-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> ---
>  drivers/net/memif/memif.h         |  4 +--
>  drivers/net/memif/rte_eth_memif.c | 53 +++++++++++++++++++++--------
> ----------
>  2 files changed, 31 insertions(+), 26 deletions(-)
> 
> diff --git a/drivers/net/memif/memif.h b/drivers/net/memif/memif.h
> index 3948b1f..a4d88c0 100644
> --- a/drivers/net/memif/memif.h
> +++ b/drivers/net/memif/memif.h
> @@ -169,9 +169,9 @@ typedef struct {
>  	uint32_t cookie;			/**< MEMIF_COOKIE */
>  	uint16_t flags;				/**< flags */
>  #define MEMIF_RING_FLAG_MASK_INT 1		/**< disable interrupt
> mode */
> -	volatile uint16_t head;			/**< pointer to ring buffer
> head */
> +	uint16_t head;			/**< pointer to ring buffer head */
>  	MEMIF_CACHELINE_ALIGN_MARK(cacheline1);
> -	volatile uint16_t tail;			/**< pointer to ring buffer
> tail */
> +	uint16_t tail;			/**< pointer to ring buffer tail */
>  	MEMIF_CACHELINE_ALIGN_MARK(cacheline2);
>  	memif_desc_t desc[0];			/**< buffer descriptors */
>  } memif_ring_t;
> diff --git a/drivers/net/memif/rte_eth_memif.c
> b/drivers/net/memif/rte_eth_memif.c
> index a59f809..b1c871e 100644
> --- a/drivers/net/memif/rte_eth_memif.c
> +++ b/drivers/net/memif/rte_eth_memif.c
> @@ -275,8 +275,14 @@ eth_memif_rx(void *queue, struct rte_mbuf **bufs,
> uint16_t nb_pkts)
>  	ring_size = 1 << mq->log2_ring_size;
>  	mask = ring_size - 1;
> 
> -	cur_slot = (type == MEMIF_RING_S2M) ? mq->last_head : mq-
> >last_tail;
> -	last_slot = (type == MEMIF_RING_S2M) ? ring->head : ring->tail;
> +	if (type == MEMIF_RING_S2M) {
> +		cur_slot = mq->last_head;
> +		last_slot = __atomic_load_n(&ring->head,
> __ATOMIC_ACQUIRE);
> +	} else {
> +		cur_slot = mq->last_tail;
> +		last_slot = __atomic_load_n(&ring->tail,
> __ATOMIC_ACQUIRE);
> +	}
> +
>  	if (cur_slot == last_slot)
>  		goto refill;
>  	n_slots = last_slot - cur_slot;
> @@ -344,8 +350,7 @@ eth_memif_rx(void *queue, struct rte_mbuf **bufs,
> uint16_t nb_pkts)
> 
>  no_free_bufs:
>  	if (type == MEMIF_RING_S2M) {
> -		rte_mb();
> -		ring->tail = cur_slot;
> +		__atomic_store_n(&ring->tail, cur_slot,
> __ATOMIC_RELEASE);
>  		mq->last_head = cur_slot;
>  	} else {
>  		mq->last_tail = cur_slot;
> @@ -353,7 +358,7 @@ eth_memif_rx(void *queue, struct rte_mbuf **bufs,
> uint16_t nb_pkts)
> 
>  refill:
>  	if (type == MEMIF_RING_M2S) {
> -		head = ring->head;
> +		head = __atomic_load_n(&ring->head,
> __ATOMIC_ACQUIRE);
>  		n_slots = ring_size - head + mq->last_tail;
> 
>  		while (n_slots--) {
> @@ -361,8 +366,7 @@ eth_memif_rx(void *queue, struct rte_mbuf **bufs,
> uint16_t nb_pkts)
>  			d0 = &ring->desc[s0];
>  			d0->length = pmd->run.pkt_buffer_size;
>  		}
> -		rte_mb();
> -		ring->head = head;
> +		__atomic_store_n(&ring->head, head, __ATOMIC_RELEASE);
>  	}
> 
>  	mq->n_pkts += n_rx_pkts;
> @@ -398,14 +402,16 @@ eth_memif_tx(void *queue, struct rte_mbuf
> **bufs, uint16_t nb_pkts)
>  	ring_size = 1 << mq->log2_ring_size;
>  	mask = ring_size - 1;
> 
> -	n_free = ring->tail - mq->last_tail;
> +	n_free = __atomic_load_n(&ring->tail, __ATOMIC_ACQUIRE) - mq-
> >last_tail;
>  	mq->last_tail += n_free;
> -	slot = (type == MEMIF_RING_S2M) ? ring->head : ring->tail;
> 
> -	if (type == MEMIF_RING_S2M)
> -		n_free = ring_size - ring->head + mq->last_tail;
> -	else
> -		n_free = ring->head - ring->tail;
> +	if (type == MEMIF_RING_S2M) {
> +		slot = __atomic_load_n(&ring->head, __ATOMIC_ACQUIRE);
> +		n_free = ring_size - slot + mq->last_tail;
> +	} else {
> +		slot = __atomic_load_n(&ring->tail, __ATOMIC_ACQUIRE);
> +		n_free = __atomic_load_n(&ring->head,
> __ATOMIC_ACQUIRE) - slot;
> +	}
> 
>  	while (n_tx_pkts < nb_pkts && n_free) {
>  		mbuf_head = *bufs++;
> @@ -464,11 +470,10 @@ eth_memif_tx(void *queue, struct rte_mbuf
> **bufs, uint16_t nb_pkts)
>  	}
> 
>  no_free_slots:
> -	rte_mb();
>  	if (type == MEMIF_RING_S2M)
> -		ring->head = slot;
> +		__atomic_store_n(&ring->head, slot, __ATOMIC_RELEASE);
>  	else
> -		ring->tail = slot;
> +		__atomic_store_n(&ring->tail, slot, __ATOMIC_RELEASE);
> 
>  	if ((ring->flags & MEMIF_RING_FLAG_MASK_INT) == 0) {
>  		a = 1;
> @@ -611,8 +616,8 @@ memif_init_rings(struct rte_eth_dev *dev)
> 
>  	for (i = 0; i < pmd->run.num_s2m_rings; i++) {
>  		ring = memif_get_ring(pmd, proc_private,
> MEMIF_RING_S2M, i);
> -		ring->head = 0;
> -		ring->tail = 0;
> +		__atomic_store_n(&ring->head, 0, __ATOMIC_RELAXED);
> +		__atomic_store_n(&ring->tail, 0, __ATOMIC_RELAXED);
>  		ring->cookie = MEMIF_COOKIE;
>  		ring->flags = 0;
>  		for (j = 0; j < (1 << pmd->run.log2_ring_size); j++) {
> @@ -627,8 +632,8 @@ memif_init_rings(struct rte_eth_dev *dev)
> 
>  	for (i = 0; i < pmd->run.num_m2s_rings; i++) {
>  		ring = memif_get_ring(pmd, proc_private,
> MEMIF_RING_M2S, i);
> -		ring->head = 0;
> -		ring->tail = 0;
> +		__atomic_store_n(&ring->head, 0, __ATOMIC_RELAXED);
> +		__atomic_store_n(&ring->tail, 0, __ATOMIC_RELAXED);
>  		ring->cookie = MEMIF_COOKIE;
>  		ring->flags = 0;
>  		for (j = 0; j < (1 << pmd->run.log2_ring_size); j++) {
> @@ -734,8 +739,8 @@ memif_connect(struct rte_eth_dev *dev)
>  				MIF_LOG(ERR, "Wrong ring");
>  				return -1;
>  			}
> -			ring->head = 0;
> -			ring->tail = 0;
> +			__atomic_store_n(&ring->head, 0,
> __ATOMIC_RELAXED);
> +			__atomic_store_n(&ring->tail, 0,
> __ATOMIC_RELAXED);
>  			mq->last_head = 0;
>  			mq->last_tail = 0;
>  			/* enable polling mode */
> @@ -750,8 +755,8 @@ memif_connect(struct rte_eth_dev *dev)
>  				MIF_LOG(ERR, "Wrong ring");
>  				return -1;
>  			}
> -			ring->head = 0;
> -			ring->tail = 0;
> +			__atomic_store_n(&ring->head, 0,
> __ATOMIC_RELAXED);
> +			__atomic_store_n(&ring->tail, 0,
> __ATOMIC_RELAXED);
>  			mq->last_head = 0;
>  			mq->last_tail = 0;
>  			/* enable polling mode */
> --
> 2.7.4