[dpdk-dev] [PATCH 09/10] net/enetc: improve prefetch in Rx ring clean

[Hemant Agrawal <hemant.agrawal@nxp.com>]

From: Alex Marginean <alexandru.marginean@nxp.com>

LS1028A does not have platform cache so any reads following a hardware
write will go directly to DDR.  Latency of such a read is in excess of 100
core cycles, so try to prefetch more in advance to mitigate this.
How much is worth prefetching really depends on traffic conditions.  With
congested Rx this could go up to 4 cache lines or so.  But if software
keeps up with hardware and follows behind Rx PI by a cache line then it's
harmful in terms of performance to cache more.  We would only prefetch
data that's yet to be written by ENETC, which will be evicted again anyway.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
---
 drivers/net/enetc/enetc_rxtx.c | 38 +++++++++++++++++++++++++++++-----
 1 file changed, 33 insertions(+), 5 deletions(-)

diff --git a/drivers/net/enetc/enetc_rxtx.c b/drivers/net/enetc/enetc_rxtx.c
index 1acc43a08..e57ecf2d4 100644
--- a/drivers/net/enetc/enetc_rxtx.c
+++ b/drivers/net/enetc/enetc_rxtx.c
@@ -14,6 +14,8 @@
 #include "enetc.h"
 #include "enetc_logs.h"
 
+#define ENETC_CACHE_LINE_RXBDS	(RTE_CACHE_LINE_SIZE / \
+				 sizeof(union enetc_rx_bd))
 #define ENETC_RXBD_BUNDLE 16 /* Number of buffers to allocate at once */
 
 static int
@@ -321,18 +323,37 @@ enetc_clean_rx_ring(struct enetc_bdr *rx_ring,
 		    int work_limit)
 {
 	int rx_frm_cnt = 0;
-	int cleaned_cnt, i;
+	int cleaned_cnt, i, bd_count;
 	struct enetc_swbd *rx_swbd;
+	union enetc_rx_bd *rxbd;
 
-	cleaned_cnt = enetc_bd_unused(rx_ring);
 	/* next descriptor to process */
 	i = rx_ring->next_to_clean;
+	/* next descriptor to process */
+	rxbd = ENETC_RXBD(*rx_ring, i);
+	rte_prefetch0(rxbd);
+	bd_count = rx_ring->bd_count;
+	/* LS1028A does not have platform cache so any software access following
+	 * a hardware write will go directly to DDR.  Latency of such a read is
+	 * in excess of 100 core cycles, so try to prefetch more in advance to
+	 * mitigate this.
+	 * How much is worth prefetching really depends on traffic conditions.
+	 * With congested Rx this could go up to 4 cache lines or so.  But if
+	 * software keeps up with hardware and follows behind Rx PI by a cache
+	 * line or less then it's harmful in terms of performance to cache more.
+	 * We would only prefetch BDs that have yet to be written by ENETC,
+	 * which will have to be evicted again anyway.
+	 */
+	rte_prefetch0(ENETC_RXBD(*rx_ring,
+				 (i + ENETC_CACHE_LINE_RXBDS) % bd_count));
+	rte_prefetch0(ENETC_RXBD(*rx_ring,
+				 (i + ENETC_CACHE_LINE_RXBDS * 2) % bd_count));
+
+	cleaned_cnt = enetc_bd_unused(rx_ring);
 	rx_swbd = &rx_ring->q_swbd[i];
 	while (likely(rx_frm_cnt < work_limit)) {
-		union enetc_rx_bd *rxbd;
 		uint32_t bd_status;
 
-		rxbd = ENETC_RXBD(*rx_ring, i);
 		bd_status = rte_le_to_cpu_32(rxbd->r.lstatus);
 		if (!bd_status)
 			break;
@@ -353,11 +374,18 @@ enetc_clean_rx_ring(struct enetc_bdr *rx_ring,
 			i = 0;
 			rx_swbd = &rx_ring->q_swbd[i];
 		}
+		rxbd = ENETC_RXBD(*rx_ring, i);
+		rte_prefetch0(ENETC_RXBD(*rx_ring,
+					 (i + ENETC_CACHE_LINE_RXBDS) %
+					  bd_count));
+		rte_prefetch0(ENETC_RXBD(*rx_ring,
+					 (i + ENETC_CACHE_LINE_RXBDS * 2) %
+					 bd_count));
 
-		rx_ring->next_to_clean = i;
 		rx_frm_cnt++;
 	}
 
+	rx_ring->next_to_clean = i;
 	enetc_refill_rx_ring(rx_ring, cleaned_cnt);
 
 	return rx_frm_cnt;
-- 
2.17.1