[dpdk-dev] [PATCH 2/2] net/tap: add Rx trigger

[Adrien Mazarguil]

This commit adds a signal-based trigger to the Rx burst function in order
to avoid unnecessary system calls while Rx queues are empty.

Triggered Rx bursts put less pressure on the kernel, free up CPU resources
for applications and result in a noticeable performance improvement when
sharing CPU threads with other PMDs.

Measuring the traffic forwarding rate between two physical devices in
testpmd (IO mode, single thread, 64B packets) before and after adding two
tap PMD instances (4 ports total) that do not process any traffic and
comparing results yields:

Without Rx trigger:

 -15% (--burst=32)
 -62% (--burst=1)

With Rx trigger:

 -0.3% (--burst=32)
 -6% (--burst=1)

Signed-off-by: Adrien Mazarguil <adrien.mazarguil at 6wind.com>
Acked-by: Pascal Mazon <pascal.mazon at 6wind.com>
---
 drivers/net/tap/rte_eth_tap.c | 59 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/drivers/net/tap/rte_eth_tap.c b/drivers/net/tap/rte_eth_tap.c
index c757a7c..d5d467a 100644
--- a/drivers/net/tap/rte_eth_tap.c
+++ b/drivers/net/tap/rte_eth_tap.c
@@ -31,6 +31,8 @@
  *   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <rte_atomic.h>
+#include <rte_common.h>
 #include <rte_mbuf.h>
 #include <rte_ethdev.h>
 #include <rte_malloc.h>
@@ -42,6 +44,9 @@
 #include <sys/socket.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
+#include <errno.h>
+#include <signal.h>
+#include <stdint.h>
 #include <unistd.h>
 #include <arpa/inet.h>
 #include <linux/if.h>
@@ -72,6 +77,8 @@ static const char *valid_arguments[] = {
 
 static int tap_unit;
 
+static volatile uint32_t tap_trigger;	/* Rx trigger */
+
 static struct rte_eth_link pmd_link = {
 	.link_speed = ETH_SPEED_NUM_10G,
 	.link_duplex = ETH_LINK_FULL_DUPLEX,
@@ -89,6 +96,7 @@ struct pkt_stats {
 
 struct rx_queue {
 	struct rte_mempool *mp;		/* Mempool for RX packets */
+	uint32_t trigger_seen;		/* Last seen Rx trigger value */
 	uint16_t in_port;		/* Port ID */
 	int fd;
 
@@ -111,6 +119,13 @@ struct pmd_internals {
 	struct tx_queue txq[RTE_PMD_TAP_MAX_QUEUES];	/* List of TX queues */
 };
 
+static void
+tap_trigger_cb(int sig __rte_unused)
+{
+	/* Valid trigger values are nonzero */
+	tap_trigger = (tap_trigger + 1) | 0x80000000;
+}
+
 /* Tun/Tap allocation routine
  *
  * name is the number of the interface to use, unless NULL to take the host
@@ -175,6 +190,43 @@ tun_alloc(struct pmd_internals *pmd, uint16_t qid)
 		goto error;
 	}
 
+	/* Set up trigger to optimize empty Rx bursts */
+	errno = 0;
+	do {
+		struct sigaction sa;
+		int flags = fcntl(fd, F_GETFL);
+
+		if (flags == -1 || sigaction(SIGIO, NULL, &sa) == -1)
+			break;
+		if (sa.sa_handler != tap_trigger_cb) {
+			/*
+			 * Make sure SIGIO is not already taken. This is done
+			 * as late as possible to leave the application a
+			 * chance to set up its own signal handler first.
+			 */
+			if (sa.sa_handler != SIG_IGN &&
+			    sa.sa_handler != SIG_DFL) {
+				errno = EBUSY;
+				break;
+			}
+			sa = (struct sigaction){
+				.sa_flags = SA_RESTART,
+				.sa_handler = tap_trigger_cb,
+			};
+			if (sigaction(SIGIO, &sa, NULL) == -1)
+				break;
+		}
+		/* Enable SIGIO on file descriptor */
+		fcntl(fd, F_SETFL, flags | O_ASYNC);
+		fcntl(fd, F_SETOWN, getpid());
+	} while (0);
+	if (errno) {
+		/* Disable trigger globally in case of error */
+		tap_trigger = 0;
+		RTE_LOG(WARNING, PMD, "Rx trigger disabled: %s\n",
+			strerror(errno));
+	}
+
 	if (qid == 0) {
 		if (ioctl(fd, SIOCGIFHWADDR, &ifr) == -1) {
 			RTE_LOG(ERR, PMD, "ioctl failed (SIOCGIFHWADDR) (%s)\n",
@@ -204,7 +256,13 @@ pmd_rx_burst(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 	struct rx_queue *rxq = queue;
 	uint16_t num_rx;
 	unsigned long num_rx_bytes = 0;
+	uint32_t trigger = tap_trigger;
 
+	if (trigger == rxq->trigger_seen)
+		return 0;
+	if (trigger)
+		rxq->trigger_seen = trigger;
+	rte_compiler_barrier();
 	for (num_rx = 0; num_rx < nb_pkts; ) {
 		/* allocate the next mbuf */
 		mbuf = rte_pktmbuf_alloc(rxq->mp);
@@ -563,6 +621,7 @@ tap_rx_queue_setup(struct rte_eth_dev *dev,
 	}
 
 	internals->rxq[rx_queue_id].mp = mp;
+	internals->rxq[rx_queue_id].trigger_seen = 1; /* force initial burst */
 	internals->rxq[rx_queue_id].in_port = dev->data->port_id;
 
 	/* Now get the space available for data in the mbuf */
-- 
2.1.4