Re: [PATCH 5/6] examples/l3fwd-power: recheck Rx queues before sleeping

After arming the Rx interrupts rx_interrupt_wait() sleeps in
rte_epoll_wait(). A packet that arrived between the last poll and the arm
can be missed on PMDs whose interrupt is edge-triggered on an empty to
non-empty transition, such as dpaa2: arming a queue that is already
non-empty raises no notification, so the lcore sleeps until the next
packet.

Before sleeping, check whether any queue already holds traffic and, if
so, skip the wait and go back to polling. NICs with level-triggered
interrupts are unaffected: a pending packet keeps the interrupt asserted,
so rte_epoll_wait() would return immediately anyway.

Signed-off-by: Maxime Leroy <redacted>
---
 examples/l3fwd-power/main.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/examples/l3fwd-power/main.c b/examples/l3fwd-power/main.c
index fa64e28e5c..c423b0dba5 100644
--- a/examples/l3fwd-power/main.c
+++ b/examples/l3fwd-power/main.c

@@ -949,6 +949,26 @@ static int event_register(struct lcore_conf *qconf)
        return 0;
 }

+static bool
+rx_queue_pending(struct lcore_conf *qconf)
+{
+       struct lcore_rx_queue *rx_queue;
+       uint16_t queue_id;
+       uint16_t port_id;
+       int i;
+
+       for (i = 0; i < qconf->n_rx_queue; ++i) {
+               rx_queue = &(qconf->rx_queue_list[i]);
+               port_id = rx_queue->port_id;
+               queue_id = rx_queue->queue_id;
+
+               if (rte_eth_rx_queue_count(port_id, queue_id) > 0)
+                       return true;
+       }
+
+       return false;
+}
+
 static int
 rx_interrupt_wait(struct lcore_conf *qconf, int *intr_registered)
 {

@@ -968,7 +988,12 @@ rx_interrupt_wait(struct lcore_conf *qconf, int *intr_registered)
                *intr_registered = 1;
        }

-       sleep_until_rx_interrupt(qconf->n_rx_queue, rte_lcore_id());
+       /*
+        * A packet that arrived during the arm window raises no new wakeup on
+        * edge-triggered PMDs, so skip the sleep if a queue already has traffic.
+        */
+       if (!rx_queue_pending(qconf))
+               sleep_until_rx_interrupt(qconf->n_rx_queue, rte_lcore_id());
        rx_intr_disable_all(qconf);
        return 0;
 }