Under low rate traffic conditions, there can be 2 issues.
  (1) Packets potentially can get stuck in the intermediate queue.
  (2) Latency of the packets can increase significantly due to
       buffering in intermediate queue.

This commit handles the (1) issue by flushing the tx port queues using
dp_netdev_flush_txq_ports() as part of PMD packet processing loop.
Also this commit addresses issue (2) by flushing the tx queues after
every rxq port processing. This reduces the latency with out impacting
the forwarding throughput.

   MASTER
  --------
   Pkt size  min(ns)   avg(ns)   max(ns)
    512      4,631      5,022    309,914
   1024      5,545      5,749    104,294
   1280      5,978      6,159     45,306
   1518      6,419      6,774    946,850

  MASTER + COMMIT
  -----------------
   Pkt size  min(ns)   avg(ns)   max(ns)
    512      4,711      5,064    182,477
   1024      5,601      5,888    701,654
   1280      6,018      6,491    533,037
   1518      6,467      6,734    312,471

PMDs can be teared down and spawned at runtime and so the rxq and txq
mapping of the PMD threads can change. In few cases packets can get
stuck in the queue due to reconfiguration and this commit helps flush
the queues.

Suggested-by: Eelco Chaudron <echau...@redhat.com>
Reported-at: 
https://mail.openvswitch.org/pipermail/ovs-dev/2017-April/331039.html
Signed-off-by: Bhanuprakash Bodireddy <bhanuprakash.bodire...@intel.com>
Signed-off-by: Antonio Fischetti <antonio.fische...@intel.com>
Co-authored-by: Antonio Fischetti <antonio.fische...@intel.com>
Signed-off-by: Markus Magnusson <markus.magnus...@ericsson.com>
Co-authored-by: Markus Magnusson <markus.magnus...@ericsson.com>
Acked-by: Eelco Chaudron <echau...@redhat.com>
---
 lib/dpif-netdev.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 51 insertions(+), 1 deletion(-)

diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
index e2cd931..bfb9650 100644
--- a/lib/dpif-netdev.c
+++ b/lib/dpif-netdev.c
@@ -340,6 +340,7 @@ enum pmd_cycles_counter_type {
 };
 
 #define XPS_TIMEOUT_MS 500LL
+#define LAST_USED_QID_NONE -1
 
 /* Contained by struct dp_netdev_port's 'rxqs' member.  */
 struct dp_netdev_rxq {
@@ -500,7 +501,13 @@ struct rxq_poll {
 struct tx_port {
     struct dp_netdev_port *port;
     int qid;
-    long long last_used;
+    int last_used_qid;        /* Last queue id where packets got
+                                 enqueued. */
+    long long last_used;      /* In case XPS is enabled, it contains the
+                               * timestamp of the last time the port was
+                               * used by the thread to send data.  After
+                               * XPS_TIMEOUT_MS elapses the qid will be
+                               * marked as -1. */
     struct hmap_node node;
 };
 
@@ -3101,6 +3108,25 @@ cycles_count_intermediate(struct dp_netdev_pmd_thread 
*pmd,
     non_atomic_ullong_add(&pmd->cycles.n[type], interval);
 }
 
+static void
+dp_netdev_flush_txq_ports(struct dp_netdev_pmd_thread *pmd)
+{
+    struct tx_port *cached_tx_port;
+    int tx_qid;
+
+    HMAP_FOR_EACH (cached_tx_port, node, &pmd->send_port_cache) {
+        tx_qid = cached_tx_port->last_used_qid;
+
+        if (tx_qid != LAST_USED_QID_NONE) {
+            netdev_txq_flush(cached_tx_port->port->netdev, tx_qid,
+                         cached_tx_port->port->dynamic_txqs);
+
+            /* Queue flushed and mark it empty. */
+            cached_tx_port->last_used_qid = LAST_USED_QID_NONE;
+        }
+    }
+}
+
 static int
 dp_netdev_process_rxq_port(struct dp_netdev_pmd_thread *pmd,
                            struct netdev_rxq *rx,
@@ -3667,6 +3693,9 @@ dpif_netdev_run(struct dpif *dpif)
                         dp_netdev_process_rxq_port(non_pmd,
                                                    port->rxqs[i].rx,
                                                    port->port_no);
+
+                    dp_netdev_flush_txq_ports(non_pmd);
+
                     cycles_count_intermediate(non_pmd, process_packets ?
                                                        PMD_CYCLES_PROCESSING
                                                      : PMD_CYCLES_IDLE);
@@ -3854,6 +3883,8 @@ reload:
             process_packets =
                 dp_netdev_process_rxq_port(pmd, poll_list[i].rx,
                                            poll_list[i].port_no);
+            dp_netdev_flush_txq_ports(pmd);
+
             cycles_count_intermediate(pmd,
                                       process_packets ? PMD_CYCLES_PROCESSING
                                                       : PMD_CYCLES_IDLE);
@@ -3879,6 +3910,9 @@ reload:
 
     cycles_count_end(pmd, PMD_CYCLES_IDLE);
 
+    /* Flush the queues as part of reconfiguration logic. */
+    dp_netdev_flush_txq_ports(pmd);
+
     poll_cnt = pmd_load_queues_and_ports(pmd, &poll_list);
     exiting = latch_is_set(&pmd->exit_latch);
     /* Signal here to make sure the pmd finishes
@@ -4481,6 +4515,7 @@ dp_netdev_add_port_tx_to_pmd(struct dp_netdev_pmd_thread 
*pmd,
 
     tx->port = port;
     tx->qid = -1;
+    tx->last_used_qid = LAST_USED_QID_NONE;
 
     hmap_insert(&pmd->tx_ports, &tx->node, hash_port_no(tx->port->port_no));
     pmd->need_reload = true;
@@ -5051,6 +5086,14 @@ dpif_netdev_xps_get_tx_qid(const struct 
dp_netdev_pmd_thread *pmd,
 
     dpif_netdev_xps_revalidate_pmd(pmd, now, false);
 
+    /* The tx queue can change in XPS case, make sure packets in previous
+     * queue is flushed properly. */
+    if (tx->last_used_qid != LAST_USED_QID_NONE &&
+               tx->qid != tx->last_used_qid) {
+        netdev_txq_flush(port->netdev, tx->last_used_qid, port->dynamic_txqs);
+        tx->last_used_qid = LAST_USED_QID_NONE;
+    }
+
     VLOG_DBG("Core %d: New TX queue ID %d for port \'%s\'.",
              pmd->core_id, tx->qid, netdev_get_name(tx->port->netdev));
     return min_qid;
@@ -5146,6 +5189,13 @@ dp_execute_cb(void *aux_, struct dp_packet_batch 
*packets_,
                 tx_qid = pmd->static_tx_qid;
             }
 
+            /* In case these packets gets buffered into an intermediate
+             * queue and XPS is enabled the flush function could find a
+             * different tx qid assigned to its thread.  We keep track
+             * of the qid we're now using, that will trigger the flush
+             * function and will select the right queue to flush. */
+            p->last_used_qid = tx_qid;
+
             netdev_send(p->port->netdev, tx_qid, packets_, may_steal,
                         dynamic_txqs);
             return;
-- 
2.4.11

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