svm_fifo rework to avoid contention on cursize

Problems Addressed:
- Contention of cursize by producer and consumer.
- Reduce the no of modulo operations.

Changes:
- Synchronization between producer and consumer changed from cursize
  to head and tail indexes
  Implications: reduces the usable size of fifo by 1.
- Using weaker memory ordering C++11 atomics to access head and tail
  based on producer and consumer role.
- Head and tail indexes are unsigned 32 bit integers. Additions and
  subtraction on them are implicit 32 bit Modulo operation.
- Adding weaker memory ordering variants of max_enq, max_deq, is_empty
  and is_full Using them appropriately in all places.

Perfomance improvement (iperf3 via Hoststack):

iperf3 Server: Marvell ThunderX2(AArch64) - iperf3 Client: Skylake(x86)
   ~6%(256 rxd/txd) - ~11%(2048 rxd/txd)

Change-Id: I1d484e000e437430fdd5a819657d1c6b62443018
Signed-off-by: Sirshak Das <sirshak.das@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/vnet/session/session_node.c b/src/vnet/session/session_node.c
index 2eac5152e16..c894c437cde 100644
--- a/src/vnet/session/session_node.c
+++ b/src/vnet/session/session_node.c
@@ -82,7 +82,8 @@ session_mq_accepted_reply_handler (void *data)
     {
       old_state = s->session_state;
       s->session_state = SESSION_STATE_READY;
-      if (!svm_fifo_is_empty (s->rx_fifo))
+
+      if (!svm_fifo_is_empty_prod (s->rx_fifo))
 	app_worker_lock_and_send_event (app_wrk, s, SESSION_IO_EVT_RX);
 
       /* Closed while waiting for app to reply. Resend disconnect */
@@ -557,7 +558,7 @@ session_tx_set_dequeue_params (vlib_main_t * vm, session_tx_context_t * ctx,
 			       u32 max_segs, u8 peek_data)
 {
   u32 n_bytes_per_buf, n_bytes_per_seg;
-  ctx->max_dequeue = svm_fifo_max_dequeue (ctx->s->tx_fifo);
+  ctx->max_dequeue = svm_fifo_max_dequeue_cons (ctx->s->tx_fifo);
   if (peek_data)
     {
       /* Offset in rx fifo from where to peek data */
@@ -788,7 +789,7 @@ session_tx_fifo_read_and_snd_i (vlib_main_t * vm, vlib_node_runtime_t * node,
 	svm_fifo_overwrite_head (ctx->s->tx_fifo, (u8 *) & ctx->hdr,
 				 sizeof (session_dgram_pre_hdr_t));
       /* More data needs to be read */
-      else if (svm_fifo_max_dequeue (ctx->s->tx_fifo) > 0)
+      else if (svm_fifo_max_dequeue_cons (ctx->s->tx_fifo) > 0)
 	if (svm_fifo_set_event (ctx->s->tx_fifo))
 	  vec_add1 (wrk->pending_event_vector, *e);
     }
@@ -956,7 +957,8 @@ session_queue_node_fn (vlib_main_t * vm, vlib_node_runtime_t * node,
 	   * and the tx queue is still not empty, try to wait for some
 	   * dispatch cycles */
 	  if (!e->postponed
-	      || (e->postponed < 200 && svm_fifo_max_dequeue (s->tx_fifo)))
+	      || (e->postponed < 200
+		  && svm_fifo_max_dequeue_cons (s->tx_fifo)))
 	    {
 	      e->postponed += 1;
 	      vec_add1 (wrk->pending_disconnects, *e);