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, 2 insertions, 2 deletions

diff --git a/src/vnet/session/application_interface.h b/src/vnet/session/application_interface.h
index 8323e72e768..b49744c0b29 100644
--- a/src/vnet/session/application_interface.h
+++ b/src/vnet/session/application_interface.h
@@ -459,7 +459,7 @@ app_send_dgram_raw (svm_fifo_t * f, app_session_transport_t * at,
   session_dgram_hdr_t hdr;
   int rv;
 
-  max_enqueue = svm_fifo_max_enqueue (f);
+  max_enqueue = svm_fifo_max_enqueue_prod (f);
   if (max_enqueue <= sizeof (session_dgram_hdr_t))
     return 0;
 
@@ -533,7 +533,7 @@ app_recv_dgram_raw (svm_fifo_t * f, u8 * buf, u32 len,
   u32 max_deq;
   int rv;
 
-  max_deq = svm_fifo_max_dequeue (f);
+  max_deq = svm_fifo_max_dequeue_cons (f);
   if (max_deq < sizeof (session_dgram_hdr_t))
     {
       if (clear_evt)